66. PHYSICS. 
cation of pressure, is called a gasometer.. Such vessels are constructed in 
various ways, according to the use to which they are to be applied. The . 
principle of construction in all, however, consists of a vessel filled first with 
water, into which the gas is then admitted,displacing the water. By the direct 
application of a weight, or by means of a column of water which exerts a 
pressure upon the gas, this is forced out through tubes attached to the vessel. | 
Pl. 19, fig. 1, represents a large apparatus of this kind, such as is used in 
gas works. It consists of a cylinder, B, of tin, closed above and open below, 
which sits in a great water reservoir of masonry. ‘T'wo tubes, D and E, 
rise into the cylinder from below, their upper extremities standing above the 
surface of the water; the one tube comes from the apparatus in which the 
gas 1s prepared, and serves to fill the gasometer; the other, D, is closed by a 
cock during filling, and serves for the exit of the gas. At some distance 
from the gasometer it divides into several branches, which carry the gas to 
the various points where it may be required. The tube E has also a cock, 
which is open during filling, and closed when the cock in D is open. It is 
evident that only one can be open at a time. The pressure exerted by the 
tin cylinder upon the gas, and which may be increased by the superposition 
of weights, causes the escape of the gas, and may be regulated by the coun- 
terpoise, C. 
To produce a regular stream of air, bellows and blowers are employed. 
A common bellows is the simplest means of producing a strong. stream of 
atmospheric air. This consists of an air-tight leather or wooden box, whose 
inclosed space may be increased or diminished ; air passing in through one 
small opening during the former, and passing out through a second aperture 
during the latter. A simple bellows of this kind cannot produce an uninter- 
rupted stream of air, as it acts only intermittingly. To produce a continuous 
blast, a double or compound bellows must be employed, as represented in fig. 6. 
This consists of two sections, a and b. Press down the lower plate of 
the section b, and the air enters through a valve ; press the plate up again, 
and the air compressed in b opens a valve between the two, and passes into 
the upper division, where it is compressed by superincumbent weights, and 
must escape through the opening at c. 
These bellows are only used by hand, or at most in small forges and 
organs. If avery powerful and intense stream of air be required, as, for 
instance, in smelting furnaces, &c., large blowers are employed, driven by 
steam or water power. These form a kind of condensing air-pump, except- 
ing that they have an escape aperture. ‘The most convenient and generally 
employed of these contrivances is the cylindrical blower represented in 
pl. 19, figs. 2 and 8. A is a cast iron cylinder, in which a piston, cc, , 
fitting air-tight, may be moved up and down by a piston rod, a. Through 
the upper valve at , and the lower at d, the inside of the cylinder is in 
communication with the external air, while the valves at f and g unite the 
cylinder with a four-cornered box, Ii. At all the openings are valves, of 
which those at b and d open inwards; those at f and g, outwards. When 
the piston descends, it closes the valve at d, while the air penetrates through 
the opening > into the upper part of the cylinder. The reverse takes place 
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