LECTURES AND ESSAYS READ AT INSTITUTES. 275 



a lower level than the inlet drains. The silt can be removed by dippers. It 

 is my opinion that, so far as the deposit of silt is concerned, the silt basins 

 are of little practical value. If the main is properly proportioned, any silt 

 that passes through the laterals, will readily pass through that, and reach the 

 outlet. Silt basins have an important office, however, viz.: that of affording 

 a means of ascertaining the working of the drains, and consequently giving 

 valuable information. 



Silt basins may be constructed of stone or brick entirely, and of any dimen- 

 sions needed for especial cases. If designed only as peep-holes, it is not 

 necessary to excavate below the main drain, or even to make a break in the 

 grade of the drains. Covers should always be provided, to prevent the frost 

 reaching the drains. 



SIZE OE TILE. 



One of the important considerations to be decided is the size of the tile to 

 be used. Economy requires that small tile as will answer be employed, while 

 other considerations imperatively demand that the tile be not too small. 

 The important thing is to get just the correct size. There are many circum- 

 stances which effect the carrying capacity of tiles, such as the fall, area of 

 cross-section, length, directness, and smoothness of bore. Directness makes 

 more difference than many imagine. Thus, if resistance in a straight line 

 be ninety, on a true curve it will be one hundred, and be increased by a right 

 angle to one hundred and forty. The friction in tiles is proportionately less 

 in large than small tiles; if this were not so then the capacity would vary as 

 the square of the diameter, and a two-inch tile would carry one-fourth as 

 much water as a four-inch tile; but as an actual fact, it will carry only about 

 one-sixth as much, while a ten-inch tile has about ten times the capacity of a 

 four-inch tile. The variation of tile up or down from the grade line has 

 much to do with its capacity; a line of tile with open joints at each foot, is 

 never under the same pressure as a perfectly tight pipe having the same fall. 

 When the ground above it is saturated with water, the current may be 

 urged on by some head, but since the joints are open, it is evident that 

 the pressure on the water on the inside of the tile cannot rise much above 

 the pressure on the water in the soil outside the tile. If this were not true, 

 the water would flow out from the tile at each joint, and as the area of a 

 limited number of joint openings soon eqiials the capacity of the tile, the tile 

 would empty itself. This may take place under some circumstances until the 

 pressure is equal, both inside and out; but in any event, the pressure on water 

 flowing through a line of tile drain can never exceed that due to a depth of 

 water equal to or less than the depth of the drain at any given place. Usually 

 there is no pressure urging the water onward except that due to gravity, which 

 depends only on the fall of the drain. This pressure, or force, acting con- 

 stantly, would make the water flow, like a ball rolling down hill, faster and 

 faster. Were it not for the continual resistance caused by the rubbing of the 

 water against the sides of the tile, and by irregularities and bends in the pipe 

 its velocity would be excessive. This rubbing, or friction, is very great, and 

 may almost entirely destroy the current in small pipes of great length, so that 

 pipes on a given grade, and of a given size, should not exceed certain lengths. 



PROPORTIONING OF TILES FOE A LONG DRAIN. 



This subject is an interesting one, from the fact that opposite views are said 

 to be held. It was stated at the 1884 meeting of the Michigan Tile Drainage 



