ON PNEUMATIC MACHINES. 3i3 



heated air is suffered to escape by a stopcock at the upper part of the bell. 

 When proper care is taken to lower the machine gradually, the diver can 

 support the pressure of an atmosphere of twice or thrice the natural density. 

 It would be advisable that every diver should be provided with a float of 

 cork, or with a hollow ball of metal, which might be sufficient to raise him 

 slowly to the surface, in case of any accident happening to the bell; for want 

 of a precaution of this kind, several lives have been lost from confusion in 

 the signals. (Plate XXIV. Fig. 330.) 



Bellows are commonly made of boards connected by leather, so as to 

 allow of alternately increasing and diminishing the magnitude of their cavities, 

 the air being supplied from without by a valve. The blast must be inter- 

 mitted while the cavity is replenished ; and in order to avoid this inconveni- 

 ence, 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 not- 

 withstanding 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 em- 

 ployed may be much diminished, and the operation expedited, by introduc- 

 ing, 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 sup- 

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



