122 DESCRIPTION or A TACHOMETER. 



drawn in elevation at X, fit;. 2. Tiie whiter parts of the 

 section, in fig. 1, refireseut what is solid, and the dark parts 

 what is hollow. This cup is filled with merciny up to the 

 level L L; fig. 1. Into the mercury is immersed the lower 

 part of the upright glass tube A B, which is filled with co- 

 loured spirits of wine, and open at both ends, so that some 

 of the mercury in the cup enters at the lower orifice, and 

 when every thing is at rest, supports a long column of spi- 

 rits, as represented in the figure.' The bottom of the cup is 

 fastened by a screw to a short vertical spindle D, so that 

 when the spindle is whirled round, the cup, (the; figure of 

 which is a solid of revolution) revolves at the same time 

 round its axis, which coincides with that of the spindle. 



In consequence of this rotation, the mercury in the cup 

 acquires a centrifugal force, by -which its particles are 

 thrown outwards, and that with the greater intensity, ac- 

 cording as they are more. distant from the axis, and accord- 

 ing as the angular velocity is greater. Hence, on account 

 of its fluidity, the mercury rises higher and higher as it re- 

 cedes from the axis, and consequently sinks in the middle 

 of the cup ; this elevation at the sides, and consequent 

 depression in the middle, increasing always with the velo- 

 city of rotation. Now the mercury in the tube, though it 

 does not revolve with the cup, cannot continue higher than 

 the mercury immediately surrounding it, nor indeed so 

 high, on account of the superincumbent column of spirits. 

 Thus the mercury in the tube will sink, and consequently 

 the spirits also ; but as that part of the tube which is 

 within the cup is much wider than the part above it, the 

 depression of the spirits will be much greater than that of 

 the mercury, being in the same proportion in which the 

 square of the larger diameter exceeds the. square of the 

 smaller. 

 MetTroi'd of Let us now suppose, that, by means of a cord passing 



round a small pulley F, and the wheel G, or H, or in any 

 other convenient way, the spindle D is connected with the ■ 

 machine, the velocity of which is to be ascertained. In 

 forming this connection, we must be careful to arrange 

 matters so, that, when the machine is moving at its quickest 

 rurte, the angular velocity of t|je cup shall not be so great 



as* 



uMn^rt. 



