302 



Mr. J. Y. Buchanan. 



[Feb. 11, 



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 f miction 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 other 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 

 compressibility 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 are, in a way, antagonistic, because for the 

 microscope the finest possible point is desirable, while for the handling 

 of the wire a sensible thickness is essential. Only in the case of glass 

 can a good working compromise be effected, because the wire which 

 enters the glass terminal can be drawn out at the end to the finest 

 possible hair, and the end of the hair can be fused into the minutest 

 possible sphere, which can then be observed in the microscope with the 

 sharpness with which a barometer can be read with a good telescope. 



When the substance under observation is in the form of a wire, 

 it lies in a s^lass tube which fits the bore of the steel tube as closely 

 ^as possible. Its bore is a very little larger than that of the glass 

 terminals, or about 1 mm. This tube acts as a bearer, and its length 

 Is as nearly as possible equal to the distance which separates the inside 

 ends of the glass terminals when in position. When the pressure in 

 the apparatus is raised, both the wire and the glass tube which carries 

 it are shortened, while the steel tube which carries both of them is 

 lengthened, and when the pressure is relieved the reverse takes place. 

 The glass tube behaves exactly like the glass rod, that is, it is liable 

 to a slight motion of translation. Similarly, the wire, which is carried 

 by the glass tube, generally expands and contracts under pressure at 

 a less rate than does the glass, producing again a slight apparent 

 motion of translation. But again, as in the case of the rod, the 

 algebraic sum of the observed motions is the expansion or contraction 

 of the wire. 



