The topic of ventilation has been discussed and re-discussed, and a library might be collected of books and lectures and reports of learned societies upon the subject; yet churches, theaters, school houses, and private houses, judging from universal complaints, still remain unventilated. We hear, indeed, from time to time, of success in the use of apparatus for rentilating capital buildings, or parliament houses, but when circumstances compel us (as they do occasionally) to visit some such place, we find but little to praise in this respect. We however moderate our disappointment when we reflect how very difficult it must be to keep a pure atmosphere in these places. For the most part buildings in which people congregate are ventilated about as well as a certain horse car, the unwonted brilliancy of whose lamps elicited some inquiry on the part of a curious passenger. The phenomenon was explained by the scientific conductor's pointing out some flues admitting fresh air from the outside to the small cells inclosing the lamps, in order, as he learnedly stated, that the foul air from the lungs of the passengers might not totally extinguish them. The amount of learning displayed in discourses on ventilation would make even our scientific conductor open his eyes. Few indeed who have not given a lifetime to the study of this important subject, can be aware of the intimate relations existing between the geological periods and the scientific mode of getting bad air out of a room and replacing it with pure air. It would be still more preposterous to suppose that ordinary practical minds could be able to grasp this subject without being first well grounded in the cosmical theories of La Place, and the "Principia" of Newton. In short the subject embraces, if the harangues and discourses of Professor this or Doctor that are any index to its magnitude all the knowledge as yet attained by mankind, with a very large proportion of what is yet unattained. An English journal states that Dr. Edward Smith, F. R. S., read a paper on ventilation before the Society of Arts, on the evening of the 24th of February, in which he treated the subject comprehensively without recommending any particular plan. Treating the subject comprehensively is the mode. Of what use is it to descend to particulars when so much science can be displayed in generalities ? Of what use is it to teach others if we fail to show that we ourselves are learned ? The fact is that the science of ventilation is small, the art is easy, and the learned discourses which have lately dragged their tedious lengths along in the Journal of the Franklin Institute, and have burdened the pages of many other scientific journals, as well as the patience of their readers, are called for no more than learned discussions upon the problem how to avoid cutting two holes in a back door to let out two cats, one being a large one and the other a small one. We have many times urged the supreme folly of treating the subject in the ridiculous manner described, and have given rules, the simple observance of which will insure well ventilated apartments. To apply thesa rules requires common sense mechanical skill in construction, and arithmetic ; "only these ana* nothing more/1 It seemsj however, timt upon this as upon many other subjects upon which, we write frequently, we must repeat our lessons often. There is no subject upon Which we receive so many inquiries. First, then, the fundamental law upon which ventilation is Ibased is, that liot air rises and cold air descends. It follows if the pure air admitted to a room b heated by a furnace, the impure air which is cooler will settle to the bottom of the apartment, at which the registers for its escape ought to be placed. If the room be heated by radiation, as with steam apparatus, stoves, etc., and the pure air be admitted cold, the registers should be at the top of the room. Second, good ventilation can not be secured by using long flues, unless mechanical appliances, as fans, etc., or apparatus for heating them are employed. The air gets cold before it passes through them, and consequently ceases to rise, or rises but slowly. The best thing for this purpose is an open grate at the bottom of the room having for its chimney the flue through which the foul air is desired to pass. Third, strong winds over the unprotected external mouths of flues, are apt to reverse or obstruct currents. The mouth of every flue should be covered with a hood so adjusted that it can rotate with the wind. The. winds blowing from any quarter will thus aid rather than impede the egress of air from them. Fourth, they, as well well as the flues for the admission of pure air, should be made of a size proportionate to the requirements of each particular case. Here the arithmetic comes in, and the data are as follows: * The number of respirations in a healthy adult per minute, is from 14 to 18. The average amount of air taken into the lungs at each respiration is about twenty inches. From this air the oxygen is removed, and its plaoe supplied with carbonic acid at the mean rate of '0485. From these figures it is easy to calculate the rate at which fresh air must be admitted to supply the demand or (as admission of fresh air implies in any proper system of ventilation the removal of foul air) the rate at which the foul air ought to be removed. The size of the escape flues ought to be proportioned to the size of the room, and the number of people it is intended to contain, which can be easily done by any competent architect. To those who are not competent we say, err if you must on the saf sidej make the hole large enough for the adult cat and the kitten will also b e accommodated. Of course if a building is not constructed so as to admit of proper ventilation, it will be1 impossible to ventilate it properly, a statement so logical that even Dr. Edward Smith, F. R. S., will not dispute it; lifthj the admission of pure air should be so adjusted when the air ib not previously heated that all sharp drafts shall be avoided-. This can easily be done by causing it to enter through Wire gauze, breaking the currents by screens, etc., in the application of which means, common sense is of much more value than large scholastic acquirements. Thus ends our discourse upon ventilation, which if not so learned, will, we are confident, do more good than that of Dr. Edward Smith, F. R. S., before the Society of Arts, above mentioned.