Tuomson.—On the Cleansing of Towns. 41 
“By this arrangement the sewage is not only at once diluted by the 
large volume of water in the Thames at high-water, but is also carried by 
the ebb tide to a point in the river twenty-six miles below London Bridge, 
and its return by the following flood tide within the metropolitan area is 
effectually prevented.” 
Again: “At the threshold of my (Sir J. W. Bazalgette’s) enquiry into 
this subject the following important points required to be solved :— 
“1st. At what point and state of the tide ean the sewage be discharged 
into the river, so as not to return within the more densely inhabited por- 
tions of the metropolis ? 
* 2nd. What is the minimum fall which should be given to the inter- 
cepting sewers ? 
“Ərd. What is the quantity of sewage to be intercepted, and does it 
pass off in a uniform flow at all hours of the day and night, and in what 
manner ? 
“4th. Is the rainfall to be mixed with the sewage? In what manner 
and quantities does it flow into the sewers; and, also, is it to be carried 
off in the intercepting sewers, and how is it to be provided for ? 
' 6th. Having referred to all these points, how are the sizes of the 
intercepting and main drainage sewers to be determined ? 
' 6th. What description of pumping engines and of pumps are best 
adapted for lifting the sewage of London at the pumping stations? So 
comprehensive a subject, involving not only the above but many other 
important topics, cannot be fully considered within the limits of an 
ordinary paper, in which these questions can only be briefly touched 
upon." 
Experiments by floats were now made on the river Thames, by 
whieh it was found that *the excess of the ebbs over the floods was 
only five miles in four days," and “that a substance in suspension, works 
up the river about one mile a day at each high water, as the springs 
strengthen, and down the river two miles a day as they fall off." Again: 
that “the delivery of the sewage at high water into the river at any 
point, is equivalent to its discharge at low water at a point twelve miles 
lower down the river; therefore the construction of twelve miles of sewer 
is saved by discharging the sewage at high instead of at low water." 
The flow of sewage in the drains was then determined by reference to 
the data afforded by the works of well-known authorities, and it was con- 
cluded by the engineer to regard that “a mean velocity of one-and-a-half 
miles per hour in a properly protected main sewer, when running half 
full, is sufficient, more especially when the contents have passed through 
8 pumping station,” 
