372 On the Compressibility of Solids 



I had recognised to be fatal to absolute measurements. The 

 problem had, therefore, come to be: to design a piezometer 

 which should contain no liquid ; and it was the solution of this 

 problem which occurred to me on the evening of March 23, 



The form which the instrument took was very simple. In 

 my laboratory outfit I had included some lengths of tubing 

 suitable for the stems of piezometers, of which I had to make a 

 number during the voyage. In order to be able to use the 

 indices of broken deep-sea thermometers, the tubes had the 

 same internal diameter as the stems of these instruments, about 

 I mm. On the outside of the tubes a scale of millimetres was 

 etched. I took the greatest available length of this tube, 

 namely, 60 cms. I then drew out a wire of the same glass and 

 passed it into the tube until it appeared at the other end of 

 the tube. This end of the tube was then sealed up, and the 

 end of the glass wire was fused into it, so that, while free 

 throughout its whole length, longitudinal motion was pre- 

 vented. The length of the glass wire was 57 cm., so that there 

 was an empty space in the tube of 3 cm. above it. The magnetic 

 index of a broken deep-sea thermometer was re-haired and 

 passed into the tube above the glass wire. The open end of 

 the tube was then sealed up. The result was a piezometer 

 consisting of nothing but glass. In principle it was precisely 

 the same as any of the other piezometers. The indices of these 

 give the difference between the compression produced by the 

 pressure on the contents and on the envelope. In the case of 

 the other piezometers, which contained liquids, the balance was 

 on the side of the contents. In the all-glass piezometers the 

 contents, besides being of the same material as the envelope, 

 were completely protected from pressure, and the whole of the 

 change of length measured fell to the envelope. It has, there- 

 fore, a feature which is possessed by no other instrument ; with 

 it the absolute compressibility of a solid is determined by one 

 measurement. 



Before the instrument was attached to the sounding-line, 

 the index was brought down by means of a magnet to rest on 

 the end of the internal glass wire, exactly in the same way as 



