reason that all such tables are without practical
value. The smoothness and uniformity of the bore; the rate of fall; the
depth of the drain, and consequent "head," or pressure, of the water; the
different effects of different soils in retarding the flow of the water to
the drain; the different degrees to which angles in the line of tile
affect the flow; the degree of acceleration of the flow which is caused by
greater or less additions to the stream at the junction of branch drains;
and other considerations, arising at every step of the calculation, render
it impossible to apply delicate mathematical rules to work which is, at
best, rude and unmathematical in the extreme. In sewerage, and the water
supply of towns, such tables are useful,--though, even in the most perfect
of these operations, engineers always make large allowances for
circumstances whose influence cannot be exactly measured,--but in land
drainage, the ordinary rules of hydraulics have to be considered in so
many different bearings, that the computations of the books are not at all
reliable. For instance, Messrs. Shedd & Edson, of Boston, have prepared a
series of tables, based on Smeaton's experiments, for the different sizes
of tile, laid at different inclinations, in which they state that
1-1/2-inch tile, laid with a fall of one foot in a length of one hundred
feet, will discharge 12,054.81 gallons of water in 24 hours. This is equal
to a rain-fall of over 350 inches per year on an acre of land. As the
average annual rain-fall in the United States is about 40 inches, at least
one-half of which is removed by evaporation, it would follow, from this
table, that a 1-1/2-inch pipe, with the above named fall, would serve for
the drainage of about 17 acres. But the calculation is again disturbed by
the fact that the rain-fall is not evenly distributed over all the days of
the year,--as much as six inches having been known to fall in a single 24
hours, (amounting to about 150,000 gallons per acre,) and the removal of
this water in a single day would require a tile nearly five inches in
diameter, laid at the given fall, or a 3-inch tile laid at a fall of more
than 7-1/2 feet in 100 feet. But, again, so much water could not reach a
drain four feet from the surface, in so short a time, and the time
required would depend very much on the character of the soil. Obviously,
then, these tables are worthless for our purpose. Experience has fully
shown that the sizes whi