AIR-PUMP

A pneumatic machine for removing the air out of a ve el. The principle of this important philosophical instrument is very simple, and may be easily comprehended from a brief explanation. The e ential part of the machine consists of an exhausting syringe (a), formed of a tube or barrel of bra , closed at one end with the exception of a small orifice, to which a valve (6), opening inwards, is attached. \JL jf I I r/ An air-tight piston is worked up ^^— 'J "^ and down in the b.^rrel by a rack d and pinion turned by a winch. The piston has also an orifice with a valve (c) which opens upwards, or in the same direction as the valve of the tube. The syringe communicates, by means of a small pipe (d) fitted into the opening at its lower extremity, with a ve el (c) called the receiver, from which the air is to be extracted. The receiver is placed on a bra plate (/, g,) over a A [s. 39]