DESCRIPTION OF A TACHOMETEK. ]Q3 



as to depress the spirits below C into the wider part of the 

 tube. We are also, as io the figure, to have a scale of 

 inches and tenths applied to A C, the upper and narrower 

 part of the tube, the numeration being; carried downward 

 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 mark 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 manycases, and particularly in steam-engines, 

 there is a continued osciilatioB of velocity, in these cases 

 we have to note the two points between which the column 

 oscillates durinjj the most advantageous movement of the 

 machine. 



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

 lumn of spirits will vary with the temperature, when other ^'"P*'*^"'** 

 circumstances are the same. On this account the scale 

 ought to be movable; 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 adju!<ted to the particular temperature 

 with the utmost facility, and with sufficient precision. The 

 essential parts of the tachometer have now been mentioned, 

 as well as the method of adjustment ; but certain circum- 

 stances remain to be stated. 



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

 tity of mercury sufficient, than what must have been era- 

 ployed either with a cylindrical or hemispherical vessel. 

 In every case two precautions are necessary to be observed : Precautions. 

 First, That, when the cup is revolving with its greatest ve- 

 locity, the mercury in the middle shall not sink so low as to 

 allow any of the spirits in the tube to escape from the 

 lower orifice; and that the mercury, when most distant 

 from the axis, shall not be thrown out of the cup. Secondly, 

 That, when the cup is at rest, the mercury shall rise so high 

 above the lower end of the tube, that it may support a 

 coluipn of spirits of the proper length. 



Now in order that the quantity of mercury, consistent 

 1»ith these conditions, may be reduced to its minimum, it is 

 ^e^eesary — first, that if M M, fig. 1, is the level of the mer» 



cury 



