SIR,--Your favor of the 4th was received on my return to this place, and the proposition of your correspondent on the subject of fortification was referred to the Secretary at War, where office and qualifications make him the proper judge of it. I enclose you his answer. The same prudence which in private life would forbid our paying our own money for unexplained projects, forbids it in the dispensation of the public moneys. It is not enough that an individual and an unknown one says and even thinks he has made a discovery of the magnitude announced on this occasion. Not only explanation, but the actual experiment must be required before we can cease to doubt whether the inventor is not deceived by some false or imperfect view of his subject. Still your patriotic attention to bring such a proposition under our notice, that it might be applied to the public good, if susceptible of it, is praiseworthy, and I return you thanks for it with the assurances of my esteem and respect. TO CHRISTOPHER COLLES. WASHINGTON, June 19, 1808. SIR,--I thank you for the pamphlet containing your ideas on the subject of canals constructed of wood; but it is not in my power to give any definite opinion of its national importance. If there exists a cement which used as a lining for cisterns and aqueducts, renders them impermeable to water, (and it is affirmed that in France they are in the possession and use of such an one,) then it becomes the common question whether constructions of wood, brick, or rough stone are cheapest in the end? A question on which every man possesses materials for forming his judgment. I suspect it is the supposed necessity of using hewn stone in works of this kind which has had the greatest effect in discouraging their being undertaken. I tender you my salutations and respects. TO JAMES PEMBERTON. WASHINGTON, June 21, 1808.