586 



GAS, COAL 



IV. APPARATUS FOR THE STORAGE OF COAL-GAS. 



The gasholder serves as a magazine for receiving the gas when it is purified, and 

 keeping it in store for use. It consists of two essential parts : 1, of an under cistern, 

 open at the top and filled with water ; and 2, of the upper floating cylinder or chest, 

 which is a similar cistern, inverted, and of somewhat smaller dimensions (see F, jig. 

 1025, p. 573). The best form of this vessel is the round or cylindrical ; both because 

 under equal capacity it requires least surface of metal, and it is least liable to be 

 warped by its own weight or accidents. Since a cylindrical body has the greatest 

 capacity with a given surface when its height is equal to its semi-diameter, its dimen- 

 sions ought to be such that when elevated to the highest point in the water, the 

 height may be equal to the radius of the base. For example, let the capacity of the 

 gasholder in cubic feet be k, the semi-diameter of its base bo x, the height out of the 

 water be h ; 



snr 



h = x = This height may bo increased by one or two feet, according to it 



3*14 



magnitude, to prevent the chance of any gas escaping beneath its under edge, when 

 it is raised to its highest elevation in the water. 



The size of the gasholder should be proportional to the quantity of gas to be con- 

 sumed in a certain time. If 120,000 cubic feet be required, for instance, in 10 hours 

 for street-illumination, and if the gas-retorts be charged 4 times In 24 hours, 30,000 

 feet of gas will be generated in 6 hours. Hence the gasholder should have a capa- 

 city of at least 70,000 cubic feet, supposing the remaining 50,000 cubic feet to be pro- 

 duced during the period of consumption. If the gasholder has a smaller capacity, it 

 must be supplied from a greater number of retorts during the lighting period, which is 

 not advantageous, as the first heating of the supernumerary retorts is wasteful of fuel. 



The water cistern is usually constructed in this country with cast-iron plates bolted 

 together, and made tight with rust cement. 



1047 



1048 



In cases where the weight of water required to fill such a cistern might bo incon- 

 venient to sustain, it may be made in the form represented in Jig. 1048 ; which, how- 

 ever, will cost nearly twice as much. Parallel with the side of the cistern, a second 

 cylinder c, of the same shape but somewhat smaller, is fixed in an inverted position t-> 

 the bottom of the first, so as to leave an annular space B n between them, which is 

 filled with water, and in which the floating gasholder A plays up and down. The 

 water must stand above the cover of the inverted cylinder, a and b are the pipes for 

 leading the gas in and out. Through an opening in the masonry upon which the ap- 

 paratus rests, the space c may be entered, in order to make any requisite repairs. 



The water cistern may also bo sunk in the ground, and the sides made tiirht with 

 hydraulic mortar, as is shown in Jig. 1047, and to make it answer with less water a 



