The first want of every living being is fresh air, and unless the human lungs are supplied with such air constantly at the rate of from ten to thirty cubic feet every minute, by night as well as by day, perfect health and vigor cannot be preserved. Then, too, there are exhaled from the surface of the body and from the lungs, such quantities of waste organic matter, which tend to immediate putridity, that it, together with the carbonic acid, would keep the human body immersed in a deadly vapor of these exhalations, were not fresh air supplied. The illustrations by which you have made these truths easily understood, are admirably given in your lectures, and the method, by which you would best insure success in removing the foul and supplying the pure fresh air in every place where persons live or sleep, are, as I believe, from my own careful studies of this subject, most correct and trustworthy. Indeed, I am able to say that, in my examinations of the vast number of hospitals and buildings which you ventilated during the late war, under authority from the intelligent and humane Quartermaster-General of the army, the proof of entire success in your work was everywhere witnessed. Simplicity, invariable certainty and a liberal sufficiency characterizes these admirable methods of yours. I wish every family in the land had a copy of these lectures. Sincerely yours, ELISHA HARRIS, M. D., Corresponding Secretary Metropolitan Board of Health. To LEWIS W. LEEDS, Esq. * * * * * VAUX, WITHERS & Co., Architects, No. 110 Broadway, New York, August 27th, 1867. Dear Mr. Leeds. I am glad to receive your Lectures in printed form, and trust that they may be widely read throughout the community. Having been in the habit for several years past, of consulting with you professionally in regard to the arrangements to be made for heating and ventilation in plans for public and private buildings, I take this opportunity to acknowledge the value of the aid thus given; and as I feel assured, from a lengthened personal experience, that your thorough knowledge of the subject, both theoretically and practically, is calculated to render your assistance particularly valuable in the adjustment of complex and intricate plans, I trust that one result of the circulation of your interesting pamphlet may be to introduce you more widely to members of the architectural profession. I remain, Dear Mr. Leeds, Yours faithfully, CALVERT VAUX. LEWIS W. LEEDS, Heating and Ventilating Engineer. * * * * *