298 



Mr. J. Y. Buchanan. 



[Feb. 11, 



with the appliances at hand to measure such small distances with 

 sufficient accuracy to enable a definite value to be determined. 



On the return of the ship I embodied the principle in an instrument 

 of precision, which I had constructed in the early part of 1880, and I 

 used it in the month of June of that year for the exact determination 

 of the compressibility of the glass which had been used in the 

 construction of my " Challenger " piezometers. 



It is this instrument, and without any alteration, which I have used 

 for the purpose of the present research. 



Fig. 1. 



With the assistance of fig. 1, its features, and the distribution of its 

 parts, will be apparent without any lengthy description. It consists of 

 three parts : the force pump on the left, the receiver for the reception 

 of piezometers or other bodies on the right, and behind these, the block 

 with tubes projecting on either side to receive the rod or wire of the 

 solid, the compressibility of which is to be determined. Every part of 

 the instrument is made of steel. The part which most concerns the 

 present research is the steel block, in the rear, with tubular prolongations. 

 When the rod or wire to be experimented with has been introduced, the 

 ends of the tubes are closed with thick glass tubes, which are kept in 

 their places by open steel caps. Each of these tubes is commanded by a 

 microscope with micrometer eye-piece. In 1880, when the instrument 

 was housed in a room with a stone floor, these microscopes stood on 

 three-legged stools, as shown in the figure. As the room with the 

 stone floor was no longer available, I had to instal the instrument 

 close to the windows of the laboratory, which has a wooden floor, and 

 fix metal brackets in the wall to carry the microscopes. In both cases the 

 micrometers, which measure the expansion or contraction of the body 

 under examination, are independent of the instrument which holds it. 



The manometer, which indicates the pressure in the instrument, is 



