"The use of collars is by no means general, although those who have used them speak highly of their advantages. Except in sandy soils, and in those that are subject to sudden alteration of character, in some of the deposits of red sand-stones, and in the clayey subsoils of the Bagshot sand district, for instance, collars are not found to be essential to good drainage. In the north of England they are used but seldom, and, in my opinion, much less than they ought to be; but this opinion, it is right to state, is opposed, in numerous instances of successful drainage, by men of extensive practice; and as every cause of increased outlay is to be avoided, the value of collars, as general appliances, remains an open question. In all the more porous subsoils in which collars have not been used, the more successful drainers increase the size of the pipes in the minor drains to a minimum size of two inches bore." _The form of the bore, or water passage_, in tiles, is a point of more importance than at first appears. At one of our colleges, certain plank sewers, in the ordinary square form, were often obstructed by the sediment from the dirty water. "Turn them cornerwise," suggested the professor of Natural Philosophy. It was done, and ever after they kept in order. The pressure of water depends on its height, or head. Everybody knows that six feet of water carries a mill-wheel better than one foot. The same principle operates on a small scale. An inch head of water presses harder than a half inch. The _velocity_ of water, again, depends much on its height. Whether there be much or little water passing through a drain, it has manifestly a greater power to make its way, to drive before it sand or other obstructions, when it is heaped up in a round passage, than when wandering over the flat surface of a tile sole. Any one who has observed the discharge of water from flat-bottomed and round tiles, will be satisfied that the quantity of water which is sufficient to run in a rapid stream of a half or quarter inch diameter from a round tile, will lazily creep along the flat bottom of a sole tile, with hardly force sufficient to turn aside a grain of sand, or to bring back to light an enterprising cricket that may have entered on an exploration. On the whole, solid tiles, with flat-bottomed passages, may be set down among the inventions of the adversary. They have not the claims even of the horse-shoe form to respect, because they do not admit water better than round pipes, and are not united by a sole on which the ends of the adjoining tiles rest. They combine the faults of all other forms, with the peculiar virtues of none. [Illustration: Fig. 33--FLAT-BOTTOMED PIPE-TILE.] From an English report on the drainage of towns, the following, which illustrates this point, is taken: