STREET-MAINS. 165 



The depth of mains below the surface of the street ought to be about one 

 foot six, not less. Their course should be as nearly as possible in straight 

 lines, inclining at the rate of 1 in 100. 



In order to guard against the danger of water remaining that enters from 

 the external surface into the pipes, and the deposition of other condensed 

 matter, a reservoir should always be placed at the lowest point, where two or 

 more descending mains meet and form an angle, to receive the water, etc. that 

 may happen to collect at this angular point, an accumulation of which would 

 obstruct the passage of the gas through the mains. These receivers ought to 

 be at least twice the diameter of the mains between which they are interposed, 

 and four times that diameter in depth. These receivers afford the best indication 

 of the sound or leaky state of the system of mains. In all instances where the 

 pipes are perfectly sound, observation has shown that hah a mile of gas-mains 

 three inches in diameter, does not deposit more than a quart of condensed 

 vapour or water in the year ; on the other hand, when the mains are leaky, the 

 water of the reservoir requires to be pumped out, particularly in wet weather, 

 as frequently as once a fortnight. The loss of gas by such leakage is much 

 greater than is generally imagined. In order to keep the common air out of 

 the faulty mains, a constant influx of gas is often necessary ; this is of course 

 so much gas lost to the ceconomy of the establishment. 



In all wide streets, where there are a number of houses to be supplied on 

 both sides, it is more economical to employ a separate gas-main for each side 

 of such street than one large main, because much smaller pipes may then be 

 laid down, and the collateral branch-pipes leading to the houses are shorter, 

 which circumstance more than compensates for the additional small main. 



