new Automatic Gas-Pump. 219 
The chamber is thus connected with the mechanical pump, 
the mercury falls, and the cycle is repeated automatically. 
The gas removed by the Toepler would accumulate in the 
trap T were it not that the mechanical pump is periodically 
connected directly therewith. Thisis achieved by the control 
valve, when permitting air to enter L also placing the 
mechanical pump in communication with the trap. The 
pressure in T is thus kept fairly constant, and the difference 
between it and that within the Toepler-pump head may at 
any moment be observed by measuring the height of mercury 
in the capillary-fall tube C. 
The control device consists essentially of a hollow slider 
moving over a smooth flat bed, and coyering or uncovering 
