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In 1938 the bathythermograph (BT) came into use; this ingenious device for determining temperature below the

ocean surface was the brainchild of Athelstan F. Spilhaus, then at Woods Hole Oceanographic Institution. The original device was modified during World War II especially to gather data on the temperature structure of the ocean for the Navy; it consists of a temperature-sensing element to which is attached a stylus that scratches a trace on a glass slide, recording temperature against pressure. The early slides were prepared “by rubbing a bit of skunk oil on with a finger and then wiping off with the soft side of one's hand,” followed by smoking the slide over the flame of a Bunsen burner.[6] The mechanical BT is lowered by means of a small winch on the ship. The instrument drops nearly freely through the water, dragging out wire as it sinks; when the maximum depth is reached, a brake is applied, and the BT is drawn to the surface. Over the years BTs were developed for lowering to 900 feet. But Li'l Abner's Skunk Works was put out of business — at least for BT slides — by replacing skunk oil with an evaporated metal film.

The first BT slides handled by Scripps were a few sent to the institution by Woods Hole in late 1940; Eugene LaFond made prints of these — the first time this had been done — and returned them. During World War II LaFond was in charge of the BT center at the University of California Division of War Research and carried out the analysis of the slides on the Scripps campus. LaFond's group compiled sonar charts of the BT data for the Pacific and Indian oceans for use by Navy ships. After the war Scripps was designated the repository for the BT soundings in the Pacific Ocean that had been taken by Navy and Coast Guard ships — some 100,000 of them, accumulated especially for studies of underwater sound. When LaFond transferred to Navy Electronics Laboratory in 1947, graduate student Dale Leipper was put in charge of the group of Scripps technicians who converted the BT readings into depths, and he

was followed by graduate students Wayne V. Burt and John Cochrane.

In the latter 1940s Margaret Robinson entered the scene, first as a draftsman in the BT section. She soon enrolled as a graduate student — in spite of Director Sverdrup, who told her that “women will never be accepted as oceanographers.”[7] Mrs. Robinson received her M.S. in 1951, and in 1957 became supervisor of the Bathythermograph Temperature Data Analysis Section, a post that she filled until her retirement in 1974. With “energy, ingenuity and perseverance,” Margaret Robinson and her assistants (all women) processed vast amounts of ocean temperature data, from BTs and from shore stations.

The early slides processed by the BT group were gathered laboriously. On Midpac Expedition in 1950, for example, according to watchstander Edward S. Barr:

The BT winch … was used every hour. … [It] was operated from the side of the ship. One would lower the recording device — looking like a rocket — over the side, and let it drop, free wheeling, to a predetermined depth. Then the brake would be applied, stopping its descent. Winching power was then applied to reel the device back to the surface and aboard. … In any kind of rough weather, this BT position was frequently subject to waves making a clean sweep of the deck. In spite of breaking waves over the side, the operator had to hold his station, because the equipment was already over the side. One couldn't run for shelter as the brake and hoisting power were combined in a single hand lever. To let go of this lever would cause all the wire on the winch to unwind, sending the recording device and all its cable to the ocean bottom forever. It was not at all uncommon, from the protective position of the laboratory door, to look back and

see your watchmate at the BT winch completely disappear from sight as a wave would come crashing over the side. … We also took turns taking BT readings. It wasn't fair for only one person to get wet consistently.[8]

Crashing waves were not the only problem. As B. King Couper and Eugene C. LaFond noted of the wartime technique: “Probably the greatest hazard was a swinging BT after it left the water. A familiar saying was: ‘sight, surface, oh that son of a gun,’ or words to that effect, as the instrument swung in circles around the boom.”[9] James M. Snodgrass noted that “Oftentimes, in somewhat heavy — and not so heavy — weather, [the BT] behaves as a tethered lethal missile. The bathythermograph cable with bathythermograph attached has occasionally been wrapped around the ship's funnel.”[10]

