REPORT Or THE PRESSURE ERRORS OF THE THERMOMETERS. 21 



steel cylinder, and the l: eight to which it rose could be easily measured. Comparative 

 experiments were made several times by putting one of the glass gauges, whose scale had 

 been carefully ascertained:, inside the apparatus, while this newly-described gauge was 

 also connected with it. In this way the external gauge was accurately calibrated. But, 

 lest an accident should happen to one of the gauges, or to its index (as sometimes was 

 the case) no experiment was made without the presence of at least three gauges. The 

 way in which these worked together during the whole course of the experiments is the 

 best possible proof of their value. This form of gauge, also, is greatly improved by 

 inserting a glass tube closed at both ends into the bulb ; for the temperature changes 

 produced by pressure in mercury are greater than those in water at ordinary tempera- 

 tures. 



Results of the Experiments. The true correction for pressure is very small. 



Having described the apparatus I proceed to the results. As soon as I applied pres- 

 sure to the Challenger thermometers I found I reproduced pretty nearly the results 

 obtained by Captain Davis. I had already seen one proof that at least a large part of the 

 result was in all probability not due directly to pressure. The experiment with the long 

 thermometer tube showed that my theoretical calculations had been correct. The epiestion 

 thus became : — Is this a pressure effect of any kind ; and, if so, how does it originate ? 

 and if it is not a direct pressure effect, to what is it clue ? There are many ways of answer- 

 ing such questions. One answer was furnished by one of the thermometers (A 3), whose 

 degrees (especially on the maximum side) are very short. The whole effect (in degrees) 

 on this thermometer was not very markedly greater for a given pressure than on the others, 

 as it would certainly have been had the effect been entirely due to pressure directly. 

 Another is, if it be not a direct pressure effect it must be a heating effect. With Sir 

 Wyville Thomson's permission I got from Mr Casella, the maker of the Challenger 

 thermometers, a couple of others of exactly the same form and dimensions, but with the 

 bulbs plugged after the manner of the gauges already described, so as to diminish their 

 susceptibility to changes of temperature. When I put one of these into the pressure 

 apparatus along with one of the Challenger thermometers, I found the effects on the new 

 form very much smaller than on the old. Thus it was at once proved that the effect 

 could not be due to wry-neckedness produced by the fitting on of the protecting bulb ; 

 which would have been an effect due to pressure directly ; but that it must be an effect 

 due to heat. That is to say, it was now completely established that the large results 

 obtained by Captain Davis are due in the main to causes which can produce no effect 

 when the thermometers are let down gradually into the deep sea ; they are due to causes 

 connected with the thermometers, and perhaps also with the pump, but solely under the 

 circumstances of a laboratory experiment. 



