THE "CHALLENGER" THERMOMETERS 83 



in 1876, Sir Wyville Thomson consulted Tait as to the corrections to be 

 applied to the readings of the deep sea temperatures given by the self-record- 

 ing thermometers which had been used. Experiments made by Captain 

 J. E. Davis, R.N., before the Expedition started on its four years' voyage, 

 indicated that a correction of at least half a degree Fahrenheit for every mile 

 depth under the sea had to be applied. A careful examination of the thermo- 

 meters with their protected bulbs convinced Tait that only very slight corrections 

 would be required ; and the necessity arose for retesting the thermometers. 

 In these laboratory experiments, as conducted first by Captain Davis and then 

 by Professor Tait, the conditions are very different from those under which 

 the thermometers record the temperatures of the ocean deeps. For example, 

 under the increasing pressure in the hydraulic press the temperature of the 

 surrounding water will be raised. Captain Davis and Professor Miller tried 

 to determine this rise of temperature by direct experiment ; and after taking 

 it into account they found a correction still to be applied, and this they referred 

 to the direct effect of pressure. This pressure correction accordingly was to 

 be applied to the readings obtained in deep sea observations. Tail's acuter 

 physical instinct saw no necessity for such a correction ; and after a prolonged 

 investigation into all the possible causes of temperature change he found that 

 the vulcanite mounting of the thermometers was the principal source of the 

 change which Davis and Miller failed to account for. The heating of the 

 vulcanite mounting due to compression would be of no consequence in the 

 deep sea experiments ; consequently no correction was needed. Or, to put 

 it quite accurately, the correction due to pressure was of an order distinctly 

 smaller than the errors of observation and therefore negligible. See Tail's 

 Report, "Challenger" Narrative, Vol. u, Appendix A; Sci. Pap. Vol. i, 



P- 457- 



The beautiful hydraulic apparatus designed by Professor Tait and supplied 



by the Admiralty for making these tests was utilised by him in making further 

 investigations in the realm of high pressures. Some of these investigations 

 form the substance of a second " Challenger" Report (Physics and Chemistry, 

 Vol. ii, Part iv, 1888), bearing upon certain physical properties of fresh 

 water and of sea-water (Sci. Pap. Vol. n, p. i). The wide scope of this 

 enquiry may be best indicated by a few quotations from his own summary of 

 results. The compressibility of the glass of the piezometers was measured 

 by means of J. Y. Buchanan's apparatus, and found to be O'ooooo26 per atmo- 

 sphere. By a modified form of piezometer the compressibility of mercury was 



ii a 



