24 THE VOYAGE OF H.M.S. CHALLENGER. 



the temperature of the maximum density of water), I found the results to vary in accord- 

 ance with calculation. 



(3.) Heat due to friction during pumping. This from its very nature was unavoid- 

 able unless we could have got an apparatus into which (by enormous pressure) the 

 plug could have been forced directly. This could not, however, have been done in my 

 laboratory, even if the apparatus had been adapted to such a form of experiment. But 

 it was very easy to calculate the extreme possible amount of this effect. 



(4.) The peculiar heating effect due to the vulcanite mounting. I verified this effect 

 of vulcanite by taking a thermometer which had no vulcanite about it and measuring the 

 effect produced upon it by a definite pressure, and then putting loosely round the bull) 

 (in a test-tube, which had itself been previously experimented on) a small quantity of 

 vulcanite in thin plates. I found that so little as 8 grammes of vulcanite round the 

 protecting bulb raised the effect produced by a pressure of 3 '2 tons weight from 0°'5 F. 

 to 1°"1 F. The vulcanite was in thin strips about a millimetre and a half in thickness. 

 The effect of the vulcanite on the Challenger thermometers (in the hydrostatic press) 

 must, from the mode of their construction and mounting, in all cases be considerably 

 greater than this. 



Under these circumstances, we might without farther inquiry fairly attribute the whole 

 outstanding effects to the massive vulcanite slabs on which these thermometers are framed. 

 But there still remains 



(5.) The most difficult question of all, the temperature effect produced by pressure 



upon the protecting bulb, which is under different circumstances altogether from the 



vulcanite ; for the vulcanite is simply compressed, while the glass sheath is under 



pressure on one side and not on another, and is therefore subject to shear as 



well. In its interior the glass is extended in a radial and compressed in a tangential 



direction. Nobody has yet made any approximation to an answer to the question what 



effect in the way of heating or cooling will be produced by deformation which consists 



partly of compression and partly of change of form. We know that in indiarubber a 



cooling effect is produced by traction, and it may happen that a similar change of form 



in glass also produces a reduction of temperature. This is a question, however, which is 



not capable of answer by the help of my present apparatus ; — though it will probably be 



answered by experiment before theory is able to touch it. The results of my experiments 



on the thermometers with plugged bulbs show that, on the whole, a heating effect results 



from the combined compression and shear in a bulb exposed to external pressure only. 



This has been verified by cutting down a thermometer, an exact counterpart of the 



Challenger thermometers but without aneurisms, taking out the greater part of the 



mercury and inserting a second (now a maximum) index in the minimum side of the 



tube. When this instrument was stripped of its vulcanite, the effect of pressure at 40° P. 



was considerably greater than that due to compression of the tube. 



