ISO PNEUMATICS. 



ing into the centre of the brass plate at a. The 

 glass vessel K, to be emptied or exhausted of air, 

 has its rim ground quite flat, and rubbed with a 

 little pomatum, or lard, to make it fit more closely 

 upon the brass plate of the pump. These vessels 

 are called receivers. Having shut the cock I, the 

 pistons are worked by the winch, and the air 

 escapes by its elasticity when the piston is forced 

 down ; but the valve opening upwards, the air is 

 prevented from returning into the vessel ; and 

 thus the receiver is gradually exhausted, and will 

 then be fixed fast upon the pump-plate. By 

 opening the cock I, the air rushes again into the 

 receiver. 



" As light as air," is a common saying ; yet air 

 can be shown to have more weight than is gene- 

 rally supposed. Take a hollow copper ball, or 

 other vessel, which holds a wine quart, having a 

 neck to screw on the plate of the air-pump, and 

 after weighing it when full of air, exhaust it, and 

 weigh it when empty ; it will be found to have lost 

 16 grains, which shows that this is the weight of a 

 quart of air. But a quart of water weighs 14,621 

 grains: this divided by 16, quotes 914 in round 

 numbers; so that water is 914 times as heavy as 

 air near the surface of the earth. This supposes 

 air at a medium temperature and density: for 

 these, as will be seen afterwards, are variable. 



When the receiver is placed upon the plate of 

 the air-pump without exhausting it, it may be re- 

 moved again with the utmost facility, because there 

 is a mass of air under it, that resists by its elasticity 

 the pressure on the outside ; but exhaust the re- 

 ceiver, thus removing the counter pressure, and it 

 will be held down to the plate by the weight of the 

 air upon it. 



