It will be noted that the efficiency of small pipes is much greater than that of larger ones, and in these days of high-pressure steam, much may be said in favour of the use of comparatively small wrought-iron steam-pipes instead of the larger cast metal ones. The first cost is small, the pipes are easily obtained ready screwed, and in the lengths required, and may be put together by any intelligent workman. The risk of fracture by the concussion of condensed water is very trifling, as compared to that of metal, and much lighter pipes are safe for high pressures. Steam-pipes must always be laid with an incline of say 1 in. in 10 ft. from the end where the steam is admitted, so that the condensed water may get away, and at the lowest point a steam-trap must be provided for its escape. In the writer's experience, the best form is that of Holman, made by Tangye of Birmingham, of which the principle will readily be understood from Fig. 67. The cup-shaped vessel _a_ floats on the water in the outer casing, and so closes the valve _b_ until _a_ gets full, when it sinks and allows the water to escape until it floats up again. It is important that this trap should be set level, or the valve will not close properly. Each pound of condensed water is equivalent to about 1000 units of heat given off (see Table III.). In planning steam-pipes, it is not necessary that they should be arranged in a single line. Even if in gridiron form the steam will still reach every part, in proportion to the condensation which takes place. A series of large pipes may be supplied by small pipes from a common main, and discharge their condensed water into a common waste-pipe with branch from each. A 1/2-in. pipe from a high-pressure boiler will supply a considerable range, say 100 ft. of 4-in. pipe, though a larger size is advisable. At the farther end of a range of steam-pipes a small tap must be provided to let out the air which accumulates in them. In employing the exhaust steam of an engine for heating purposes, the pipes must be of ample size and freely open at the ends to avoid back-pressure. For this purpose the gridiron form is a very good one.