2136. The improvements which have been made in the mode of heating by hot water are various. At first, the water was chiefly circulated in tubes perfectly horizontal in their direction. Soon after, it was found that it might be circulated in tubes irregular in point of horizontal direction, and both below and above the level of the boiler. Two engineers, Kewley of London, and Fowler of Devonshire, have circulated water in the two legs of a siphon, which is found to increase the rapidity of its motion; and broad flat pipes have been used, instead of cylindrical ones, as giving out the heat more rapidly. It is but justice to the memory of Count Chabannes to state, that most of these methods seem to have been known to him; and as it is certain that he merely echoed the inventions of Bonnemain, they were in all probability anticipated by that engineer. The most remarkable improvement that has been made with hot water is perhaps, however, that of circulating it in hermetically sealed tubes (ï¾º 2144.), by which the water may be raised to a temperature considerably above the boiling point; and thus not only the heat is conveyed to as great a distance as it can be by steam, but much smaller pipes may be employed in heating. Thus, a pipe of water of 1 in. in diameter, outside measure, heated to the temperature of 280ï¾¦, will give out as much heat as one of 4 in. in diameter heated to 180ï¾¦. Hence the great economy of this mode of heating, besides other advantages. These are, the little attention that is required to keep the apparatus supplied with water; the certainty that, while the apparatus is in repair, no steam will escape from the joints; the more agreeable appearance of these small pipes than that of the large ones, which must necessarily be employed in circulating water not under compression; and the convenience of being able to introduce them in situations where there is not room for pipes of larger dimensions. Perhaps to these advantages may be added that of no boiler being requisite; the pipes forming a coil round the fireplace in such a manner that, while the heated water passes out at the top of the coil, the cold enters at the bottom, to be reheated in ascending to the top. The theory of the circulation of hot-water in open vessels will be found laid down in great detail by Mr. Tredgold in the Transactions of the London Hort. Soc. vol. vii. part iv. The power of imitating other climates and other seasons, Tredgold observes, than those which nature affords us, is known and valued as it ought to be; yet it remains difficult even to imagine the extent to which this power may be applied: in this age it produces luxuries, of which few can enjoy more than the commonest species; but in the next, nay, even in our own, there is a reasonable expectation of a considerable addition to the quantity and quality of these artificial productions, as well as to the vast sources of pleasure and information they afford to the admirers and the students of nature. The vehicle employed to convey and distribute heat in the new process is water; for it has been found that, in an arrangement of vessels connected by pipes, the whole of the water these vessels and pipes contain may be heated by applying heat to one of the vessels; and that in this manner a great extent of heating surface, and a large body of hot water to supply it, may be distributed so as to maintain an elevated and regular temperature in a house for plants, or, indeed, in any other place requiring heat. The obvious advantages of this method are, first, the mild and equal temperature it produces, for the hot surface cannot be hotter than boiling water; secondly, the power of heating such a body of water as will preserve the temperature of the house many hours without attention; and, thirdly, the freedom from smoke, or the other effluvia of smoke flues. In houses for plants, these advantages are most important.