ARRANGEMENT MUST HAVE REFERENCE TO SYSTEM.
The absolute necessity of some regularity of plan in our work, must be manifest. Without system, we can never, in the outset, estimate the cost of our operation; we can never proportion our tiles to the quantity of water that will pass through them; we can never find the drains afterwards, or form a correct opinion of the cause of any failure that may await us.
We prefer, in general, where practicable, parallel lines for our minor drains, at right angles with the mains, because this is the simplest and most systematic arrangement; but the natural ravines or water-courses in fields, seldom run parallel with each other, or at right angles with the slope of the hills, so that regular work like this, can rarely be accomplished.
If the earth were constructed of regular slopes, or plains of uniform character, we could easily apply to it all our rules; but, broken as it is into hills and valleys, filled with stones here, with a bank of clay there, and a sand-pit close by, we are obliged to sacrifice to general convenience, often, some special abstract rule.
We prefer to run drains up and down the slope; but if the field be filled with undulations, or hills with various slopes, we may often find it expedient, for the sake of system, to vary this course.
If the question were only as to one single drain, we could adjust it so as to conform to our perfect ideal; but as each drain is, as it were, an artery in a complicated system, which must run through and affect every part of it, all must be located with reference to every other, and to the general effect.
Keeping in mind, then, the importance of some regular system that shall include the whole field of operation, the work should be laid out, with as near a conformity to established principles as circumstances will permit.
ARRANGEMENT MUST HAVE REFERENCE TO THE FALL.
In considering what fall is necessary, and what is desirable, we have seen, that although a very slight inclination may carry off water, yet a proportionably larger drain is necessary as the fall decreases, because the water runs slower.
"It is surprising," says Stephens, "what a small descent is required for the flow of water in a well-constructed duct. People frequently complain that they cannot find sufficient fall to carry off the water from the drains. There are few situations where a sufficient fall cannot be found if due pains are exercised. It has been found in practice, that a water-course thirty feet wide and six feet deep, giving a transverse sectional area of one hundred and eighty square feet, will discharge three hundred cubic yards of water per minute, and will flow at the rate of one mile per hour, with a fall of no more than six inches per mile."