Sometimes (to show the ingenuity which people often expend upon bad things) this waste pipe has a trap, and a little pipe from the water supply fixed to feed the trap; but all these ingenious plans have been devised in order to improve upon a principle radically wrong. The pipe should be carried through the wall and end outside the house as a warning pipe. Scarcely any water ever comes out at all; if any does come out, it shows there is something wrong, so that this pipe should pass through the wall, and be made to discharge outside the house. In order to prevent wind blowing up the pipe, it is usual to put a small brass flapper on the end. Its weight keeps it shut, and the pressure of water opens it. Underneath the safe-tray you will find as a rule a trap of some kind, and generally the trap that is found is a D trap, a trap whose name indicates its shape, and which cannot be washed out by the water that passes through it. The pipe from the closet passes so far in it that it dips below the level of the out-going pipe, and thus forms a sort of dip-trap. The pipe which is the inlet from the closet is not placed close to the edge, but a little way in, to form a receptacle for all kinds of filth! You will see it is impossible for the water that passes through it to clear the contents out, so that the trap is simply a small cesspool, nothing more nor less. Into that trap various waste pipes are frequently connected. There is another form of D trap in which there are two waste pipes going into the water near the bottom of the trap (probably the waste pipe of the safe and the waste pipe of the cistern). The D trap, then, is a bad form of trap, because it is not self-cleansing. The water cannot possibly keep it clear of sediment. So that some trap should be used which is self-cleansing, and the water which passes through it is capable of keeping it clean. Now that trap is a mere ∾-shaped bend in the pipe, to which we give the name of siphon, not because we want it to act as a siphon--for if it acts as a siphon it is of no use! A curious thing about siphon traps and pan closets is, that the form of trap which was used first in connection with water-closets was the siphon trap, which we now praise; and the form of trap which supplanted it was the D trap, which we are now condemning and taking out wherever we can. A still more curious thing is that the form of water-closet which we now condemn (the pan closet) was the form of closet which supplanted the closet we are now using (the valve closet). The valve closet was invented long before the pan closet. Bramah valve closets fixed forty years ago often act tolerably well now, and at the present day they are only taken out because they are really actually worn out.