OBSERVATIONS AND COLLECTIONS

Positions at Sea

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The geographical location at which an observation is made must be known. The greater frequency with which observations are now taken makes it necessary to know the locations of sampling very accurately, and hence special methods of determining positions at sea have been developed. Accurate knowledge of locations is particularly necessary in surveying, where the introduction of sonic sounding methods has made it possible to take large numbers of soundings that must be precisely plotted in order to bring out the true configuration of the bottom. The specialized techniques developed by such organizations as the U. S. Coast and Geodetic Survey have not yet been used for general oceanographic work, but the methods may be adopted for the study of special problems.

When in sight of land where recognizable features are accurately located, the position of the vessel may be determined by means of horizontal angles and bearings on shore features. Out of sight of land the position can be determined by astronomic sights or by radio direction-finder bearings. Between positions established in these ways the location at any time is obtained by dead reckoning—namely, from the course steered and the distance run. Such methods are adequate for most oceanographic work, but, where greater accuracy is required, as in offshore surveying, positions found in this way are not commensurate with the accuracy and frequency of soundings. In certain cases an anchored vessel or buoy whose position can be exactly established by repeated astronomic observations is used as a point of reference. Since 1923 the U. S. Coast and Geodetic Survey has experimented with sonic methods of locating positions and has developed them to a high degree of accuracy. In radioacoustic ranging (usually designated as R.A.R.), the surveying vessel drops a depth bomb that is fired by fuse or electricity. The sound of the explosion is picked up by a hydrophone on the vessel and recorded on a chronograph. The impulse of the explosion, which travels in all directions, is picked up by hydrophones attached to shore stations, anchored vessels, or buoys whose positions are accurately known. The hydrophones are connected to radio transmitters, and the sound impulse received at each hydrophone is transmitted by radio to the surveying vessel, where the times of reception are automatically recorded on the chronograph. Since the time required for the transmission and reception of the radio signal is infinitesimal, the period between the bomb explosion and the return of the signal from each hydrophone is that required for the sound impulse to travel through the water from the point of the bomb explosion to the hydrophone. The velocity of sound in water can be computed from the known distribution of temperature and salinity

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(p. 77), but the waves are reflected between the surface and the bottom and may be distorted by density stratification, so that empirical tests must usually be made to establish the effective horizontal velocity. When the velocity of the sound impulse is known, the distance between the bomb and the accurately located hydrophones is readily obtained. From two or more such radii the location of the bomb explosion up to distances of 100 miles from the hydrophones can be determined to within a few hundred feet. Radioacoustic ranges are obtained at frequent intervals while the survey vessel is running sounding lines.

Taut-wire traverses are also used to determine accurately the distances between anchored buoys when detailed surveys are made out of sight of land. The distances are measured by paying out steel piano wire under controlled tension over an accurate meter wheel from drums which carry over 140 miles of wire. When the distance between a row of anchored buoys 15 or 20 miles long has been measured, the wire is cut and abandoned. With a combination of the methods outlined above, extremely accurate surveys can be made out of sight of land where depths are not too great to make it impossible to anchor the buoys satisfactorily. The methods employed by the Coast and Geodetic Survey have been described by Rudé (1938) and by Veatch and Smith (1939).

Sonic Soundings

Sonic-sounding equipment consists of three essential parts: (1) a source that will emit a sound impulse, (2) an instrument for detecting or recording the outgoing and the returning signals, and (3) a means of measuring the time required for the sound to travel to the sea bottom and for the echo to return to the ship. Sound sources are of two general types: those that emit sound of audible frequency which is nondirectional in water, or those that emit high-frequency vibrations which are nonaudible and are classed as ultrasonic. The audible type is satisfactory for general use, but for sounding in shoal water or over a bottom that is very irregular, directional ultrasonic equipment must be used. Audibletype transmitters usually consist of a diaphragm that is vibrated by an electromagnet, although other devices are employed. The ultra- or supersonic transmitter depends upon the piezoelectric property of quartz crystals which, when subjected to a high voltage, vibrate at high frequency, and, as the process is reversible, the returning echo stimulates a current through the circuit, so that the same device is used as a transmitter and as a sound detector. In the audible-type sonic sounder the outgoing signal and the returning echo are picked up by a submerged microphone called a hydrophone.

