In the forcing department of gardening, wonders have been accomplished. By this application of art, we appropriate to ourselves an almost perfect imitation of any of the warmer [p273] climates: heat, that powerful agent in the development of vegetation, we can have in any degree, by stoves, by fermenting substances, and from the steam of boiling water: light, a no less necessary agent in the maturation of fruits, we combine with the former, by glazed houses and frames. The various expedients for obtaining the necessary degree of heat, are, first, the most simple method of a stove, with its flue passing through or round the floor of the house, and this for warming the air within; but in this case, as the roots of the plants do not sufficiently, it is supposed, receive the proper degree of heat, various fermenting substances, as recent stable-yard dung, tanners’ bark, oak and other leaves of trees, &c. are formed into beds, on which the compost of earth is placed, as in hot-bed frames, or in which the pots containing the plants are placed, or plunged, as in a hot-house. To obtain the same effect, borders within houses are formed for the roots, having an excavated heat-chamber beneath, supplied by simple stove flues, or from the fermenting substances above named, or from steam admitted for the purpose. This mode of supplying an equal degree of heat to the roots, as well as to the leaves and branches of a plant, is plausible, and cannot be far wrong, because it has been attended with success: but there is, perhaps, more attributed to it than it deserves, because the region or stratum of the soil, which is naturally occupied by roots, differs, in respect of temperature, much less over the whole surface of our globe, than is commonly imagined. The heat of the air in different latitudes ranges from several degrees below zero to 110 degrees of Fahrenheit’s scale; but the temperature of the earth eighteen inches below the surface, it is probable, does not vary more than ten or fifteen. In England spring water varies only two degrees, viz. from 42 degrees in summer to 40 degrees in winter; and the effects of our hardest frosts very rarely penetrate deeper than nine inches; but it is necessary to observe, that, in such cases, as well as in hotbeds, we _force_ as well as defend; and probably, by such mode of applying heat and moisture, nutritious gases may be communicated, which may be no small advantage. Besides, the atmosphere of the house can (as is done) be impregnated with the same qualities and degrees of heat and humidity [p274] (a most necessary accompaniment[51],) which may be generated below.