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PROFESSOR CHRYSTAL AND MR JAMES MURRAY 



Fig. 1 gives a general view of the instrument. A steel band 2"31 m. (7 feet 

 7 '4 inches) long, 6 mm. broad, and "14 mm. thick — which was, in fact, a Chesterman's 

 steel tape — is attached to the float, passes over two identical carefully bushed pulleys, 

 each of which has a diameter of 7 cm. and a flat rim 6 mm. broad. The band is 

 weighted by a counterpoise at its free end. To the horizontal part of the band is 

 fastened, by means of a screw clamp, a small waggon, whose horizontal motion is to 

 be the same as the vertical motion of the float. To secure geometrically accurate 



Fig. 1. 



horizontal motion of the waggon with as little friction and sticking as possible, it is 

 arranged to run on two horizontal rails, about 3 "6 cm. apart. On one side of the 

 waggon are two small wheels (diameter about 2 cm.), each having a V-groove, the 

 sides of which are slightly convex. These two wheels run on a rail which has a wedge- 

 shaped edge. On the other side of the waggon is another small wheel with a flat or 

 slightly convex rim, which runs on a flat rail. If care be taken to adjust the band 

 correctly on the pulleys before tightening the waggon clamp, it will have no tendency 

 to leave the pulleys ; lateral guides, except as a precaution against accident, are un- 

 necessary, and thus one source of friction is avoided. It is necessary that the waggon 



