DESCRIPTION OF A TACHOMETER. 125 



of rotation, unless the sides were raised to an inconvenient 



height; but the lid, by obstructing the elevation at the 



sidesof the cup, will diminish the depression in the middle, 



and consequently the depression of spirits in the tube: on 



this account a cavity is formed in the block immediately 



above the level L L, where the mercury stands when the 



cup is at rest ; and thus a receptacle is given to the fluid 



which would otherwise disturb the centrifugal force, and 



impair the sensibility of the instrument. 



It will be observed, that' the lower orifice of thiettibe i? Curyeatthe 

 , ■ Tk , ■ r 1 111 bottom of the 



turned upwards. . JBy this means, after the tube has: be6n-tube. 



tilled with spirits by suction, and its upper orifice stopped 

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

 irnmersed in the qaicksilver without any danger of the 

 spirits escaping,. a circumstance which otherwise it would be 

 be extremely difficult to prevent, since no part of the tube 

 can be made capillary, consistently with that free passage to 

 the fluids, which is essentially necessary to the operation of 



the instrument. - —---.•:-■ 



We have next to attend to the method of ptftting the Method of set- 

 tachometer in motion, whenever we wish to examine the ve- ^'"S^hem- 

 . . strumeal in 



locity of the machine. The pulley F, which is contmAally moiion, 



whirling during the motion of the machine, has no connec- 

 tion whatever with the cup, so long as the lever Q R 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 embracing a solid cone on the spindle of the 

 cup, communicates the rotation by friction. When our ob- 

 servation is made, we have only toallow the lever to drftp by 

 its^ own weight, and the two cones will be disengagedi, and ..; t.- 



the cup remain at rest. V* '-: 



The lever Q R is connected by a vertical rod to another 

 lever S, having at the extremity S a valve, which, v/hea 

 the lever Q R is raised, and the tachometer is in motion, is 

 lifted up from the top of the tube, so as to admit the exter- 

 nal air upon the depression of the spirits; on the other 

 hand, when the lever Q R falls, and the cup is at rest, the 

 valve at S closes the tube, and prevents %h*i spirits from 

 ■being wasted by evaporation. 



It is lastly to be remarked, that both the seftsibility and Increase of tha 



the 