Snodgrass, in fact, found himself “appalled at the primitive nature of the BT lowerings”; a physics graduate of Oberlin College, he had served at the University of California Division of War Research during World War II, and after two years in industry, he returned to Scripps at the invitation of Roger Revelle in the fall of 1948. He participated in the development of several of the new oceanographic tools of the early 1950s. By 1958 he was in frequent correspondence with B. King Couper of the Navy Bureau of Ships over the possibility of developing an easier method of taking BTs, specifically an expendable bathythermograph. Snodgrass's idea was to use a wire-connected device, instead of an acoustic signal. As he described it to Couper:

Briefly, the unit would break down in two components, as follows: the ship to surface unit, and surface to expendable unit. I have in mind a package which could be jettisoned, either by the “Armstrong” method, or some simple mechanical device, which would at all times be connected to the surface vessel. The wire would be payed out from the surface ship and not from the surface float unit. The surface float would require a minimum of flotation and a small, very simple sea anchor. From this simple platform the expendable BT unit would sink as outlined for the acoustic unit. However, it would unwind as it goes a very fine thread of probably neutrally buoyant conductor terminating at the float unit, thence connected to the wire leading to the ship.[11]


Louis Garrison taking a mechanical bathythermograph recording on Midpac Expedition, 1950.

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A number of companies were entering the growing field of ocean industry, and Snodgrass urged them toward developing an expendable bathythermograph (quickly called an XBT), which was much desired by the Navy. He discussed with engineers from various companies his ideas on a feasible approach. In the early 1960s the Navy called for bids on an XBT, and three companies, including Sippican Corporation of Marion, Massachusetts, received contracts to provide a small number of units. Within a short period of time, Sippican became the sole supplier of XBTs.

Their unit, based on Snodgrass's original idea, consists of: a probe that falls freely at 20 feet per second in the ocean; a wire link; and a shipboard canister that remains in the launcher until the measurement is made. The wire link, which is wound on a spool in the probe and a second spool in the canister, is unreeled from both spools as the probe sinks and the ship moves away, so that the wire remains stationary. A thermistor temperature sensor in the probe is connected electrically to a chart recorder through the three-conductor fine wire and a cable from the launcher to the recorder. The sinking rate of the probe determines its depth and so yields a temperature-depth trace on the recorder.


Over the years the quality of XBTs has been improved well beyond the accuracy of the laborious mechanical BTs, and at a cost per unit measurement that is considerably less. Also, the depth to which XBTs can be used is much greater.

In 1967, Jeffery D. Frautschy, Marston C. Sargent, and Phillip R. Mack, supervisor of the staff shop, designed and built the BT digitizer, a machine to digitize analog BT traces automatically. Computer programs were developed to speed up the data reduction and to conform to the National Oceanographic Data Center system. The programs could also be used to derive annual temperature distribution from 125 meters to the ocean bottom and to determine annual salinity distribution from surface to bottom.

The processing group handled BT data from many other institutions in several countries, as well as Scripps and Navy BTs. From the mass of material Margaret Robinson prepared “carefully compiled atlases of the temperature distribution of the oceans and seas of the world.” The first one, on the North Pacific Ocean, was prepared in 1971 for the U.S. Naval Oceanographic Office; it provided monthly temperature distribution at the surface and at levels of 100, 200, 300, and 400 feet. Atlases for the Gulf of Mexico, the Caribbean, the Red Sea, the Mediterranean, and the Black Sea followed; these brought the comment to Mrs. Robinson, upon her retirement, from Chief of Naval Research Rear Admiral M. D. Van Orden that her compilations had “formed the basis for the Fleet's oceanographic forecasting capability, the source material for many scientific evaluations of the physical ocean, especially the North Pacific, and provided the framework for many oceanographic expeditions.” By that time Margaret Robinson — and “her girls” — had processed 477, 483 bathythermograph slides.

Couper and LaFond noted:


Although the accuracy of [BT] data is not always as good as desired, they have proven extremely useful to both the Navy and others in understanding water structures and the physical, chemical and biological processes which occur in the upper layers of the sea. For instance, the correlation between time-lapse photographs of moving slicks and BT layer-depth information established the relationship of surface slicks to internal waves. Even in meteorology, heat transfer can be more accurately established with BT data.[12]

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