ON PNEUMATIC MACHINES. 263 



be intermitted while the cavity is replenished ; and in order to avoid this 

 inconvenience, a second cavity is sometimes added, and loaded with a weight, 

 which preserves the continuity of the stream. If great uniformity be 

 required in the blast, it will be necessary to take care that the cavity be so 

 formed as to be equally diminished while the weight descends through equal 

 spaces ; but notwithstanding this precaution, there must always be an 

 additional velocity while the new supply of air is entering from the first 

 cavity. Sometimes the construction of the bellows resembles that of a 

 forcing pump ; and then, if the barrel is single, a second barrel, loaded with 

 a weight, must be provided, in order to equalise the blast : or a vessel 

 inverted in water, and either loaded or fixed, may supply the place of the 

 second barrel. The first cavity may also be formed of a similar inverted 

 vessel, suspended to a beam, so as to be moved up and down in the water, 

 and such a machine is much used, in large founderies, under the name of 

 hydraulic bellows. The quantity of water employed may be much dimi- 

 nished, and the operation expedited, by introducing, in the centre of the 

 inverted vessel, a fixed solid, or an internal inverted vessel, capable of 

 nearly filling up the cavity of the moveable vessel when it is in its lowest 

 position, so that the water only occupies a part of the interstice between the 

 vessels. (Plate XXIV. Fig. 331.) 



The gasometer differs little from the hydraulic bellows, except that it is 

 provided with stopcocks instead of valves, and the moveable cylinder is 

 supported by a counterpoise, which, in the best kind, acts on a spiral fusee, 

 calculated to correct the difference of pressure arising from the greater or 

 less immersion of the cylinder. (Plate XXIV. Fig. 332.) 



A shower of water, or even an irregular stream, being conveyed through 

 a descending pipe, plunged into the water of a reservoir, a large quantity of 

 air is carried down with the water, and rises to the upper part of an 

 inverted vessel which surrounds the pipe, whence it may be conveyed 

 through another pipe, in a rapid stream, for any required purpose ; and 

 the water escapes at the bottom of the air vessel into the general reser- 

 voir, from the surface of which it runs off. The quantity of air sup- 

 plied by these shower bellows is, however, small. (Plate XXIV. Fig. 

 333.) 



The velocity of the blast produced by any pressure, forcing the air 

 through a pipe of moderate dimensions, may readily be determined from 

 the height of a column of air equivalent to the pressure. Thus if the 

 hydraulic bellows were worked with a constant pressure of four feet of 

 water, the velocity would correspond to a height of about 3300 feet, 

 and the air would move through a space of about 460 feet in a second. But 

 in this calculation no allowance is made for any of the causes which 

 diminish in all cases the discharge of fluids, and the velocity actually 

 observed is only five eighths as great as that which corresponds to the 

 height ; that is, in the example here given, 285 feet in a second, when the 

 air escapes through a small orifice ; but when it moves in a pipe, about 

 three fourths, or 345 feet. If the pipe were of considerable length, there 

 would also be a diminution of velocity on account of friction. In 

 some bellows actually employed, a pressure equivalent to nine feet of 



