352 On the Construction and 



and tempered in the same way as the knife-edge, and worked 

 carefully by hand into cones, the sides of which form an 

 angle of 45° with a point as perfect as can be produced. 

 Upon each screw is an octagon-shaped nut, by screwing 

 which up against the bottom of the cross brass bearing piece, 

 the screws are clamped steady and firm, when the adjust- 

 ments are complete, and any chance of shake prevented. 



Into the upper part of the central brass piece, a vertical 

 steel screw 1| inch long is fixed, upon which turns freely a 

 cross piece of brass 2 inches long, the ends of the arms of 

 which have a screw thread worked upon them, and carry two 

 little balls the size of a large pea, which traverse freely. 

 The whole weight of this adjusting piece is about 100 

 grains, and by screwing it up on the vertical screw, the centre 

 of gravity of the system is raised, and the vibrations made as 

 slow as requisite, after the adjustment has been roughly made 

 by screwing up the points of suspension. A nut which is 

 loose upon the screw can be screwed down upon the adjust- 

 ing piece, so as to clamp it tight when the adjustment is 

 complete. The little balls serve to adjust the beam to the 

 horizontal position, by screwing one of them nearer or further 

 from the centre, the adjusting piece being kept parallel with 

 the beam. Two little projecting points of wire on the sur- 

 face of the balls allow them to be turned by the touch of 

 a pointed piece of wire, without disturbing the motion of the 

 beam. 



The bob counterpoise is a polished brass ball with a square 

 hole through it, which slips on to a square extremity 

 of the beam about -\h inch square, on which it is firmly fixed 

 by a clamping screw. Of these I use five, different sized, 

 capable of counterpoising 3000, 1800, 900, 500, and 200 

 grains each, having the central hole adjusted by filing off a 

 portion from the top or bottom of the ball, so as to bring its 

 centre of gravity into the proper position for making the 

 beam vibrate slowly, whatever bob may be attached. 



