116 Dr. Hare on the Cojistrudmi and Applications of the 



contained in an iron cup. At C is a cock, the key of which, 

 in addition to the perforation usual in cocks, has another at 

 right angles to, and terminating in the ordinary perforation. 

 When the lever D, attached to the key, is situated as it ap- 

 pears in the figure, the tube containing the sliding-rod com- 

 municates with the receiver, but not with the mercury in the 

 cup. Supposing the lever moved through a quarter of a cir- 

 cle to the other side of the glass, the tube in which the rod 

 slides will communicate at the same time with the receiver E 

 and the mercury. F is a steel spring, which has a disk of 

 oiled leather let into it, so as to correspond with the surface of 

 the apex of the receiver E, which is ground as true as possible. 

 Hence, a slight pressure from the screw G renders the joint 

 made between the apex of the receiver and the spring air- 

 tight; while at the same time the bore of the cock H com- 

 municates with the cavity of the receiver by means of a per- 

 foration through the leather and spring. On the other hand 

 the relaxation of the screw permitting the spring to rise, opens 

 a communication between the cavity of the receiver and the 

 external air. The cock H, supported by the spring, carries a 

 gallows with a screw I, which serves to fasten a small bi'ass 

 casting, so perforated and fitted as to produce a communica- 

 tion between the cock H, and two others K L, with which the 

 ends of the casting are severally furnished. The cock K serves 

 to open or close the communication with the gauge M, and 

 bell-glass within the jar N. The bell-glass is furnished with 

 a cock, upon which the socket O of the gauge screws. 



Description of the Water-Gauge. 

 The gauge consists of three tubes, the interstices between 

 which are partially supplied with water. In the first place a 

 larger and outer glass tube O M, open at the upper end, is at 

 the lower end cemented into a socket attached to the cock O 

 of the bell-glass. Secondly, a small tube of varnished copper, 

 the axis of which is made to coincide with that of the larger 

 tube, is inserted into the bore of the cock. Lastly, a glass 

 tube, in size and situation intermediate between the tubes just 

 mentioned, and open at the lower end, at the upper end en- 

 ters the pipe Q, which communicates with the 'bore of the 

 cock K, and of course, when this is open, with the cavity of 

 the receiver. When water is poured into the tube M, if the 

 pressure within and without be in equilibrio^ it rises in the in- 

 terstices between the three tubes to the same height ; but when- 

 ever there is any diversity of pressure between the air of the 

 inner and outer glass tubes, it is indicated by a consequent 

 difference in the height of the liquid columns included. 



Descrip- 



