A Neiv Penduhnn Apparatus. 231 



angles to a double pair of V grooves in the base, on which a 

 block of cast-iron carrying a grinding cylinder and running 

 parallel to the knife-edges can be traversed as in a planing 

 machine. The cylinder receives rapid rotation from any outside 

 motor, while the grooved block is traversed to and fro in its 

 grooves by hand. A means of approaching the knife-edge block 

 towards the grinder is supplied by a fine pushing screw. 



To obtain accurate horizontality of the knife-edges, a delicate 

 level resting on agate planes similar to the pendulum cross-heads, 

 with a light rod and bob below, forming a small pendulum, is 

 used, and is lowered on the knife-edges exactly as the pendulums 

 themselves are. 



A second stand of cast-iron is used when swings are made at 

 ordinary atmospheric pressures. This is formed of two A-shaped 

 uprights joined at the top by a rectangular platform, and 

 a heavy circular base, resting on three rounded feet. On the 

 platform, lOOmra. square, is the platform carrying the knife-edges 

 and other arrangements as described already ; but in this case the 

 final levelling of the knife-edge is done by levelling the platform 

 on the stand by special levelling screws. 



The coincidence apparatus consists of a stand with levelling 

 screws carrying a rectangular metallic box, within which are an 

 electro-magnet, armature and lever, a mirror, and a mechanical 

 shutter. Horizontally over the metal box is mounted a telescope 

 with a horizontal wire at its focus. In front of the box is a 

 narrow horizontal slit, about an inch and a half below the object- 

 glass of the telescope, and on one side of the box a circular 

 opening admits light from a lamp, candle, or other source on ta 

 the mirror within the box, whence it is reflected on to the slit in 

 front ; the shutter, however, occults the slits except at tlie 

 instant the electro-magnet acts on the lever, when an instan- 

 taneous flash is projected through the slit. A flash would occur 

 both at the rising and return of the lever but for the shutter 

 (a modification of an ingenious arrangement described by Menden- 

 hall, report cited above) which keeps the slit occulted for either 

 the up or down stroke of the lever, as may be desired. There is 

 a black and white scale, divided to three millimetres spaces in front 

 of the box, with an opening for the slit above-mentioned in its 

 centre. 



