2125. One of the most economical modes of applying steam to the heating of hothouses is to apply it to a bed or mass of loose stones. This mode appears to have been first adopted by Mr. Hay, of Edinburgh, in 1807, and has been subsequently applied by the same eminent garden architect to a number of pine and melon pits in different parts of Scotland. It was also adopted in England, and on a very extensive scale, in connection with heating pipes and cisterns of water, at the nursery of Miller and Co. at Bristol. Nothing can be more simple than this mode of applying steam. The bed of stones to be heated may be about the usual thickness of a bed of tan or dung; the stones may be from 3 to 6 inches in diameter, hard round pebbles being preferred, as less liable to crumble by moisture, and having larger vacuities between. The pipe for the steam is introduced at one end of the bottom of this bed, and is continued to the opposite end. It is uniformly pierced with holes along the two sides, so as to admit of the equal distribution of the steam through the mass of stones. The steam-pipe may be of any dimension, it being found that the only difference between a large pipe and a small one is, that the steam proceeds from the latter with greater rapidity. The steam only requires to be introduced once in twenty-four hours in the most severe weather, and in mild weather, once in two or three days is found sufficient. After the steam is turned on, it is kept in that state till it has ceased to condense among the stones, and, consequently, has heated them to its own temperature. This is known by the steam escaping, either through the soil over the stones, or through the sides of the pit; or when a mass of stones is enclosed in a case of masonry, as in the stone flues of the Bristol nursery, the point of saturation is known by the safety valve of the boiler being raised. When we consider the small-sized pipes that may be used for conveying and delivering steam by this mode of its application, there can be no doubt that this is the cheapest mode of heating on a large scale known; and it has also the advantage of never requiring to be applied oftener than once in the twenty-four hours, and thus rendering all night-work unnecessary.
