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



where t is the temperature centigrade, and p the pressure in tons weight per square 

 inch. This, of course, cannot be the true formula, but it is sufficient for ordinary 

 purposes within the limits of temperature and pressure above stated. It represents the 

 value of 



" With a new set of compression apparatus, very much larger and more sensitive 

 than those employed in the above research, I have just obtained the following mean 

 values for the single temperature 15° - 5 C. : — 



Pressure in Tons. 1 1-| 2 3 



Fresh water, . . . 0-00678 663 657 638 



Sea-water, . . . 0-00627 618 609 593 



v — /) / 1 dv \ 



" These are the values of -j^— , and they give, for the true compressibility (-- ^-j 

 at any pressure, and temperature, 15° "5 C, the formulae, 



Freshwater, ..... 0-00698(1 -0 05 p) 



Sea-water, ..... 0-00645(1 - 0-05 .p) 



"The ratio is 0"925, i.e. the compressibility of sea- water at the above temperature 

 is only 92'5 per cent, of that of fresh water." 



The new and larger piezometers referred to were made when Mr. Murray requested 

 me to write this Report. They are those whose form and dimensions have been detailed 

 in Section III. above. The former piezometers had no capsule containing mercury, but 

 had the stem simply cut off flat at the end, and when filled with water were merely 

 dipped in mercury. I had felt that to this was probably due the fact that my experi- 

 ments gave a value of the compressibility at 0° C. somewhat smaller than that usually 

 accepted. It will be seen that the very first data given by the new instruments at 

 once tended to set this matter right. For while the formula representing the results of 

 the smaller instruments gave the compression of water at 15°"5 C. as 0'00678 for one 

 ton weight per square inch, that for those of the new instruments gave 0'00698, i.e. 

 about l/34th more, which is much nearer to the result of my later experiments. 



For two winters after this period the apparatus was kept in working- order in the 

 hope that I might lie enabled to employ temperatures between 6° and 0° C. But a 

 single day's work at 1°"7 C, and a few days at temperatures between 3° and 5° C. were 

 all I got. Hence the reason for procuring the smaller compression apparatus, as stated 

 in Section I. But, as yet, my measurements of pressure were not satisfactory. 



In the spring of 1886 I obtained the Amagat gauge, and after a careful compara- 



1 [See Appendix B to this Report.] 



