GAS, COAL 587 



fiofcC'entric cylindrical mass of masonry may be built at a distance of 2 or 3 inches 

 within it. 



Every large gasholder must be strengthened interiorly with cross iron rods, to 

 stiffen both its top and bottom. The top is supported by rods stretching obliquely 

 down to the sides, and to the under edge an iron ring is attached, consisting of curved 

 cast-iron bars bolted together ; with which the oblique rods are connected by perpen- 

 dicular ones. Other vertical rods stretch directly from the top to the bottom edge. 



X r n the periphery of the top, at the end of the rods, several rings are made fast, to 

 :h the gasholder is suspended, by means of a common chain which runs over a 

 pulley at the centre. Upon the other end of the chain there is a counterpoise, which 

 takes off the greater part of the weight of the gasholder, leaving only so much as is 

 requisite for the expulsion of the gas. The inner and outer surfaces of the gasholder 

 should be a few times rubbed over with hot tar, at a few days' interval between each 

 application. The pulley must be made fast to a strong frame. 



If the water cistern be formed with masonry, the suspension of the gasholder may 

 be made in the following way: A ^fig. 1047, is a hollow cylinder of cast iron, stand- 

 ing up through the middle of the gasholder, and which is provided at either end with 

 another small hollow cylinder G, open at both ends, and passing through the top, with 

 its axis placed in the axis of the gasholder. In the hollow cylinder o, the counter- 

 weight moves up and down, with its chain passing over the three pulleys, B, B, B, as 

 shown in fig. 1048 ; E F are the gas-pipes made fast to a vertical iron rod. Should the 

 gasholder be made to work without a counterweight, as we shall . presently see, the 

 central cylinder A A serves as a vertical guide. 



In proportion as the gasholder sinks in the water of the cistern, it loses so much 

 of its weight as is equal to the weight of the water displaced by the sides of the sink- 

 ing vessel, so that the gasholder, when entirely immersed, exercises the least pressure 

 upon the gas, and when entirely out of the water, it exercises the greatest pressure. 

 In order to counteract this inequality of pressure, which, where no governor is used, 

 would occasion an unequal velocity in the efflux of the gas, and of course an unequal 

 intensity of light in its flame, the weight of the chain upon which the gasholder hangs 

 is so adjusted as to be equal, throughout the length of its motion, to one-half of the 

 weight which the gasholder loses by immersion. In this case, the weight which it 

 loses by sinking into the water is replaced by the portion of the chain which, passing 

 the pulley and hanging over, balances so much of the chain upon the side of the coun- 

 terweight ; and the weight which it gains by rising out of the water is counterpoised by 

 the links of the chain which, passing over the pulley, add to the amount of the counter- 

 weight. The pressure which the gasholder exercises upon the gas, or that with which 

 it forces it through the first main pipe, is usually so regulated as to sustain a column 

 of from one to two inches of water, so that the water will stand in the cistern from one 

 to two inches higher within than without the gasholder. The following computation 

 will place these particulars in a clear light : 



Let the semi-diameter of the gasholder, equal to the vertical extent of its motion 

 into and out of the water, =x ; let the weight of a foot square of the side of the gas- 

 holder, including that of the strengthening bars and ring, which remain plunged under 

 the water, =p; then 



1. the weight of the gasholder in its highest position = 3 p * X* ; 



2. the weight of the sides of the gasholder which play in the water = 2 p x 1 ; 



3. the cubical contents of the immersed portion of the gasholder =^J?l ; 



1 12 



4. its loss of weight in water p IT x* ; 



400 



5. the weight of the gasholder in its lowest position 



6. the weight of n inches height of water = n * a? ; 



7. the amount of the counterweight = IT x 2 ( 3 p -^ J ; 



8. the weight of the chain for the length x = p * a? 2 . 



800 



If we reduce the weight of the gasholder, in its highest and lowest positions, to 

 the height of a stratum of water equal to the surface of its top, this height is that of 



