44 Desaiplion of an Instrumeiit to ascertain 



axis, and consequenllv sinks in the middle of the cup; this 

 elevalioii at the sides and consequent depression in the mid- 

 dle increasing always with the velocity of rotation. Now 

 the mercury in the tube, though it does not revolve with 

 the cup, cannot continue higher than the mercury imme- 

 diately surrounding it, not indeed so hisrh, on account of 

 the superincumbent column of spirits. Thus the mercury 

 in the tube will sink, and consequently the spirits also; but 

 as that pan of the tube which is within the cuj) 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. 



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 spmdle D is connected with the 

 machine whose velocity is to be ascertained. In forming 

 this connexion, we must be careful to arrange matters so, 

 that when the machine is moving at its quickest rate, the 

 angular velocity of the cup shall not be so great as to de- 

 press the spirits below C into the wider part of the tube. 

 We are al<o. as in the figure, to have a scale of inches and 

 tenths applied to AC, the upjier and narrower part of the 

 tube, the numeration being carried downwards from zero, 

 which is to be placed at the point to which the column of 

 spirits rises when the cup is at rest. 



Then the instrument will be adjusted, if we inark on the 

 scale the point to which the column of spirits is depressed, 

 when the machine is moving with the velocity required. 

 But, as in many cases, and particularly in steam-engines, 

 there is a contiinied oscillation of velocity, in those cases 

 we have to note the two points between which the column 

 oscillates during the most advantageous movement of the 

 machine. 



Here it is proper to observe, that the height of the co- 

 lumn of spirits will vary wiih the temperature, when other 

 i;iroumsiances arc the same. On this account the scale 

 ought to be moveable, so that by slipping it upwards or 

 downwards the zero may be placed at the point to which 

 the column reaches when the cup is at rest; and thus the 

 instrument may be adjusted to the particular temperature 

 \\\\.\i the utmost facility, and with sufficient precision. 

 The essential parts of the tachometer have now been men- 

 tioned, as well as the method of adjustment ; but certain 

 circumstances remain to be slated. 



The form of the cup is adapted to render a smaller quan- 

 tity 



