DISTRIBUTION OF GAS THROUGH MAINS. 



169 



gas, increases the loss by leakage, and is otherwise injurious. None of these 

 evils will arise if the circulation through the system of mains be perfect. If 

 the general level of any division of the town or district rise more than thirty 

 feet above the datum line, it must be supplied from the lower level, but the 

 pressure equalized by a small governor, similar in construction to that at 

 Plate XVIII., but of smaller dimensions : the principle of its action is the same 

 in both cases. This governor need only be applied where the high level is 

 uniform, and where there are no means of equalizing the pressure by uniting 

 its mains with those of a sufficiently low division. Should there be two distinct 

 portions of the town rising one above the other, like a terrace, each maintain- 

 ing a general level, they will be more conveniently lighted from different 

 leading mains, the entire arrangements being kept separate. 



By these few examples I have endeavoured to convey my ideas of the most 

 economical arrangements for street-mains. Though practical knowledge is 

 necessary, theory will lend us considerable assistance in determining the 

 diameters of the various mains necessary to supply the requisite quantities of 

 gas for any length and under any pressure. The following data will furnish 

 every facility for the calculations. 



All gases are elastic, or capable in a greater or less degree of compression 

 and expansion. The elastic force is exactly equal to the force of compres- 

 sion. Heat increases and cold diminishes the elasticity of gases ; in other 

 words, heat expands and cold contracts. The effect of pressure upon gas is 

 the same as upon any non-elastic fluid ; and the velocity with which it is dis- 

 charged through an orifice or tube will depend upon the pressure, and will be 

 as the square root of the perpendicular height or head of the water giving 

 that pressure. For example, carburetted hydrogen gas of the specific gravity 

 420 will flow through a circular orifice one-fourth of an inch diameter, with 

 a pressure equal to five-tenths of an inch head of water at the rate of eighty 

 cubic feet per hour, and under different pressures, as follows : 



