46 On ike VtlocUie<i cf ISlachinery. 



form in \" at its quickest motion ; let v be the number of 

 inches wh^ch a botiv would describe uniformly in l", with 

 the velocity acquired in falling fiom rest, through a height 



= to GV, and make Gil = --^. — . Then, the parabola to 



be determined is that which has v for its vertex, VG for its 

 axis, and GH for its ordinate at G. The cup has a lid to 

 jirtvenl tlie niertury from being thrown out of it, an event 

 whicii would take place with a very moderate velocity of 

 rotation, imlcss the sides were raised to an inconvenient 

 height; but the lid, bv obstructing the elevation at the sides 

 of the cup, will diminibh the depression m the middle, and 

 consequently the dtpression of spiriis in ihe tube: on this 

 account a cavitv is iormed in the block mimediately above 

 ihe level LL, where tlie mercury stands when the cup is at 

 rest; and thus a receptacle is given t;"> the fluid which would 

 otherwise disturb ihe centrifugal force, and imjiair the sen- 

 sibility of the instrument. 



U will be observed, that the lower orifice of the tube is 

 turned tiji.vards. By thi? means, after the tube has been 

 filled wuh spirits bv suction, and its upper onfice stopped 

 with the finger, it mav easily be conveyed to the cup and 

 inin)..rsed ni the quicksilver with(;ul anv danger of the 

 spnits escaping, a circumstance which oiherui^e it would 

 be extrtmelv ditficuli to prevent, since no part of the tuue 

 can be made capillary, consistenily with that free passage to 

 the fluids whicii is esstnlially neccssuy to the operation of 

 the instrument. 



We have next to attend to the method of putting the 

 tachometer in motion whenever we wish to examine the 

 velocity of the machine. The pulley F, which is continually 

 whirling during the motion ot- the machine, has no con- 

 nexion whatever v^ilh the cup, so long as the lever QR is 

 left to itself. But when this lever is raised, the hollow cone 

 T, which is attached to the pulley and whirls along with it, 

 is also raised, and enibracing a solid cc^ne on the spindle of 

 the cup, coniinunicates tiie rotation by friction. When 

 our observation is madt, we iiave only to allow the lever to 

 drop by its own wciglit, and the two cones will be disen- 

 |.aged, and the cup lemain at rest. 



The kver Oil is connected by a vertical rod to another 

 lever 53, having, at the extremity S, a valve, which, vvlien 

 the lever OK is raised, and the tachometer is in nioiion, is 

 lilted up hum the top of the tube, so as to admit the exler- 

 i,..! air upo.i the depression of ti»e spirits; en the other 



Land. 



