2143. Kewley's method of circulating hot water on the siphon principle is one of the very best modes which has yet been applied to the heating of hothouses, but as it requires some scientific knowledge, as well as practical skill, on the part of the mechanic who fits it up, and as this branch of engineering is in the hands of ironmongers who know very little of its principles, it has by no means become so general as it deserves. Mr. Kewley's mode will be understood by the following diagram (fig. 636.): a c e represent the two legs of the siphon; the upper log, commencing at c, being that through which the heated water ascends, and the lower leg being that by which it returns. The point e may be at, any distance from the boiler containing the open ends of the siphon. On the upper part of the pipe, at e, a small orifice is connected with an air-pump, by a pipe which need not be more than half an inch in diameter. The air-pump may be placed in any convenient situation, and the pipe connecting it with the siphon may be bent in any direction. The use of the air-pump is twofold: first, to exhaust the siphon, in order that it may be filled with water; and, secondly, to exhaust the air which always collects in the highest part of any tube in which water is circulated. The expense of a good air-pump, suitable for this purpose, is about 2l.; but for those who think this too much, there may be a small brass cock introduced on the upper surface of the highest part of the pipe, and, a funnel being put over it, and stoppers of any kind in the open ends of the siphon,' it may be filled through the funnel. Air which collects at this highest point may also be let off by opening the cock, after having previously stopped the open ends of the siphon, and pouring in as much water as will supply the place of the air, after which the cock is to be shut.