4 ‘ ROYAL SOCIETY OF CANADA 
giving either a positive or negative reading of the scale of the observing » 
telescope, Fig. B. 
The two bars are held in contact with the mirror roller by a small 
suspension spring, which offers a minimum resistance to any longi- 
tudinal movement of the bars. This arrangement constitutes one of 
the novel features of the instrument. Parallelism of the bars in a 
horizontal position combined with freedom of movement is maintained 
by making the bent end pass into the opposite slot; parallelism of the 
bars relatively to the face of the beam is maintained by means of an 
additional roller placed between the bars as far as possible from the 
suspension spring. The suspension spring is situated as near the 
mirror roller as is practicable with the requisite range of movement. 































. Fig. C. 
The frame carrying the mirror, being attached to the top of the 
roller, is bent down in front of the bars, so that the mirror is thus 
pivoted directly in front and allows the extensometer to be made less 
than 1-inch in width—an important consideration when it is desired to 
use as many instruments as possible at one time. The length of surface 
over which the distortion is measured with the present instruments 
may vary from 8 to 12 inches. 
The value of the rotation of the mirror produced by the extension 
or compression in that part of the beam over which it is situated is 
observed by means of an ordinary reading telescope, with cross-hairs, 
Fig. C. The telescope is carried on a suitable head, which can be moved 
