163 



AIR-.GUN. 



AIR-PUMP. 



166 



read a description of it before the Institute of Civil Engineers in 1846. 

 In this engine two strong air-tight vessels are connected with the oppo- 

 site ends of a cylinder, in which a piston works in the usual manner. 

 About four-fifths of the ulterior space in these vessels is occupied by 

 two similar air-vessels, or plungers, suspended to the opposite extre- 

 mities of a beam, and capable of being alternately moved up and down 

 to the extent of the remaining fifth. By the motion of these interior 



- the air to be operated upon is moved from one end of the 

 exterior vessel to the other ; and as one end is kept at a high tempera- 



nd the other as cold as possible, when the air is brought to the 

 hot end it becomes heated, and has its pressure increased, whereas its 

 heat and pressure are diminished when it is forced to the cold end. 

 Now, as the interior vessels necessarily move in opposite directions, it 

 follows that the pressure of the enclosed air in the one vessel is in- 

 creased, while that of the other is diminished ; a difference of pressure 

 is produced on opposite sides of the piston, which is made to move 

 from one end of the cylinder to the other. The piston is connected 

 with a fly-wheel, and motion communicated in the usual way. A large 

 bakery has been established on this principle at New York, the 

 machinery being worked by the ascensive force of a current of air 

 heated by the baking ovens. At a certain height within a lofty 

 vertical shaft is a horizontal wheel, with wings or vanes attached 

 at an angle of 10; the ascending air causes this wheel to revolve 

 horizontally ; drums are fixed on the spindle of the wheel, and straps 

 or bands from these drums drive the machinery for grinding the flour 

 and kneading thorough. 



Baron Von Kathen made trial of an air-locomotive on a common 

 road, in 1848. It travelled from Putney College (where it was con- 

 structed) to Wandsworth, at the rate of 10 or 12 miles an hour. The 

 air-reservoir (substitute for a steam boiler) had a capacity of 75 cubic 

 feet ; it was capable of being charged with air to a pressure of fifty 

 atmospheres, by a 6-horse power engine. 



An air-engine of peculiar construction was employed in 1851, in the 

 Govan Colliery, near Glasgow, invented by Mr. Randolph. There 

 were circumstances connected with the shape and arrangement of the 

 shafts of the mine which rendered the supply of steam difficult; 

 whereupon Mr. Randolph, acting on a suggestion made by Mr. David 

 Elder, contrived the air-engine. He makes use of compressed air, 

 supplied by a compressing steam-engine at the surface, and conveyed 

 down by a pipe to work an engine at the top of the second shaft, in the 

 manner of a non-condensing steam-engine ; the discharged air being 

 thrown into the workings to aid in the ventilation of the mine. The 

 air-engine, supplied by the compressing apparatus, is employed for 

 working the winding and pumping apparatus of the second shaft. The 

 compressed air is conveyed to it through a cast-iron shaft 10 inches in 

 diameter, carried down the main shaft. At a meeting of the Institu- 

 tion of Mechanical Engineers, in 1857, it was stated that this machine 

 had been working admirably for six years, and had illustrated many 

 points connected with the useful employment of air-engines. 



There are certain forms of air-engines, especially Ericsson's, which 

 will be better noticed under CALOKIC ENGINE. 



AIR-GUN. An instrument for projecting bullets, in which the 



moving power is the rush of condensed air allowed to escape, instead 

 of the formation of gases arising from the ignition of gunpowder. 

 The air-gun and the common gun are therefore the same in principle. 



In the stock of the gun is a condensing syringe, the piston of which 

 condenses air into a cavity, which has a valve opening inwards, just 

 behind the bullet. The barrel ia open, and the bullet (which should 

 just fit it) in inserted hi the usual way. The trigger opens the valve 

 behind the bullet, and permits the rush of the condensed air, which 

 propels the bullet. The moment the finger is withdrawn from the 

 trigger, the air closes the valve, and remains, somewhat less condensed 

 than before, for the next discharge. 