Various devices are used to determine the time required for the sound impulse to travel to the bottom and return to the hydrophone. For sounding in deep water the simplest method is to measure the

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interval by means of a stopwatch. However, this method is not very accurate and is not practical in shallow water, where the time interval is small. Most instruments depend upon visual signals or a combination of visual and auditory signals to measure the time interval, and in some devices the interval is recorded automatically upon a moving tape. In practice, an instrument is set for a constant sounding velocity (p. 79), usually between 800 and 820 fathoms per second (1463 and 1500 m/sec), and hence the time interval is a direct measure of the depth obtained, using a constant sounding velocity. In accurate work the depths obtained in this way must be adjusted to allow for the vertical distribution of temperature and salinity. The time interval is commonly measured by means of a rotating disk revolving at a constant speed. This speed is determined by the graduations on the disk and the sounding velocity. For example, the disk may be graduated to read from zero to 1500 m, and, if rated for a sound velocity of 1500 m/sec, it will require 2 sec for one rotation of the disk; that is, 2 sec is the time required for the sound to travel to the bottom and back when the depth is 1500 m. The outgoing signal is activated automatically each time the disk is at zero. When earphones are used in deep-water instruments of this type, the position of the disk is noted at the instant the return echo is heard. The recorded depth will usually represent the average of several such measurements. For work in shallow water (less than 500 m) a flashing light signal activated by the outgoing sound impulse and the returning echo is commonly used. In such instruments the light is on a revolving arm that is mounted behind a graduated disk with a circular slit through which the light is visible. The outgoing sound impulse is emitted automatically each time the light passes zero on the depth scale, and the returning echo causes the light to flash, the depth being indicated by the graduations on the dial. In automatic recording devices the depths are marked upon a moving paper, and the plot obtained in this way represents an accurate profile of the bottom. Details concerning the construction and operation of sonic depth finders are given in the Hydrographic Review, published periodically by the International Hydrographic Bureau. The instruments used by the U. S. Coast and Geodetic Survey are described by Rudé (1938) and by Veatch and Smith (1939).

Wire Soundings

Since the introduction of sonic methods, relatively few wire soundings are taken for the sole purpose of measuring depths. Checks must be made from time to time to see that the sonic equipment is operating successfully, but in most cases wire soundings are now made for the express purpose of obtaining samples of the sea-bottom sediments. The weight of equipment used for collecting bottom samples has increased, and, as a consequence, the piano-wire sounding machines are no longer

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adequate. Instead, the hydrographic cable and winch, or even the heavy winch, must be used for this purpose. The amount of weight attached to the end of the wire rope or mounted on the bottom-sampling device depends upon the strength of the cable, the depth of water (hence, the weight of wire suspended in the water), and in many cases upon the type of sampling device. It must be sufficient, when the weight itself reaches the bottom, to permit the detection of the reduced load by means of the motion of the accumulator, dynamometer, or other device. Unless the winch is stopped immediately, the wire rope will pile up on the bottom and will be badly tangled or kinked when hauled in again. If iron weights are used, they may be dropped by a release mechanism when striking the bottom, but, if lead weights are used, they are permanently secured. For sounding with 5/32-inch steel rope, about 50 lb must be attached to the end of the cable for depths less than 1000 m, and for depths of about 4000 m, double the amount of lead is needed.

An unprotected reversing thermometer for measuring pressure, used in conjunction with a protected reversing thermometer and attached to the sounding wire some 50 m above the weight, is sometimes used as a check on the depth (p. 351).

Bottom-Sampling Devices

Devices used for collecting specimens of the sea-bottom sediments depend upon the character of the deposit, the depth of water, and the strength of the wire rope available. Certain apparatus that is suitable for work in soft, cohesive sediments cannot be used where the bottom material is coarse-grained or where it is rocky. Similarly, other types that can be operated in shallow depths are too heavy for general use in deep water. Methods have been devised for sampling the superficial layers of the sediments, but since the Meteor Expedition, 1925–1927, greater emphasis has been placed upon obtaining core samples. Instruments are now in use that will take cores several meters long, and much thought is being given to the development of instruments that will take even longer samples. Unlike the equipment used in most of the other fields of oceanography, the devices for taking samples of the marine sediments are not standardized. Every investigator uses his own type of bottom sampler, but all samplers are based on certain basic designs. Hough (1939) has listed and described the various types and gives an exhaustive bibliography.

