SOME EEMAEKS ON SEWERAGE SYSTEMS. 109 



the exact cost of lifting sewage by ejectors is not generally 

 known, no details having ever, to my knowledge, been 

 published. It is stated, however, in a paper read last year 

 before the Society of Engineers by Mr. Ault (Mr. Shone's 

 partner), that the usual effect obtained from the ejector 

 when using air at a pressure of 11 lbs. per square inch is 

 58 per cent, of the I.H.P. of the driving engine (in this 

 case an Atkinson gas-engine). 



The 77 of a good centrifugal pump is about ... 52 per cent. 

 „ „ „ direct-acting steam-pump for 



water is about ... ... 88 per cent. 



An average yj for a good direct-acting steam sewage pump 

 would probably be about QQ per cent., so that where pumping 

 is necessary the ejector appears to be a little less economical 

 than a steam-pump. Of course, also, where a system de- 

 pends on gravitation only, and the sewers are so flat as to 

 be sewers of deposit, a great set-off to the expense of ejec- 

 tion is the reduction in cost of cleansing the sewers, and 

 their improved sanitary condition consequent upon better 

 gradients. 



With regard to the second objection — 



The estimated loss due to compression (iso- 

 thermal) is about ... ... ... ... 31 per cent. 



The estimated loss due to leakage and fric- 

 tion in engines, mains, and ejectors ... 11 per cent. 



Total ... ... ... 42 per cent. 



This loss, however, compares favourably with that in 

 other classes of pumps ; and it has been found that when 

 the air is not compressed to more than 40 lbs. per square 

 inch, the leakage and friction in the mains is not so large as 

 to form a serious item of loss. 



