1864.] 



and Air-pump. 



323 



brancli r ; and in an intermediate position it is completely shut off. These 

 glass cocks have this great advantage over those of metal, that it can always 



be ascertained if they are air-tight ; their transparency permits us to see if 

 the key and shell are in optical contact ; and the slightest air-way there is 

 at once detected. They should not be lubricated with oil, which grips, 

 and may perhaps find its way into the bell and soil its interior. I find the 

 best material to be castor oil with rosin dissolved in it. A hole is drilled 

 down the axis of B, which communicates by a tube (sunk in the wood and 

 therefore not visible in the figure) with the cast-iron cylinder D. This is 

 13 inches high and 3*2 in internal diameter ; its top and bottom are secured 

 to it air-tight by screws ; in it works a plunger of boxwood well varnished 

 10*4 high, and moving so loosely that mercury may pass it easily* The 

 plunger is wrought by a rod passing through the collar of leather H. In 

 the top of the cylinder is a stopcock E, to which is fixed a tube of vul- 

 canized caoutchouc (varnished with a solution of caoutchouc in benzidine), 

 which is shown hanging down ; it has a coupling to connect it with an 

 ordinary air-pump. There is also in the top a screw S for admitting air, 



VOL. XIII. 2 B 



