132 Lieut-Col. A. R. Clarke on the Elasticity of Brass* 

 mens of brass, of which the dimensions and weights are shown 

 in the following Table : — 



I 



Rod. 



Length. 



Breadth of 



opposite faces 



I. and III. 



Breadth of 



opposite faces 



11. and IV. 



Weight. 



1 



No. 1. 



"3 



„ 4. 



in. 



39-85 

 39-85 

 39-85 



39-85 



in. 



02370 

 02392 



0-2875 



in. 



0-2400 

 0-2385 

 02127 



lb. 



072161 

 073018 

 076143 



109018 



Circumference =1*0610 



Nos. 1 and 2 were cut from the same sheet of brass, but the 

 latter was well hammered ; No. 3 is cut from a different sheet. 

 These three specimens were very carefully filed, and their di- 

 mensions are very uniform throughout. The quantities in the 

 Table result each from ten measures at equal intervals ; and the 

 probable error of each quantity may be ±0*0003 in. The round 

 rod No. 4 is very fairly circular in section ; it is probably 

 drawn, not rolled. The probable error of its circumference 

 may be taken at ±0*001 in. 



In order to ascertain their elasticity, the rods were provided 

 with three supports, one at each extremity, and a third at the 

 centre capable of movement by a micrometer-screw in a ver- 

 tical direction. Thus, when the rod is straight, or when the 

 centre support is in line with the others, the bar rests at the 

 left for one fortieth of an inch of its length, and over its entire 

 breadth, upon the brass rectangular support ; the central mo- 

 vable support is a knife-edge ; and at the right end the contact 

 is on a surface of one fortieth of an inch square, being in the 

 mid breadth of the rod. Now if w, h, k, I are the weight, 

 breadth, depth, and length, and E the modulus of elasticity of 

 the rod, \ln the breath of each support, so that 1(1 — n) is the 

 distance between the edges of the supports, then 



wl 3 , . 



where e is the space (measured by the micrometer-screw) 

 moved over by the centre of the rod between its position of 

 resting wholly on the supports at the ends, and the position of 

 being carried entirely by the centre support and having just 

 contact only with the extreme supports. The three supports 

 were fixed in a window-sill, the greatest rigidity being requi- 

 site in order to obtain trustworthy results. The experiments 

 were all made at nearly the same temperature (65° Fahrenheit); 

 and great care was taken to avoid change of temperature du- 

 ring the measures. The faces of the rods were numbered I., 



