The following memorandum respecting the heating of the hot-houses and pits at this place has been kindly supplied by Mr. Harrison. "The hot-water apparatus first used in the small house in the walled garden, 3 in fig. 159. in p. 652., was put up by a regular mechanic from London, but worked so ill and with such frequent failures, requiring night-watching, that soon after Mr. Pratt became head gardener the whole was taken to pieces, and the materials, with additions and alterations, and removing the boiler from the house into an adjoining place where the fire and flue were built, were reconstructed, and the house has completely answered ever since. "Since that time no person supplying hot-water apparatus has ever been consulted, or even employed, except in casting boilers and other iron work, according to plans and drawings made to a scale and sent to London; these materials being put together by a smith in the country, who has learned to cut and join pipes. Three fires heat eight distinct houses; and if the whole had been erected, or its erection contemplated, at the same time, two fires would have been amply sufficient, by placing the houses requiring most heat nearest the fires. One of the boilers is a common large iron coal box, which has now been in use seven years, without the slightest failure. Simplicity in construction, with large bodies of water and iron (the pipes in the largest house being 6 in. internal diameter, in the others 4 in., and going entirely round the pine and orchideous houses, requiring the greatest heat, and round three sides of the botanic house), has been the basis of all the plans; and experience derived from the house in the walled garden has led to the exclusion of all the boilers from the other houses, the delivering and return pipes being in every instance carried from the boilers through the wall, and close fitted; and experience has proved that the advantages of this mode more than compensate, in various ways, for the loss of heat which the boilers would give in the houses. "Experiments have also proved that the 6-inch pipes in the botanic house, which is considerably more than double the area of the pine-house which is heated from the same boiler by 4-inch pipes, give a greater heat than would be given by a double set of 4-inch pipes, making full allowance even for the excess of water beyond the double quantity carried by the 6-inch pipes above the 4-inch, calculated on the squares of their diameters. This proves that the same quantity of water is more effectual when distributed in large pipes, than in a greater number of smaller pipes. No deficiency has been found in the regular diffusion of heat, and great advantage in the continuance. The pipes through, the wall delivering and returning the water in the orchideous houses, which supply two sets of service pipes branching off at opposite right angles, were cast with the boiler, on the calculation of the squares of the diameters, the service pipes being 4 in., and the others nearly 6 in. "These arrangements have now had sufficient trial to prove their efficiency. Mr. Pratt can give and secure the continuance of whatever heat he considers necessary, without any attention to the fires during the night. It was necessary to look at the fires in the night during the use of a very highly praised boiler, which was obliged to be used in consequence of failure in procuring a large coalbox, and the founders not being able to send the present boiler in time. Since the present boiler has been fixed, neither Mr. Pratt nor any one under him has ever looked at a fire during the night; the temperature of the houses in the morning, after the severest night, proving the absolute security of the mode of supplying the heat under these houses.