Bottom samplers used for oceanographic work fall into three general categories; dredges (drag buckets), snappers, and coring tubes. Dredges patterned after the naturalist dredge (fig. 89), but constructed of stronger material and with a chain-mesh bag, are used for procuring samples of rock where the sea bottom is covered with rock fragments or where there are outcrops of solid rock. Smaller cylindrical dredges with solid bottoms

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are sometimes used for collecting nonconsolidated material in relatively shallow water. Snapper samplers of the clamshell type have been widely used for obtaining samples of the superficial layers of the sediments. The telegraph snapper (fig. 84) has been widely used, particularly in routine wire-sounding work by surveying vessels. The drawback to the use of this relatively simple device is that it takes so small a sample. The same principle has been employed in the Ross snapper, which will hold several hundred cubic centimeters of sediment. Other samplers of this general type have been devised for sampling the benthic fauna (fig. 89 and p. 375). The disadvantage of the clamshell type of sampler is that its contents are likely to be washed out while it is being hauled to the surface. This is particularly true when it is used in areas where the bottom is sandy or contains coarse fragments, since a fragment caught in the jaws may prevent them from closing completely.

Coring devices (fig. 85) are essentially long tubes that are driven into the sediment, either by their own momentum or by the discharge of an explosive. The latter principle is used in the coring instrument developed by Piggot (1936). A momentum-type coring instrument weighing about 600 lb with the weights attached will take cores up to about 5 m in length in soft sediments at depths as great as about 2000 m (Emery and Dietz, 1941). The Piggot coring tube has been used to take cores about 3 m long at depths greater than 4000 m. In coarse-grained deposits the coring tubes are not capable of penetrating more than about 0.5 m.

The momentum-type core sampler is allowed to run out freely when it approaches the bottom. The depth of penetration is determined by the weight of the instrument, the character of the sediment, the diameter of the tube, and the type of cutting nose. The factor that determines the size and type of coring tube which can be used on any wire rope is not the weight of the instrument alone, because the greatest strain is developed when the coring tube is pulled out of the sediment, and this may be several times the weight of the instrument. The Emery-Dietz sampler is constructed of galvanized iron pipe of 2- or 2.5-inch diameter. It is connected through a reducing coupling to a smaller pipe on which the weights are mounted. The reducing coupling is perforated to permit the passage of water, and is sometimes fitted with a ball valve. The cutting nose is sharp, with a slightly smaller internal diameter than the tube itself so as to reduce the internal wall friction and to hold in the inner liner, which further reduces the wall friction and facilitates the removal of the core samples. Inner liners are sheets of metal or celluloid rolled up and inserted in the pipe, or they are glass or metal pipes that are cut open after the sample is taken. In some coring devices a “core catcher” is fitted to the nose of the coring tube to prevent the core from sliding out of the tube after the instrument is raised from the bottom. The depth of penetration of coring tubes is generally considerably greater than the length of the sample obtained. The amount of “compaction” varies between about 25 and 50 per cent, depending upon the characteristics of the coring tube and the type of sediment. The nature of the compaction and the effects that it may have upon the stratification of the sample have been investigated by Emery and Dietz (1941). Smaller instruments based on an original design by Ekman and developed by Trask (Hough, 1939), but of the same general type described above, can be used on fairly light gear and will take samples between 0.5 and 1.0 m in length.

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The care and treatment of bottom samples depend to a large extent upon the nature of the examinations that are to be made later (chapter XX). For most purposes it is sufficient to place the specimens in mason jars fitted with rubber washers. No preservative or additional water is necessary. Labels should be placed on the outside of the bottle, since mechanical abrasion and the activities of microorganisms may destroy or render illegible any paper in contact with the sediment sample. Core samples may be cut in sections and carefully placed in bottles, or the entire specimen may be kept in the inner liner.

Temperature Measurements

Three types of temperature-measuring devices are used in oceanographic work. Accurate thermometers of the standard type are employed for measuring the surface temperature when a sample of the surface water is taken with a bucket and for determining the subsurface temperatures when the water sample is taken with a thermally insulated sampling bottle (p. 354). The thermometers used for measuring temperatures at subsurface levels are of the reversing type and are generally mounted upon water-sampling bottles so that temperatures and the water sample for salinity or other chemical and physical tests are obtained at the same level. The third type are temperature-measuring instruments that give a continuous record, such as thermographs, which are employed at shore stations and on board vessels to record the temperature at some fixed level at ot near the sea surface. Many devices have been invented for

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obtaining the temperature as a continuous function of depth, but most of these, for one reason or another, have proved impractical or of insufficient accuracy. The bathythermograph (p. 352) developed by Spilhaus has overcome many of the difficulties that rendered previous designs ineffective.

