The regularity of the blast in the double bellows, is effected by means of a weight pressing on the air contained in the second copartment of the bellows ; just in the same way as a stream of air is made to issue regularly from a tube fixed in the mouth of an inflated bladder, when a 'weight is placed on the bladder. A regular stream of air may also be ob tained, by subjecting inclosed air to the pressure of a column of water, mercury, or some other liquid. If a vessel containing air, and open at the mouth, be plunged into water with the mouth downwards, and if the water on the outside of the vessel rise higher than the surface of the water within the ves sel, then the column of water, whose height is the difference of level,' exercising its pressure, as all li quids do, in every direction, acts upwards on the Inclosed air; and the inclosed air will be pressed and more condensed than the external air, and will escape in a current, by a stop-cock opened on the top of the vessel for its issue ; this issue will con tinue till the surface of the exterior and interior water come to a level ; for then the air in the vessel is come to have the same density as the external air. The force with which the inclosed air is pressed, is equal to the weight of a volume of water whose height is the difference of levels, and its base the surface of water exposed to the inclosed air. The gasometers used by Lavoisier, to afford a stream of oxygen gas and a stream of hydrogen gas, for ac complishing the composition of water, are construct ed upon this principle. An apparatus of the same nature has, for many years, been employed in the great way, in different parts of Britain, to regu late the most powerful blast used in The arts,— that for reducing ironstone to the state of cast-iron. In blast-furnaces upon this construction, the blast is raised by means of a large cast-iron cylinder which acts as a bellows, having a valve in the bot tom that opens inwards, and that admits the air during the ascent of the piston ; when the piston • descends, the valve shuts, and the air is driven into a large parallelopipedal vessel, less in height than in the other dimensions, immersed in water, and having its under surface closed only by the water. In this vessel the air is pressed by the column of water, whose height is the distance between the surfaces of the exterior and interior water ; a.pipe of issue ter minating in a nose-pipe, conducts the blast to the furnace.
The blowpipe of the Abb6 Melograni of Naples, for the use of the mineralogist, operates by the pres sure of water. It is composed of two hollow globes, the upper filled with water, which, by running into the lower, forces the air contained in the lower to issue through a nozle. This apparatus is described by Mr Greenough, in Nicholson's Journal, Vol. IX. p. 25 and 10. It has some inconveniencies, and does not appear ever to have come into much use.
The water-pressure apparatus, applied to the blow pipe, of which a section is given at fig. 10. was contrived by Mr Tilley, an ingenious fancy glass-blower. It consists of a tin box with a par tition in it, reaching within half an inch of the bottom ; water is poured in equal in bulk to three fourths of the capacity of the box. The water in the cavity DE is open, and subject to no other pres sure but that of the atmosphere, being only covered by the lid of the vessel; the apartment F is closed at top, so as to be air. tight, and the water in it is pressed by the elasticity of the air confined in its upper part. The tube C has its lower extremity al ways 'plunged in water, so that, when air is blown in through it, the return of the air by that tube is prevented. Before the apparatus is set to work, the surface of the water in both copartments is at the same height, both being pressed by air of the densi ty of the surrounding air ; but, when air is blown in through C, the air rises through the water to the top of the copartment F; and as the only issue for the air is through the small aperture of the blow-pipe, by which it cannot escape nearly so fast as it is blown in, the air consequently becomes condensed in the upper part of the copartment F ; and this condensed air pressing on the water in F more strongly than the atmosphere does on the water in DE, depresses the surface of the water in F, and causes it to rise in DE, which is effected by a por tion of the water passing under the partition into the open copartment DE. Thus the pressure exerted
by the column of water whose height is the differ ence of level of the water in DE, and of the water in F, forces the air from the copartment F through the blow-pipe a, which is directed against the flame of a lamp ; and this pressure keeps up a constant blast till the water in the two copartments comes nearly to the same height. The degree of conden sation of the inclosed air, and the height of the column of water pressing on the condensed air, are measures of each other, when much air is blown in, so as to occasion a considerable degree of condensa tion. The difference of level resulting is considerable; and the column of water, which is always reacting with an equal and contrary pressure on the condensed air, causes it to issue with greater velocity from the blow-pipe : when the condensation diminishes, 'so does the column of water, and the velocity of the is suing stream of air. More air is to be blown in with the mouth through the tube C, from time to time, so as to keep the blast regular. Mr Tilley is of opinion that this apparatus produces a more regu lar stream of air than a double bellows, and it has likewise the advantage that the operator is free from the trouble of moving a pedal. The dimensions of the vessel A A, which is either of tinned iron or of tinned copper, are 17 inches in height, 5 inches in width, and 9 in breadth ; the lid of the vessel opens and shuts on hinges, and supports the tallow lamp B. The bent glass tube a, which terminates in a sssall hole, is fitted into a tin tube, which is made conical, and which forms the issue from the top of the copartnient F; for this purpose, paper is wrapped round the glass tube, and then cotton wick yarn, in a conical form, so that the glass tube thus clothed may fit tight into the socket, and may nevertheless be moved round, that the blast may act properly on the flame. The bent metal tube C is also fixed into its socket in the same manner its junction with the socket is seen in fig. 10. There is a screen formed of a tin plate sliding vertically in grooves between two upright pieces of tin ; the edge of this is seen at S, in fig. 10. It is intended to pro• test the eyes of the operator from the light of the lamp, whilst, at the same time, he can see the subject of his operation over the top of the screen. This screen is not soldered to the vessel, but is held fast by its foot being placed between the lid of the ves sel and the top of the close chamber F. Two rests for supporting the operator's arms project one from each aide of the vessel; upon these the arms are placed when any substance is held to the flame. These rests are wrapped round with woollen list or leather, so as to be more convenient for leaning upon. The whole of the apparatus, including the lamp and case, weighs only three pounds and a half. When it is to be used, the vessel is fixed to a table or bench, by means of a leather strap buckled to two loops, which are on the sides of the vessel op posite to each other ; the straf is passed under the table or bench. The long flat cotton wick is pre ferred by some glass-blowers to the usual round cot ton wick. The lamp is filled with tallow, which melts after the lamp has been lighted for some time, and then it burns as freely as oil, and with a less offensive smell. When not in use, the tallow be oomes solid, and is more conveniently carried about than oil : hogs lard also does well for burning in this lamp. Some glass-blowers mix cocoa-nut oil, which is solid, at the temperature of the climate of Britain, with hogs' lard, and find it to answer well in the lamp. The lamp is placed within another vessel, marked K, which supports it at a proper height, leaving a space between all round, to receive any tallow that may run over the edge of the lamp. A wire bent at the end is convenient for trimming the wick, and form ing it into a channel through which the stream of air is to be directed. It is convenient to have seve ral lamps with wicks of different thicknesses, name ly, one to hold two flat cottons of about one inch and a quarter broad, another to hold four, a third to hold six, or as much common wick-yarn as is equal to those wicks in bulk, and glass adjutages of differ ent sized apertures to suit the different sized wicks. See Transactions of the Society for Encouraging Arts, Vol. XXXI.