The Fire Marshal of this city, in his semiannual report, just published, presents a considerable amount of information of very general interest which deserves special notice, on account of the facts and fallacies set forth. He condemns the use of hot air furnaces, now BO common, because of their dangerous character in respect to fires, but more so on sanitary considerations. "When imperfectly constructed and arranged, tlioy are frequently the cause of fires; yet it is admitted they may be so built as to avoid this danger, but they are always prejudicial to health. It is stated that in the public schools in which they are employed, they produce injury to the health of the scholars, the teachers having to allow numbers of them to go home frequently before the hour of dismissal, on account of severe headaches. For this reason they aro condemned, and the abjuration of their use recommended, and as a substitute for them, regarding both health and safety, the heating with hot water by pipes is advocated. We know that tha abuse of hot air furnaces in heating apartments is the frequent cause of fires, of nervous fevers, and lung diseases. In very cold weather the plates of these furnaces are generally heated red hot, and as a consequence the air which comes in contact with them is decomposed, and rendered unfit to be inhaled. But will the use of hot water pipes, distributed through a schoolroom or any Other apartment, remedy the evil of headaches complained of in the Fire Marshal's report, without the use of other agencies ? We are confident they will not. Ho has overlooked the main cause of the health evils in schoolrooms, namely, the absence of arrangements for proper ventilation. Unless means are employed for a constant supply of pure fresh air to rooms heated by the hot water pipes, it is evident this system must be move hurtful to the health of children in overcrowded schools than the present hot air furnaces. These latter do take in a eonstant stream of fresh air, and throw it into the rooms, and if some of it is deteriorated in passing over too highly heated surfaces, yet a portion of pure warm air is also upplied, and thus the foul air has not to be ruminated by the lungs as in rooms heated by stoves, and steam, and water pipes unprovided with the means of furnishing fresh air. The correct method of heating rooms is to throw a eonstant stream of fresh warm air into them. By keeping hot air furnaces at a moderate temperature, so as not to burn the air, they afford the means of properly heating and partially ventilating rooms, but they are too liable of abuse in being easily overheated. Hot water is undoubtedly the most safe and pleasant means of heating air for rooms, but it must be so employed as to meet the conditions requisite for health, by sending a constant supply of warm fresh air into the apartments to be heated. On page 51, Vol. XI, SCIENTIFIC AMISKI-CAN, there is an illustrated description of a hot water heating furnace, which appears to meet all the conditions necessary for heating rooms, both us it relates to safety and health; and were it combined (as it no doubt can be) with means for removing the foul air, it would, in our opinion, be a very perfect system. As many deplorable accidents have occurred in public schools from defective hot air furnaces, the attention of the Boards of Education is specially invited to this subject. It is fraught with consequences of the highest importance, and deserves ear!" rind riend inves-, tigation, as most of the public schools seem k to have been erected and arranged in violation ) of the plainest rules for heating and ventilat-S ing them properly.