The centigrade scale is the standard for the scientific investigation of the sea. A high degree of accuracy is necessary in temperature measurements because of the relatively large effects that temperature has upon the density and other physical properties and because of the extremely small variations in temperature found at great depths. Subsurface temperatures must be accurate to within less than 0.05°C, and under certain circumstances to within 0.01°. Such accuracy can be obtained only with well-made thermometers that have been carefully calibrated and rechecked from time to time. Because of the greater variability of conditions in the surface layers the standards of accuracy there need not be quite so high.

Conventional-type thermometers for surface temperatures or for use with an insulated bottle must have an open scale that is easy to read, with divisions for every tenth of a degree. The scale should preferably be etched upon the glass of the capillary. The thermometer should be of small thermal capacity in order to attain equilibrium rapidly; it should also be checked for calibration errors at a number of points on the scale by comparison with a thermometer of known accuracy, and it should be read

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with the scale immersed to the height of the mercury column. Observations of surface temperatures made upon bucket samples should be obtained immediately after the sample is taken; otherwise, heating or cooling of the water sample by radiation, conduction, and evaporation may have a measurable effect upon the temperature. Surface temperatures obtained in this way represent the conditions in, approximately, the upper 1 m of water. Samples taken from a vessel must be obtained as far away as possible from any discharge outlets from the hull, and, if the vessel is under way, they should be taken near the bow so as to avoid the churned-up water of the wake (Brooks, 1932).

Protected Reversing Thermometers. Reversing thermometers (fig. 86) are usually mounted upon the water-sampling bottles (fig. 87), but they may be mounted in reversing frames and used independently. Reversing thermometers were first introduced by Negretti and Zambra (London) in 1874, and since that time have been improved, so that well made instruments are now accurate to within 0.01°C. On the Challenger Expedition, 1873–1876, the subsurface temperatures were measured by means of minimum thermometers, which were the most satisfactory instrument available at that time.

A reversing thermometer is essentially a double-ended thermometer. It is sent down to the required depth in the set position, and in this position it consists of a large reservoir of mercury connected by means of a fine capillary to a smaller bulb at the upper end. Just above the large reservoir the capillary is constricted and branched, with a small arm, and above this the thermometer tube is bent in a loop, from which it continues straight and terminates in the smaller bulb. The thermometer is so constructed that in the set position mercury fills the reservoir, the capillary, and part of the bulb. The amount of mercury above the constriction depends upon the temperature, and, when the thermometer is reversed, by turning through 180 degrees, the mercury column breaks at the point of constriction and runs down, filling the bulb and part of the graduated capillary, and thus indicating the temperature at reversal. The loop in the capillary, which is generally of enlarged diameter, is designed to trap any mercury that is forced past the constriction if the temperature is raised after the thermometer has been reversed. In order to correct the reading for the changes resulting from differences between the temperature at reversal and the surrounding temperature at the time of reading, a small standard-type thermometer, known as the auxiliary thermometer, is mounted alongside the reversing thermometer. The reversing thermometer and the auxiliary thermometer are enclosed in a heavy glass tube that is partially evacuated except for the portion surrounding the reservoir of the reversing thermometer, and this part is filled with mercury to serve as a thermal conductor between the surroundings and the reservoir. Besides protecting

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the thermometer from damage, the tube is an essential part of the device, because it eliminates the effect of hydrostatic pressure.

Readings obtained by reversing thermometers must be corrected for the changes due to differences between the temperature at reversal and the temperature at which the thermometer is read, and for calibration errors. An equation developed by Schumacher is commonly used for correction:

ΔT is the correction to be added algebraically to the uncorrected reading of the reversing thermometer, T′, t is the temperature at which the instrument is read, V₀ is the volume of the small bulb and of the capillary up to the 0°C graduation expressed in terms of degree units of the capillary, and K is a constant depending upon the relative thermal expansion of mercury and the type of glass used in the thermometer. For most reversing thermometers, K has the value 6100. The term I is the calibration correction, which depends upon the value of T′. Where there are large numbers of observations to be corrected, it is convenient to prepare graphs or tables for each thermometer from which the value of ΔT can be obtained for any values of T′ and t and in which the calibration correction is included.

