On Drainage and Sewerage of Towns. 367 
be a risk of the crown of the sewer being forced up, unless the 
dimensions were increased in the inverse proportion to the 
velocity of the water through it. 
This isa matter to which attention should be paid, for it 
frequently occurs that the valley through which the main 
sewer runs has steep sides, and is then level, or nearly so, 
for some distance from the edge of the watercourse. ita 
such cases it is necessary to increase the dimensions of the 
branch sewer on the level parts, in order to prevent the risk 
of damage which would be certain to happen should this 
portion be choked or gorged with water. 
As to the actual construction of these sewers, this must 
depend of course very much upon the materials of which 
they are made. In some places, stone will be cheaper than 
brick ; in others, it might be possible to use cylindrical pipes 
of different diameters, made of strong earthenware. 
The engineer must of course be guided in the employ- 
ment of any or all of these by the peculiar circumstances 
of the case. A preference of one over another should 
result only from a close examination of the advantages and 
disadvantages attendant upon each,—among which of course 
their relative economy is one of the most important con- 
siderations. 
Tt is most desirable that no leakage should take place 
through the bottom or sides of the branch sewer, which 
should, therefore, always be carefully bedded in cement. 
Where the-sewers are built of brick, and the amount of 
water-way required is more than an 18-inch barrel drain would 
supply, the oval or egg form is that which combines most 
advantages. ‘The effect of the scour of a small quantity 
of water is far greater in a sewer of this shape than in one 
with a flat floor. The quantity of material required in 
