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Plates, wrought iron, cast iron, and copper tubes of various dimensions are employed to retain the heated water and to constitute a radiating surface. I believe that tubes of cast iron from 3 to 7 inches in diameter, as at once cheap, presenting a good surface, easily connected and durable, are the best modes of using hot water for heating, and that they are best distributed in large plank or brick flues in the cellar, which may admit the external air in large openings or in smaller holes, if the djffused method is judged more eligible.

The practical objection to heating by this form of apparatus, as compared with steam, is the slow manner in which the heating up must be effected, almost necessarily involving the necessity of a fire being maintained during the night.

3d. The modes of distributing steam to obtain its heating power, are essentially those of hot water. As its temperature is higher than hot water can be maintained in any apparatus, less radiating surface is requisite. The extent of tubular surface could not probably be determined by any previous calculations as it would be materially influenced by the quantity of air that would be drawn from any particular gallery to keep a perfectly pure atmosphere. If the generating boiler was of due size, the repeated additions of separate ranges of iron pipe within the air flue would enable the maximum supply to be experimentally reached, and as these ranges would be connected only by receiving at one extremity the branches of a common steam pipe, the admission of steam into more or less of them could be regulated in accordance with the temperature, and the ventilation required.

My own convictions are altogether in favor of steam managed in this manner, as a means of warming an insane asylum. Generated and applied solely to this end, it might not be so economical as the hot air furnace, but as forming one of the uses of a boiler designed also to furnish steam, for lifting water, cooking, washing, drying clothes, as I shall explain hereafter, I am inclined to believe it would be found not objectionable even in point of economy. Regarding the much higher hygienic quality of the air thus produced, I confess I regard some such mode as indispensible.

I have referred to the mode of ventilation common, indeed the only one used as far as I know in our institutions. Flues of a size inadequately small, being usually some 8 by 4 inches, carried up in a 12 inch brick wall, proceed from the bottom or top, or both of the apartments to the attic, where they open and foul air finds its escape by some form of cap or cowl. In a few instances and certainly with the utmost advantage another flue descends from near the floor to the cellar, to admit air to replace that which diminished gravity from its expansion has induced to escape. Generally, however, the only air which can supply the place of that which escapes (and none can escape unless its place is thus filled,) must enter at cracks and accidental openings.

When a large body of heated air is admitted and the wind is favorable, there are periods when these flues draw with sufficient activity, but generally their performance is very uncertain and inadequate, especially at those seasons when their use is most essential, as in the weather of our late spring and early fall, when a fire is not needed, nor is the openings of windows admissible. The internal heat of the building as compared with the external, the state of the currents of air and various other circumstances, in fact all the curious causes, apparent and latent. which affect the draft of smoke flues, produce an endless variety of cross currents, counter drafts, regurgitations from the attic, which defy all remedy. Any valvular arrangements to meet these difficulties virtually effect a stoppage of the flues in calm weather when no great amount of heated air is admitted. In the same manner, any upward or extractive force expected from cowls or caps turning to leeward will be dependent entirely on the force of the wind. When this is trifling, the obstruction from any apparatus of this kind, producing at least one right angle and more or less friction in the ascending current will actually obstruct the object.

Again, in a day room or single sleeping room, the little elevation of temperature resulting from animal heat, will not induce an upward ascent in a cold and rough flue, which instantly reduces the ascending air to a lower level than the room from which it ascended. Indeed any one who has attended carefully to the operation of these flues will readily admit that they draw any way and no way, beyond the reach of explanation. I believe that no institution can be found dependant on the natural ascent of air for ventilation, in which a pure atmosphere can be relied upon, or even generally experienced.

There is also this important objection to a natural ventilation in our hot, but uncertain, summer nights, that no sufficient change of atmosphere can be effected except through an open window. Enough air will not be carried upward through the flues to meet the want. The animal heat will not raise hot air enough to induce its ascent in contact with cool bricks.