Reversing thermometers are usually used in pairs, most commonly attached to the water-sampling bottles, but they may be mounted in special reversing frames that are either operated by messenger or that have a propeller release. The frames holding the thermometers are brass tubes which have been cut away so that the scale is visible and which are perforated around the reservoir. The ends of the tubes are fitted with coil springs or packed with sponge rubber so that the thermometers are firmly held but are not subject to strains.

Unprotected Reversing Thermometers. Reversing thermometers identical in design with those previously described but having an open protective tube are employed to determine the depths of sampling (fig. 86). Because of the difference in the compressibility of glass and mercury, thermometers subjected to pressure give a fictitious “temperature” reading that is dependent upon the temperature and the pressure. This characteristic is utilized for determining the depth of reversal. Instruments used for this purpose are so designed that the apparent temperature increase due to the hydrostatic pressure is about 0.01°C/m. An unprotected thermometer is always paired with a protected thermometer, by means of which the temperature in situ, T_w, is determined. When T_w, has been obtained by correcting the readings of the protected thermometer, the correction to be added to the reading of the unprotected thermometer, T′_u, can be obtained from the equation

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T′_u and t_u are the readings of the unprotected reversing thermometer and its auxiliary thermometer, and I is the calibration correction. The difference between the corrected reading of the unprotected thermometer, T_u, and the corrected reading of the protected thermometer, T_w, represents the effect of the hydrostatic pressure at the depth of reversal. The depth of reversal is calculated from the expression where Q is the pressure constant for the individual thermometer, which is expressed in degrees increase in apparent temperature due to a pressure increase of 0.1 kg/cm², and where ρ_m is the mean density in situ of the overlying water. For work within any limited area it is usually adequate to establish a set of standard mean densities for use at different levels. Depths obtained by means of unprotected thermometers are of the greatest value when the wire rope holding the thermometers is not vertical in the water. When serial observations are made, unprotected thermometers should be placed on the lowest sampling bottle and, if possible, one on an intermediate bottle and one near the top of the cast (p. 356). The probable error of depths obtained by unprotected thermometers is about ±5 m for depths less than about 1000 m, and at greater depths it amounts to about 0.5 per cent of the wire depth. Wüst (1933) has presented a detailed examination of the results of wire soundings, sonic depths, and depths measured by unprotected thermometers on the Meteor Expedition.

Special Devices. Temperatures measured by the methods mentioned above yield observations at discrete points in space and time. Subsurface observations with reversing thermometers are time consuming and the instruments and equipment are expensive. Many devices have been suggested for obtaining continuous observations at selected levels or as a function of depth. Thermographs are commonly used at shore stations and on vessels to obtain a continuous records at or near the sea surface. The thermometer bulb, usually containing mercury, is mounted on the ship's hull or in one of the intake pipes and connected to the recording mechanism by a fine capillary. The recording mechanism traces the temperature on a paper-covered revolving drum. Records of temperatures obtained by a thermograph should be checked at frequent intervals against temperatures obtained in some other way.

Various types of electrical-resistance thermometers have been designed to be lowered into the water and to give a continuous reading, but have not proved satisfactory. Spilhaus (1938, (1940) has developed an

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instrument called a bathythermograph which can be used to obtain a record of temperature as a function of depth in the upper 150 m, where the most pronounced vertical changes are usually found. The temperature-sensitive part activates a bourdon-type element that moves a pen resting against a small smoked-glass slide, which in turn is moved by a pressure-responsive element. As the instrument is lowered into the water and raised again, the pen traces temperature against pressure (hence, depth). This device has the great advantage that it can be operated at frequent intervals while under way, and thus a very detailed picture of the temperature distribution in the upper 150 m can be rapidly obtained. Mosby (1940) has devised an instrument called a thermosounder for measuring temperature against depth which can be used for observations to great depths. The thermal element, mounted on an invar-steel frame, is a 75-cm length of specially treated brass wire attached to a pen that makes a trace upon a circular slide. The slide is slowly turned by means of a propeller as the instrument is lowered through the water.

