On the Compressibility of Solids 373 



if it had been the mercury column in a maximum and minimum 

 thermometer. The instrument was then sent to the bottom, 

 or to whatever depth might be decided on. 



During the descent the temperature of the glass, both 

 in-ide and outside, fell with that of the water through which 

 it passed, but as the contraction produced was the same on the 

 wire and on the tube, there was no differential effect to be 

 recorded by the index. On the other hand, the increasing 

 pressure, as the instrument descended, affected only the outside 

 tube, which it shortened. In contracting, it was obliged to 

 pass the index, which was kept in its place by the internal 

 win . When the instrument was being hove up, the reverse 

 process took place; the tube lengthened, and lifted the index 

 clear of the internal wire by an amount equal to the lengthening 

 of that portion of the tube. As the whole clearance produced 



the expansion from the greatest depth did not exceed 

 I mm., its amount had to be estimated by the eye with the 

 assistance of a magnifying glass. 



The experiment made on March 29, 1875, was quite success- 

 ful, and it gave 3-74 as the cubic compressibility per million 

 per atmosphere, of the glass of which the tube was made. 

 The exact figure found in 1880 for glass from the same source 

 2-92. A number of observations were made with the 

 instrument, both on the sounding-line and in the compression 

 apparatus with which the ship was supplied, and figures from 

 3 5 per million were found. These were sufficient to give 

 the order of the constant which was sought, but it was impossible 

 with the appliances at hand to measure such small distances 

 with sufficient accuracy to enable a definite value to be deter- 

 mined. 



On the return of the ship I embodied the principle in .m 



timent <t >n. whirh I h.id oiMni<-t-d in tin- railv 



i I UM-d it in tin- month of June of that year 



for the exact determinati'm of the compressibility of the glass 



whi. h had been used in the construction of my "Challenger" 



piezometers. 



tin- iiMnun-ni. .md without any alteration, n 

 n-rd IMI tin- purpose of the present resran h. 



