AIR-GUN 30 



receiver, from whence it has to pass into the working cylinder, to force up the working 

 piston within it. As it leaves the receiver to perform this duty, it passes through what 

 is called the regenerator, where it becomes heated to about 450 ; and upon entering 

 the working cylinder, it is further heated by the supply underneath. For the sake of 

 illustration, merely, let us suppose that the working cylinder contains double the area 

 of the supply cylinder ; the cold air which entered the upper cylinder will, therefore, 

 but only half fill the lower one. In the course of its passage to the latter, however, 

 it passes through the regenerator ; and as it enters the working cylinder, wo will 

 suppose that it has become heated to about 480, by which it is expanded to double 

 its volume, and with this increased capacity it enters the working cylinder. We will 

 further suppose the area of the piston within this cylinder to contain 1,000 square 

 inches, and the area of the piston in the supply cylinder above to contain but 500. 

 The air presses upon this with a mean force, wo will suppose, of about eleven pounds 

 to each square inch ; or, in other words, with a weight of 5,500 pounds. Upon the 

 surface of the lower piston the heated air is, however, pressing upwards with a like 

 force upon each of its 1,000 square inches ; or, in other words, with a force which, 

 after overcoming the weight above, leaves a surplus of 5,500 pounds, if we make no 

 allowance for friction. This surplus furnishes the working power of the engine. It 

 will be seen that after one stroke of its pistons is made, it will continue to work with 

 this force so long as sufficient heat is supplied to expand the air in the working 

 cylinder to the extent stated ; for, so long as the area of the lower piston is greater 

 than that of the upper, and a like pressure is upon every square inch of each, so long 

 will the greater piston push forward the smaller, as a two-pound weight upon one end 

 of a balance will be sure to bear down a one-pound weight placed upon the other. 

 We need hardly say, that after the air in the working cylinder has forced up the 

 piston within it, a valve opens ; and as it passes out, the pistons, by the force of 

 gravity, descend, and cold air again rushes into and fills the supply cylinder. In this 

 manner the two cylinders are alternately supplied and discharged, causing the pistons 

 in each to play up and down substantially as they do in the steam-engine. 



The regenerator must now be described. It has been stated that atmospheric air is 

 first drawn into the supply cylinder, and that it passes through the regenerator into 

 the working cylinder. The regenerator is composed of wire net, like that used in the 

 manufacture of sieves, placed side by side, until the series attains a thickness of about 

 1 2 inches. Through the almost innumerable cells formed by the intersections of the 

 wire, the air must pass on its way to the working cylinder. In passing through these 

 it is so minutely divided that all parts are brought into contact with the wires. 

 Supposing the side of the regenerator nearest the working cylinder is heated to a iiigh 

 temperature, the air, in passing through it, takes up, as we have said, about 450 of 

 the 480 of heat required to double the volume of the air ; the additional 30 are 

 communicated by the fire beneath the cylinder. 



The air has thus become expanded, it forces the piston upwards ; it has done its 

 work valves open, and the imprisoned air, heated at 480, passes from the cylinder 

 and again enters the regenerator, through which it must pass before leaving the 

 machine. It has been said that the side of this instrument nearest the cylinder is 

 kept hot ; the other side is kept cool by the action upon it of the air entering in the 

 opposite direction at each up-stroke of the pistons ; consequently, as the air from the 

 working cylinder passes out, the wires absorb the heat so effectually, that when it 

 leaves the regenerator it has been robbed of it all, except about 30. 



The regenerator in the 60-horse engine measures 26 inches in height and width 

 internally. Each disc of wire composing it contains 676 superficial inches, and the 

 net has ten meshes to the inch. Each superficial inch, therefore, contains 100 meshes, 

 which, multiplied by 676, gives 67,600 meshes in each disc ; and, as 200 discs are 

 employed, it follows that the regenerator contains 13,520,000 meshes ; and con- 

 sequently as there are as many spaces between the discs as there are meshes, we find 

 that the air within it is distributed in about 27,000,000 minute cells. Thence every 

 particle of air, in passing through the regenerator, is brought into very close contact 

 with a surface of metal which heats and cools it alternately. "Upon this action of the 

 regenerator, Ericsson's Calorific Engine depends. In its application on the largo 

 scale, contemplated in the great Atlantic steamer called ' The Ericsson,' the result 

 was not satisfactory. We may, however, notwithstanding this result, safely predi- 

 cate, from the investigation of Messrs. Thomson and Joule, that the expansion of 

 air by heat will eventually, under some conditions, take the place of steam as a motive 

 power. 



AIR-GRATING. A kind of air-brick built into walls to admit air under the 

 floors or into close places. Air-gratings are often made of iron. 



-AIR-GtTW. This is a weapon in which the elastic force of air is made use of to 

 project the ball. It is so arranged, that in a cavity in the stock of the gun, air can be, 



