90 Mr. J. J. G-uest on the Strength of 



which is at the back of the adjuster, through which a rubber 

 band is passed to support the adjuster on the specimen while 

 the grips are being fastened. The spheres c l5 c 2 , d of the 

 grip 2 are placed in the V-groove C and on the plane D, 

 and the grip slid along into position and the screws G, H 

 nipped into the specimen ; the screw R is then adjusted to 

 touch the specimen. The other grip is adjusted in the same 

 manner, and the adjuster is then withdrawn. It will be seen 

 that the grips form one piece with the specimen, and during 

 a test the screw-points are not employed as bearings, the 

 friction at which would be an unknown variable quantity. 

 The bottom grip 2 is fitted with a trihedral hole (see fig. 9) 

 at P, in which lies a steel ball p fastened to the length-bar 

 5 (fig. 5), the pair forming a universal joint. Similar uni- 

 versal joints are to be seen at Vu, Ss, Yv, Rr, and Wio. 

 The length-bar connects the grip 2 with a bent lever 4 

 (fig. 8), which rests on the top grip 3; the length-bar being 

 connected to the lever by the universal joint Um, and the 

 lever being kinematically hinged to the top grip by the 

 universal joint Ss and a Sphere t working in a V-groove T. 

 The axis of the screws L, M intersects and bisects the line st, 

 and the grip 2 is so adjusted that the point of intersection is 

 on the axis of the specimen. To the other end of the lever 

 is connected, by the universal joint Yv, the mirror-bar 6 

 (figs. 4 & 9), to which the mirror-frame, 7, is hinged 

 kinematically by the universal joint Wiv and the V-groove 

 and sphere Xa?. The mirror-frame 7 (figs. 4 & 7) is 

 finally connected to the lower grip, 2, by the universal joint 

 Hr, the sphere B being carried on the end of the calibrating- 

 screw Q. In addition to the restraints mentioned, the mirror- 

 bar has a wing, z, which is in contact with the (blunt) end of 

 a screw Z, being kept in contact by the small weighted 

 lever Y. Any extension of the specimen wiil result in 

 an angular movement of the lever 4, thus raising the axis of 

 the hinge WX, and causing the mirror to rotate about this 

 axis. It will be noticed that the forces along all the members 

 of the instrument are practically unaffected by the movements 

 of the parts, being controlled by the position of the adjustable 

 weight shown on the mirror-frame. The mirror is readily 

 adjusted, so that the reflexion of the scale appears in the 

 field of view of the telescope by means of the screw Z, which 

 turns the mirror about a vertical axis, while the scale-reading 

 can be adjusted to zero by means of the screw Q. This 

 screw also serves, in case it should be desired to .push 

 the investigations beyond the yield-point, to readjust the 

 reading on to the scale as often as desired, and also to 