The same principle has been variously applied. In the magazine 

 , there is a reservoir of bullets, in a channel under the barrel, 

 one of which is turned in by a cylindrical cock pierced by a tube, 

 which in one position is a continuation of the reservoir of bullets, and 

 in another, of the barrel. Thus, by turning the gun upside down and 

 turning the cock, a bullet falls into it from the reservoir, which, on 

 re-turning the cock, is of course in the barrel. This is a very primitive 

 application of the now famous principle of the Revolver, or revolving 

 pistol. In some air-guns, the cavity containing the condensed air is a 

 hollow cop|ier ball, which can be screwed on to the gun after conden- 

 sation. The air-cane in so called because it is usually in the form of a 

 walking-stick. The handle contains the condensed air, and can be 

 unscrewed and filled by a separate condensing syringe. There is some 

 mention of an instrument similar in principle t*> the air-gun among 

 the ancient* ; and it is said that Ctesibius, a celebrated mechanical phi- 

 losopher, who lived, B. c. 1 20, at Alexandria, constructed an instrument, 

 in which the air, by its elastic force, discharged an arrow from a tube. 

 I M"ui . i tr|;i, ' II i.-it-iire des Mathdmatiques,' vol. i. p. 267.) The invention, 

 such as we have described it, is ascribed to Marin, a native of Lisieux, 

 in France, who is said to have presented an air-gun to Henry IV. 



Shaw's air-gun, patented in 1849, combines an endless band of vul- 

 canised india-rubber with an air-exhausting apparatus. The elasticity 

 is so applied as to compress the air by 'a single stroke of a syringe or 

 air-purnp, the moment before the discharge. The operation performed 

 by the sportsman, to bring the apparatus into a charged condition, is 

 vmilar to that adopted in the use of the cross-bow. 



Down to the present period, however, the air-gun, in all its forms, 

 has been little other than a mere toy. No power, but only an adapta- 

 tion of power, is gained by its use; for the condensation of the air 

 itself requires force. The air-gun has never been used in war, on 

 account of its expense, and the force which must be employed to con- 

 detue the air. Tin '"</,, //, exhibited in London a few years ago, 

 a Hindi more forcible agent than ah- for the propulsion of 

 bullets; but, nothing has hitherto been accomplished to render that 

 mechanism practically available, although suggestions to that effect 

 were made during the Russian war of 1854-5. 



AIR-PUMP. A philosophical instrument for removing the air out 



-iel. We shall also include under this head the apparatus for 



forcing more air into a vowel, better known by the name of the Con- 



tion of pov 

 itself requi 

 account of 

 dense the a 

 exemplified 

 bullet* : hi! 



riwje, as the two differ Very slightly in their main principle 

 and simplest construction. 



Fig. 1. 



Fig. 2 



Above we have the sections of the simplest forms of an e.diausting 

 and of a condensing syringe. Both consist of a brass tube or cylinder, 

 closed at one end, excepting an orifice to which a valve or lid is 

 attached. A piston, with a rod and handle, enters at the other end, 

 and can be moved up and down the tube. The piston is not entirely 

 closed, but has a valve opening the same way as the other valve. Both 

 are attached to vessels the air of which is to be rarefied or condensed. 

 In fig. 1, or the exhausting syringe, both valves open upwards, or let 

 air only out of the vessel and the piston : in fig. 2, or the condensing 

 syringe, both open downwards, or let air only into the vessel and 

 through the piston. 



Let the whole contents of each vessel be, for example, six times 

 that of the tube of the syringe, and let both pistons be pushed down. 

 We first take the exhausting syringe. The instant the piston begins 

 to rise, there is no air between A and B ; the valve A is kept shut by 

 the pressure of the exterior air, while the air in the vessel, pressing on 

 B from underneath at the rate of about fifteen pounds to the square 

 inch, raises it, and the air in the vessel is thus distributed between the 

 vessel and the tube. If we call the tube one measure, the vessel is six 

 measures ; so that the air which occupied six measures now occupies 

 seven, or is only six-sevenths of its former density. When the piston 

 is returned again, the air in the tube is compressed, but cannot return 

 into the vessel, because B does not open inwards. By the time the 

 piston has returned through one-seventh of its descent, the equilibrium 

 between the air in the tube and the external air will be re-established ; 



