60 CONSTRUCTION AND MANAGEMENT OF SMALL GASWORKS. 



when the rate of consumption is such that a 5-tenths, or, 

 in other words, when : 



Initial pressure - a = less than 2O-tenths. 



A very important point is that as we approach and exceed 

 the carrying limit, the value of a advances by leaps and 

 bounds. 



These simple illustrations serve to explain the distribution 

 troubles that so often arise in practice. When we consider 

 the difficulty in forecasting the maximum hour's consumption, 

 the effect of variations in altitude, of bends, curves, tees 

 and other obstructive influences, it is evident that the 

 distribution problem is a very complicated one, and that 

 the main pipes must be sufficiently large to clear the peak 

 of the load. 



We have already noted the effect of varying conditions, 

 and the problem is not simplified by the changes that are 

 continually taking place. Even if a satisfactory distribution 

 scheme is carried out, things may so alter in the course of 

 ten years that a rearrangement to suit the new conditions is 

 called for. 



Suppose a new by-road is opened up, and a main laid. 

 The first year there will not be more than half-a-dozen con- 

 sumers, as residences will not be built faster than they are 

 wanted, and the district continues to develop slowly, adding, 

 say, three or four new consumers per annum. At first, a 

 is an infinitesimal quantity, and the consumers get the full 

 initial pressure. But, as more consumers are added, and 

 the existing ones begin to adopt cookers, gas fires, 

 geysers, etc., a limit is arrived at when a comes into 

 evidence, first at periods of maximum consumption, such as 

 Saturday night, and as the carrying limit is approached the 

 advance in a becomes rapid. Can one wonder that some of 



