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



and had Challenger thermometers at the bottom. By this arrangement I hoped to get 

 over the difficulty due to the temperature of the gun, by having the inner vessel 

 enclosed in water which would be lowered in temperature to about 3° C. by the appli- 

 cation of pressure. The device proved quite successful. The result of lj tons pressure 

 per square inch maintained for 20 minutes, some ice being still left in each vessel, was 

 from a number of closely concordant trials — 



Temperature in outer vessel, . . . l 0- 7 C. 



Temperature in inner vessel, . . . 0° - 3 C. 



The direct pressure correction for the thermometers is only about — C- 1 C, and has 

 therefore been neglected. 



" The close agreement of this result with that obtained (under similar pressure 

 conditions) in the thick glass vessel leaves no doubt that the lowering of the 

 maximum-density point is somewhat under 4° C. for 1^ tons, or 2°"7 C. for 1 ton per 

 square inch. It is curious how closely this agrees with the result of my indirect 

 experiments." 



Further work of the same kind led me to the conclusion that even the double 

 vessel had not sufficiently protected the contents from conducted heat, and to state 

 in my Heat (p. 95, 1884) that "a pressure of 50 atmospheres lowers the maximum- 

 density point by 1° C." 



During the next two years I made several repetitions of these experiments, with 

 the help of thermometers protected on the Challenger plan, but very much more sensi- 

 tive. These experiments were not so satisfactory as those just described. The new 

 thermometers caused a great deal of trouble by the uncertainty of their indications, 

 which I finally traced to the fact that the paraffin oil which they contained passed, in 

 small quantities, from one end of the mercury column to the other. I was occupied with 

 an attempt to obtain more suitable instruments, when the arrival of the Amagat gauge 

 turned my attention to other matters. 



So far as I can judge from the results of the three different methods which I have 

 employed, the lowering of the maximum-density point of water by 1 ton of pressure is 

 very nearly, though perhaps a little in excess of, 3° C. 



It is peculiarly interesting to find that Amagat, by yet another process, — viz. 

 finding two temperatures not far apart at which water, at a given pressure, has the 

 same volume, — has lately obtained a closely coinciding result. He says : " A 200 atm. 

 (chiffres ronds) le maximum de densite de l'eau a retrograde vers zero et l'a, presque 

 atteint ; il parait situe entre zero et 0°'5 (un demi-degre)." ' This makes the effect of 

 1 ton slightly less than 3° C. 



As the freezing point is lowered, according to J. Thomson's discovery, by about 



1 Comptes Eendus, torn. civ. p. 1160, 1887. 



