This establishes a Torricellian vacuum in B. The cock n is now turned,
giving the position X, when air is at once exhausted from the vessel
connected to C. This process is repeated until full exhaustion is
obtained. In practice the first exhaustion is often effected by a
mechanical pump. By closing the cock on the outlet tube c but little air
need ever find its way to the chambers o and B.
Fig. 276. GEISSLER AIR PUMP.