1869.] Descriptions. 567 of the fans in the Capitol at Washing- ton, probably the most perfect in con- struction of any in this country, if not in the world. According to the report of the Engineers, the}' burn in summer, for running the fans for ventilation, 8,000 pounds of coal in eight hours. This supplies the two engines, one of 30-horse power to drive the fan for the House of Representatives, and one of 16-horse power, for the fan of the Com- mittee rooms, &c. I have never seen the record of any accurate experiments giving the actual amount of air delivered ; and the amount, as guessed at by different parties, varies greatly. Some give it as high as 60,000 cubic feet per minute, for the large fan, and 30,000 for the smaller one. But the Engineers saj', that 40,000 for the larger one, and 25,000 for the smaller, is quite as much as they ever deliver, per minute, in practice. This, for the eight hours the}' are running, gives 31,000,000 cubic feet, or a fraction over 10,000 cubic feet of ventilation for every pound of coal burned, instead of the 30,000 delivered, where the heat is applied directly to the shaft. Now this saving of the two-thirds of the coal, is but one of the very small items in favor of the heated shaft, as compared with the fan. The fan requires the constant employment of a compe- tent engineer, in addition to the fireman demanded in both cases. We must consider, as above stated, that there are scarcely six daj's, in the ! whole year, during which, such a stagna- ■ tion occurs, as would require the whole j power of the flue or fan. On the other 360 days there is sufficient difference in the motion of the external currents, to produce the required ventilation ; or the difference of temperature, within the building and out, would accomplish the same object. On all of these 360 days the shaft is doing its work quietly and efficiently, night and day, without any trouble to, or assistance from, any body or thing. But the fan requires the constant attention of the engineer. Although it ma}' be possible, so to adjust the inlets and outlets of the moving air, and the velocity of the fan, as to avoid unpleasant currents, and not to create an undue pressure ; yet, in practice, this is found to be so difficult, that it is seldom accomplished. If the outlets, from an}' cause, become partially closed, the power of the fan is such, that it drives in the air, until it becomes compressed, like the air of a diving-bell, inevitably causing headache. I think the gentle motion given to the air, by the few degrees difference in temperature, very much pleasanter ; and not liable to any such abuses. An effect will be produced, however, with just as much certainty, in propor- tion to the power used. Taking all things into consideration, I think the heated shaft is at least ten times as valuable, for the purpose of ventilation, as the fan driven by the steam-engine. Attemp;s to use the heated shaft have been frequently made by persons, who knew nothing about the principle of ven- tilation, and, of course, have very com- monly failed to produce the effect con- templated. But, because an ignorant man burns his house up, by the im- proper use of fire, it is scarcely probable that we shall all discard the use of fire in our houses. There are several important errors frequently made, in the application of the heat to the shaft, and in the general application of the shaft itself, to the ventilation of the building. I think, in the shaft, or towers, by which the Houses of Parliament are ventilated, a large proportion of the foul air is caused to pass through the fire, at the base of the tower. This requires an extravagant use of fuel. It should be burned in some furnace, or enclosure, of fire-brick, or other non-conducting substance, with a slow draught, so as to secure, first, thorough combustion, after which, it should be allowed to heat the air in the shaft, to a small degree, be- cause while an additional temperatu -e
