On the Compressibility of Solids 379 



one direction, and in this case, to the left. Therefore the 

 distance measured in the right-hand microscope is the expansion 

 of the portion of the rod which lies to the right of the point on 

 it which is motionless relatively to the tube minus the proper 

 motion of this point: and the distance measured at the left- 

 hand end is the expansion of the remainder of the rod plus 

 the proper motion of the common point. Consequently the 

 algebraic sum of the two motions measured is the expansion 

 of the rod under the relief of pressure. 



When the substance is used in the form of a rod, as, for 

 instance, in the case of glass, its ends are drawn out into wires, 

 such that they can enter and be visible in the glass terminals. 

 What we really measure then is the change of length under 

 change of pressure of the axial glass wire in the rod, which 

 may be looked on as a fascine of a very large number of similar 

 but somewhat shorter wires. The sole function of these other 

 wires is to maintain the wire that falls under observation in 

 an axial position. It is obvious that this function can.be 

 performed with equal efficiency by wires of any ether material, 

 and that the conditions are in no way altered if these are fused 

 into a tube of which the wire to be measured may be regarded 

 as the core. Consequently by my method the linear com- 

 pressibility of a solid can be determined as well on a wire as on 

 a rod ; and there is no limit to the thinness of the wire, so long 

 as it can be handled, and be perceived in the microscope. 



These two conditions arc, in a way, antagonistic, because 

 for tin microscope the finest possible point is desirable, while 

 for tin- handling of the wire a sensible thickness is essential. 

 Only in the case of glass can a good working compromise be 

 effected, because tin- win- which enters the glass terminal can 

 be drawn out at the end to the finest possible hair, .m.l the 

 end of the hair can be fused int<> tin- minutest possible spi 

 which can then be observed in the microscope with the ib 

 ness with whi< h an be read with a good telescope. 



When the substance under observation is in the l>in 



n a glass tube win* h tits the bore of the steel tube 

 as closely as possible. Its bore is a very little Uurgei than that 

 of the glass terminals. r about i mm. This tube a< ts as a 



