AIR PUMPS AND APPARATUS. 193 



represents the usual style of the large size, double barrel, 

 table air pump, and consists of two barrels of brass, accu- 

 rately bored, and finished of a uniform calibre. These, with 

 their bases, are firmly held in a perpendicular situation to 

 the square mahogany base, by the mahogany head of the 

 air pump, which is pressed upon them by screws at the top 

 of the two brass pillars, one of which is placed each side 

 of the barrels. In the centre of the base there is a true 

 ground receiver plate, having in the centre a hole, with 

 screw, for the attaching of any apparatus, and a concealed 

 channel communicating with the point, where a screw is 

 fixed to let in air occasionally. From the above mentioned 

 channel there is a perforation at right angles to the former, 

 going to the centre of the basis of each barrel ; at each of 

 these centres a valve is placed, opening upwards to admit 

 the air into the barrels ; there is a piston so fitted to each 

 barrel, that the air cannot pass between it and the sides of 

 the barrel ; to each piston there is a valve opening upwards, 

 through which the air in the lower part of the barrel mav 

 escape into the common air ; the pistons are also connected 

 to a rack, and are raised or depressed by a handle, the lower 

 part of which is fixed to the axis of a cog-wheel, whose 

 teeth take into the rack ; one piston is raised, and the other 

 depressed by the same turn of the handle. 



Two barrels are advantageous, not only as performing the 

 work quickly, but also because the weight of the atmosphere, 

 pressing upon the rising piston, is counter-balanced by the 

 same weight pressing upon the other piston descending. 



The operation of air depends on the elasticity thereof. 

 When either of the pistons is drawn upwards, a vacuum is 

 left behind it, and the pressure being thus removed from the 

 valve in the bottom of the barrels, this valve will be opened 

 by the elasticity of the air in the receiver ; and the air, 

 rushing through it, will be uniformly diffused in the receiver, 

 the canal connecting this with the lower valve, and in the 

 barrel. But upon depressing the piston, the valve at the 

 bottom of the barrel will be closed, and the air therein 

 being condensed, will open the valve in the piston and escape ; 

 thus the air contained in the barrel is discharged, and, by 

 every turn of the winch, a quantity of air equal to the con- 

 tents of the barrel, and equally dense with that in the re- 

 ceiver, is exhausted. 



Behind the large receiver, there is a small plate for sus- 



