It may be regarded as an established and universal rule, that all plants require the soil, and the atmosphere in which they are cultivated, to correspond with the natural circumstances under which they flourish; and as it has been repeatedly ascertained that the soil is naturally a degree or two above the temperature of the atmosphere, we have certain and unerring data for the application of bottom heat, and no more powerful evidence than this can be desired, to condemn at once the application of a _very powerful degree of heat_, at the roots of plants. The importance of bottom heat in the culture of tender plants, being a practical fact established beyond question, another consideration arises as to the best means of producing it, and of regulating its application. Various substances and materials have been submitted to a process of fermentation, and so employed to effect it: stable manure, tanner's bark, and the leaves of trees, are among the principal of these materials, and either of them will supply just what the plants require, as truly as these wants can be supplied by any other means; but from their very nature, they are violent, and fluctuating, and ephemeral in their action, and setting aside the labour which the employment of them necessarily involves, we have in these particulars, the special points in which the tank system of applying bottom heat far excels them: it is uniform, and constant, in its action; there need be no apprehension of the soil becoming overheated, for the source whence it derives its warmth ought never to boil; neither need there be any fear of its decline, or of a want of power, for when once thoroughly heated, a body of water will part with it in such a manner, that a very little attention to the fire, and a very little expenditure of fuel, will maintain its temperature for an almost incredible length of time; and as to power, it never should for a moment form a question, because a powerful degree of bottom heat ought never to be applied: a close attention for one or two hours during the twenty four which form a day, will maintain any apparatus in an effective state of action, if it is properly erected. How different is this, to what has been in days now past! when in rigorous weather, with the heat of his dung bed declining, the cultivator knew that at the peril of his crop, he scarcely dared to attempt to revive it, without involving a more serious because an accelerated evil; at any rate, if at an immense sacrifice of labour, his dung casings were replenished piece by piece, he knew too well, that often many days would elapse, before their action would be efficient and satisfactory, unless indeed an unlimited supply of materials, were in a constant state of preparation. By means of the tank, a fire could be lighted up, and the required effect produced in as many hours, as days would have been formerly required.