SOME REMARKS ON SEWERAGE SYSTEMS. 101 



tion chambers. A lamp can then be hung in the lamphole, 

 and the man in the manhole can see whether the sewer is 

 clear. In large sewers where men can enter, these straight 

 lines are not necessary. Manholes and lampholes are often 

 used as ventilators now-a-days, and it is then usual to give 

 the sewer a slight drop at that spot, in order to guide an 

 ascending current of air upwards as much as possible. 

 Where gradients are steep these bays are useful in 

 checking the velocity, which would otherwise cause undue 

 wear ; but as it has been found by experiment that wher- 

 ever splashing occurs foul air is generated, I am doubtful 

 if these bays, especially at ventilators, do not do more harm 

 than good. 



In addition to manholes, since the introduction of smaller 

 bore sewers, inspection pipes are placed at short intervals 

 of about 100 feet, fitted with movable caps or tops. In 

 case of stoppage these can be opened, and cleansing rods 

 passed through without disturbing the remainder of the 

 sewer. This brings us to another important point, and that 

 is, provision for flushing. This matter might not be so 

 necessary if all sewers were designed and laid out by the 

 formulae already quoted — to give a velocity of 150 feet per 

 minute. But it must be remembered that the flow of 

 sewage is often so variable that there is an insufficient 

 quantity to produce the minimum velocity, and that this 

 velocity of 150 feet per minute will not suffice to remove 

 very heavy material, such as road-stones and other heavy 

 articles which sometimes (but should not) find their way 

 into the sewers. Therefore it is best, for safety's sake, to 

 provide means of flushing ; i.e.^ means of artificially in- 

 creasing the velocity. This can be done either by storing 

 up the sewage itself and letting it go with a rush, or by 

 storing water in tanks and letting it suddenly into the 