Water-Sampling Devices

The types of water-sampling devices that will be described are those that are intended for taking samples at subsurface levels for physical and chemical studies. Samples for the enumeration of phytoplankton and for bacteriological examination may be obtained with these instruments, but for such purposes specially designed samplers are more commonly used. A water sampler for collecting at subsurface levels is so designed that it can be closed watertight at any desired depth, and thus the enclosed sample is not contaminated by water at higher levels or lost by leakage after the bottle is brought on board. Because of the great pressures encountered in deep water the sampling bottles are sent down open and then closed at the required depths by means of messengers or propeller releases. To expedite work at sea, water-sampling devices are used in series—that is, with more than one bottle on the wire rope so that samples can be taken at a number of depths on the same cast. As it is essential that temperatures and water samples be taken at the same depths, the water-sampling bottles are fitted with frames in which one or more reversing thermometers are placed. An exception to this is the insulated Pettersson-Nansen bottle, which can be used for the upper few hundred meters (p. 354). Water-sampling devices must be constructed of noncorrosive materials that will reduce contamination of the water samples to a minimum. The bottles are usually made of brass, plated inside with tin or silver or coated with a special lacquer. For removing the water sample they are fitted with a drain cock and an air vent. To be clearly visible when hauling in, the bottles should be painted white. Many types of sampling bottles have been invented, but the rigorous working conditions and the desirable features that have been listed above have reduced the types in general use to only a few of simple but rugged design.

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Messengers are essential for the operation of many types of oceanographic equipment. Although their size and shape will vary for different types of apparatus, they are essentially weights which are drilled out so that they will slide down the wire rope. In order to remove or attach them they are either hinged or slotted. The speed of travel depends upon the shape and weight of the messenger and upon the wire angle (the angle the wire rope makes with the vertical). With no wire angle the messengers used with the Nansen bottles travel approximately 200 m per minute.

Water-sampling devices are of two general types, depending upon the method of closing, which may be accomplished by means of plug valves or by plates seated in rubber. The Nansen bottle, an example of the first type, is the one most widely used in oceanographic research. The Ekman bottle is of the second type.

The Nansen bottle (fig. 87) is a reversing bottle fitted with two plug valves and holding about 1200 ml. The two valves, one on each end of the brass cylinder, are operated synchronously by means of a connecting rod fastened to the clamp that secures the bottle to the wire rope. When the bottle is lowered, this clamp is at the lower end, and the valves are in the open position so that the water can pass through the bottle. The bottle is held in this position by the release mechanism, which passes around the wire rope, but, when a messenger sent down the rope strikes the release, the bottle falls over and turns through 180 degrees, shuts the valves, which are then held closed by a locking device, and reverses the attached thermometers. After reversing the bottle, the messenger releases another messenger that was attached to the wire clamp before lowering. This second messenger closes the next lower bottle, releasing a third messenger, and so on.

The Ekman bottle, which can also be operated in series, consists of a cylindrical tube and top and bottom plates fitted with rubber gaskets. The moving parts are suspended in a frame attached to the wire rope, and, when the instrument is lowered, the water can pass freely through the cylinder. When struck by a messenger the catch is released and the cylinder turns through 180 degrees, thereby pressing the end plates securely against the cylinder and enclosing the water sample. Reversing thermometers are mounted on the cylinder.

Thermally insulated bottles such as that of Pettersson-Nansen (for illustration, see Murray and Hjort, 1912) consist of several rigidly fixed concentric cylinders each of which is fitted with end plates. When these plates are shut, a series of water samples are isolated one within the other. The outermost cylinder and end plates are constructed of brass and ebonite, the inner ones of brass and celluloid, the cylinders

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and end plates being mounted on a frame. In open position there is space between the cylinders and the upper and lower end plates, but, when the catch is struck by the messenger, the cylinders and the upper plate slide down, the cylinders are pressed against the lower end plate, and the upper one seals the bottle. Owing to its construction the bottle must be attached to the end of the wire rope. The temperature at the depth of sampling is determined by a thermometer inserted in the innermost cylinder. Corrections must be applied for adiabatic cooling and, under extreme conditions, for heat conduction.

On the Meteor Expedition a glass-lined sampling bottle with a capacity of 4 l was used to collect large samples with a minimum of contamination. This bottle was attached to the end of the wire rope, the closing mechanism being similar to that used for the insulated bottle. A special mounting that reversed when the bottle was closed was provided for thermometers (Wüst, 1932).

