Dr. Robinson on a New Mercurial Gasometer and Air-pump. 237 



figure), the joint is air-tight. The bell A is 2 inches in diameter and 

 6*5 high; it has a tubulure at the top, in which is ground a glass 

 cock C, whose construction is shown in fig. 2. The key of it is 

 pierced from its bottom to a level with the bore, with which this 

 perforation communicates occasionally by a lateral opening. In the 

 position of the figure, it will be seen that the bell communicates 

 with the branch a ; if the key be turned half round, it is connected 

 with the branch r ; and in an intermediate position it is completely 

 shut off. These glass cocks have this great advantage over those of 



FigX Crp 



r 



\_ 



\ 



y 



\ 



\ 



Fig. 3. 



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 con- 

 tact ; 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 mate- 

 rial 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; 



