HEATING. This, in connection with horticultural structures, is an absolute necessity for securing and regulating temperatures artificially, to suit the requirements of exotic plants; and for the production of flowers, fruits, and vegetables out of their natural season. Its effects may be derived from fermenting material placed inside the structure, or from causes which arise as the product of combustion by fire in the immediate vicinity, transmitted, by means of water or air, to wherever it is desired. These sources of heat, either used separately or in combination, afford the requisite temperatures for different plants, according as their admission is regulated to the various houses in which the latter are grown. Fermenting material evolves a considerable amount of heat, but by a slower process than combustion, as usually understood. A more genial and moist temperature may be secured from the former than from fire heat, but it cannot be so readily regulated. A fermenting mixture of litter and leaves greatly encourages the growth of young plants in spring, and is also preferable for starting early Vines and fruit-trees. It is advisable, in case of severe weather, to make provision for adding fire heat as well. Gentle hotbeds are also very useful for forcing vegetables, and for the raising of seeds generally. Heating by hot air is not adapted for horticultural purposes, on account of the consequent drying of the atmosphere being very injurious to plant life. Flues are but little better; still, means may be adopted for moistening the heated air transmitted by them, where it is impracticable with a continued influx of dry air. Both of these systems may, therefore, be dismissed in reference to all glass houses of modern construction, and one of the various methods of Heating by hot water should be, in all cases, adopted. Before proceeding to notice some of the most approved boilers for requirements on a large or small scale, it may be well to refer to the principles applied to Heating, as on these being properly understood and carried out in the construction of any hot-water apparatus, success or failure in its action materially depends. Heat always has a tendency to equalise itself, by communicating part of its properties to surrounding substances until they are raised to an equal temperature, so far as the original intensity admits. If generated by the combustion of fuel inside a boiler, heat may be conveyed, by water or air, to a considerable distance; the more remote it is, the less will be the amount that reaches the further extremity. Heated air or water becomes lighter than when cold, and naturally ascends in consequence. Either may be conducted in an upward incline, or in a perpendicular or horizontal direction, but not readily downwards, on account of the disposition of all heated substances to ascend. This transmission of heat in pipes containing water is usually termed circulation, and the arrangement of the pipes throughout, to allow an unimpeded circulation, is one of the main principles of Heating, but is not sufficiently recognised in many instances. The boiler must be placed below the level of any point the heat from it is intended to reach, the upper, or flow pipe, being connected on the top. The return pipe, by which the cold water enters, should be rendered free from the action of the fire by connecting it near the base-at the front preferably-and on both sides, if this is convenient. Dips in the pipes at any point should be specially avoided, as they frequently impede free circulation-generally more so when extra heat is applied. Houses erected for various purposes may have their quantity of pipes in proportion to the heat required, and still be in connection with the same mains conducting heat to others having much higher temperatures. There are no special rules applicable, in all cases, as to how many pipes a certain house will require, so much depending on stoking, and upon the amount of heat that may be available. It is best to provide for emergencies, in the first place, by insuring a sufficiency of piping, and inserting valves in the flow and return pipes, for regulating the admission of heat. In the arrangement of a Heating apparatus, an important part should be taken by the gardener in charge, as, although the workmen employed may understand the principles on which the success of their work depends, they do not similarly understand the requirements of plants. A proper system must be adopted where there are several houses to be heated and kept at different temperatures, by one or more boilers set and connected together. Main flow and return pipes should be fixed, with a gradual rise, at a point below all others in connection, and near the central part of the distance the heat is intended to reach, so that branches may be taken on either side. All houses or pits intended for Heating separately, and irrespective of the one adjoining, should be provided with check valves near the junction with the main pipes. As heat always rises most rapidly to the highest points, it should be arranged that these are in the houses required at the highest temperatures. Pipes 4in. in diameter are those most largely used for top heat; others, only 3in., are well adapted for beds or for small houses. In houses specially devoted to plants requiring a somewhat dry atmosphere in winter-Pelargoniums, for instance-an extra 2in. pipe is sometimes fixed along the lower part of the rafters for drying the air, this being generally attended with excellent results in the production of large, clean flowers, free from damp.