In connection with wire sounding and bottom sampling, it is often desirable to obtain the temperature and the water samples close to the bottom and, as an added check, to obtain the depth by means of an unprotected thermometer. In order to avoid waiting for a messenger to travel all the way to the bottom, possibly one half hour or more, special sampling devices activated by propeller releases are used (Soule, 1932; Parker, 1932). These are usually reversing bottles in which the release pin is attached to a propeller. When the apparatus is being lowered, the propeller holds the pin in place, but, as soon as hauling in is commenced, the propeller rotates and withdraws the pin.

Spilhaus (1940) has devised a multiple sampler consisting of six small valve-closing bottles that can be closed individually at predetermined depths by releases which are activated by the hydrostatic pressure. The instrument was designed to be used in conjunction with the bathythermograph (p. 352) from a vessel while under way.

The general procedure for taking water samples and temperatures by means of Nansen bottles or other serial sampling devices is as follows. In order to keep the wire rope taut and to reduce the wire angle, a stray weight is attached to the end of the wire rope, usually of 50 to 100 lb, depending upon the size of the rope, the depths at which observations are to be taken, and the general working conditions. A certain amount of the wire rope (25 to 50 m) is then payed out so that the weight will not strike the ship when the bottom bottle is being attached or detached, and also to reduce the possibility of damaging the bottle if the weight strikes bottom.

The depth at which samples are to be collected should be planned before the cast is begun. The first bottle, adjusted to the set position, is then attached to the wire rope, the thermometers are checked, and the meter wheel is set at zero. When the wire has been lowered the required

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amount, the second bottle is attached. To operate the bottles, messengers must be attached to the messenger releases on the second bottle and on all those above. The number of bottles attached on a single cast is determined by the strength of the wire and by the operating conditions. Up to twelve or more are sometimes used at one time. After all the bottles have been attached at appropriate intervals, they are lowered to the required depths. Since the meter wheel was set at zero when the first bottle was attached, the entire cast must be lowered by an amount equal to the distance between the sea surface and the height at which the bottles were put on. After the bottles are lowered to the required depths, they are allowed to remain there for about 10 minutes to permit the thermometers to reach the temperatures of their surroundings, and then the messenger is dropped. If the bottles are located at depths of less than about 500 m, their proper functioning may be checked by touching the wire, as it is usually possible to feel the jerks when the messengers strike the bottles. When the samples are taken at great depths or when the wire angle is large, it is impossible to feel the jerks, and therefore sufficient time must be allowed for the messengers to travel to the deepest bottle before hauling in. When the wire angle is large, the slower rate of travel of the messenger must be taken into consideration. The wire is then hauled in and the bottles are removed and placed in their rack in the deck laboratory, care being taken to avoid turning the bottles from the reversed position, since the reversing thermometers may set themselves again.

Just before the messenger is released the wire angle should be estimated or measured. Knowledge of the wire angle is useful when determining the depth of sampling, as will be shown later.

Certain accessories are necessary when handling water-sampling bottles. As a safety measure a light line with a harness snap should be attached to the rail. This line is snapped on the bottle before it is handed to the operator on the working platform and is not removed until the bottle is firmly clamped on the wire rope. It is also attached when removing the bottle. To hold the wire rope steady and close to the platform a short line with a large hook should be attached to the platform. The hook is placed on the wire rope when instruments are being attached to it or removed from it. When the vessel is drifting with wind or surface currents, the wire will not hang vertically and will sometimes trail so far away that the wire angle may be as much as 50 or 60 degrees. Under these circumstances the wire rope must be pulled in by means of a boat hook or block and tackle in order to attach the hook.

Because of the larger vertical gradients in the distribution of properties in the upper layers, observations are taken at relatively close intervals near the surface and at increasingly larger intervals at greater depths. The International Association of Physical Oceanography,

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in 1936, proposed the following standard depths at which observations should be obtained directly or by interpolation from the distribution at other levels. The lower limit is determined by the depth of water or the plan of operations. The standard depths, in meters, are: surface, 10, 20, 30, 50, 75, 100, 150, 200, (250), 300, 400, 500, 600, (700, 800, 1000, 1200, 1500, 2000, 2500, 3000, 4000, and thereafter by 1000-m intervals to the bottom. The depths in parentheses are optional. In addition to observations at standard depths, it is often desirable to obtain temperatures and water samples close to the bottom. In deep water this usually means within about 50 m of the bottom, but in moderate and shallow depths it may be much less, depending upon the bottom topography and the working conditions.