212 FERGUSON'S LECTURES. 



LECT. look at the top, but at the bottom of the receiver ; in or- 

 ^yj^, der to see the guinea and feather fall upon the plate : 



otherwise, on account of the quickness of their motion, 



they escape the sight of the beholders. 



2. To shew the weight of the Air. 



1. Having fitted a brass cap, with a valve tied over 

 it, to the mouth of a thin bottle or Florence flask, whose 

 contents are exactly known, screw the neck of this cap 

 into the hole i of the pump-plate : then, having exhausted 

 the air out of the flask, and taken it off from the pump 

 let it be suspended at one end of a balance, and nicely 

 counterpoised by weights in the scale at the other end : 

 this done, raise up the valve with a pin, and the air 

 will rush into the flask with an audible noise : during 

 which time the flask will descend, and pull down that 

 end of the beam. When the noise is over, put as many 

 grains into the scale at the other end as will restore the 

 equilibrium ; and they will show exactly the weight of 

 the quantity of air which has got into the flask, and 

 filled it. If the flask holds an exact quart, it will be 

 found, that 16 grains will restore the equipoise of the 

 balance, when the quicksilver stands at 29J inches in 

 the barometer ; which shows that when the air is at a 

 mean rate of density, a quart of it weighs 16 grains : it 

 weighs more when the quicksilver stands higher ; and 

 less when it stands lower. 



2. Place the small receiver O (page 207) over the 

 hole 2 in the pump-plate, and, upon exhausting the air, 

 the receiver will be fixed down to the plate by the pres- 

 sure of the air on its outside, which is left to act alone, 

 without any air in the receiver to act against it : and 

 this pressure will be equal to as many times 15 pounds, 

 as there are square inches in that part of the plate which 

 the receiver covers ; which will hold down the receiver 

 so fast, that it cannot be got off, until the air be let into 

 it by turning the cock k ; and then it becomes loose. 



