On the Compressibility of Solids 377 



hand, one division was equivalent, on the stage, to 0-0004219 

 inch. In the one on the right hand one division was equivalent 

 to 0-0004167 inch on the stage. 



As the contractions or expansions are given directly in 

 terms of the inch, the total length of the rod is given in inches 

 also. In order to bring the ends into a suitable position for 

 observation with the microscopes the length of the rod or wire 

 had to be not less than 75 or greater than 75-5 inches. The 

 actual lengths were measured exactly in each case. The 

 tge was 75-32 inches (1-913 metres). 



To facilitate the observation of the ends through the thick 

 glass tube a piece of microscopic covering glass was moistened 

 with a drop of water and laid horizontally on the tube, pro- 

 ducing the same effect as if a flat surface had been ground and 

 polished on it. 



The effect observed and measured is the lengthening of the 

 rod when the pressure is relieved. As the compressibility of 

 solids is very small, the highest pressures have been used which 

 found to be compatible with the reasonable persistence 

 of the glass terminals; the usual pressure was in the neighbour- 

 hood of 200 atmospheres. Very few of the glass terminals 

 stood over 300 atmospheres. The pressures actually chosen 

 were as nearly as possible those at which the manometer had 

 been compared with the "Challenger" piezometer. 



The body under observation is in the form either of a rod or 

 a win.-. It it is in the form of a rod then it is fitted with win- 

 ends of sufficiently small calibre to enable them to enter tin- 

 glass terminals. 



During an experiment with a n.l it (nnu-.u-ts while tin- 

 pressure is b< <1, ;m<l < -\j> .m<N attain when the pressure 



is relieved. The steel tube which Iml.ls it. however, acts in 

 the opposite sense, it expands while the pressure rises and 

 contracts \vhil< it f.tlls. If the two > were perfectly 



smooth, one half of the change of length would be measured at 

 the one end and the other lul: <>thrr end. As the 



ices are not perfectly smooth, this does not usually occur. 

 Moreover the steel tubes are prolongations of tin- < ntral steel 

 block which holds them. The block is bored with holes at 



