DEAR SIR,--Your favor of the 6th has been received, and I will beg leave to add a few supplementary observations on the subject of my former letter. I am not a judge of the best forms which may be given to the gunboat; and indeed I suppose they should be of various forms, suited to the various circumstances to which they would be applied. Among these, no doubt, Commodore Barney's would find their place. While the largest and more expensive are fitted for moving from one seaport to another, coast-wise, to aid in a particular emergency, those of smaller draught and expense suit shallower waters; and of these shallow and cheap forms must be those for Lynhaven river. Commodore Preble, in his lifetime, undertook to build such in the best manner for two or three thousand dollars. Colonel Monroe, to whose knowledge of the face of the country I had referred, approves, in a letter to me, of such a plan of defence as was suggested, adding to it a fort on the middle grounds; but thinks the work too great to be executed during a war. Such a fort, certainly, could not be built during a war, in the face of an enemy. Its practicability at any time has been doubted, and although a good auxiliary, is not a necessary member of this scheme of defence. But the canal of retreat is really a small work, of a few months' execution; the laborers would be protected by the military guard on the spot, and many of these would assist in the execution, for fatigue, rations, and pay. The exact magnitude of the work I would not affirm, nor do I think we should trust for it to Tatham's survey: still less would I call in Latrobe, who would immediately contemplate a canal of Languedoc. I would sooner trust such a man as Thomas Monroe to take the level, measure the distances, and estimate the expense. And if the plan were all matured the ensuing winter, and laborers engaged at the proper season, it might be executed in time to mitigate the blockade of the next summer. On recurring to an actual survey of that part of the country, made in the beginning of the Revolutionary war, under the orders of the Governor and Council, by Mr. Andrews I think, a copy of which I took with great care, instead of the half a dozen miles I had conjectured in my former letter, the canal would seem to be of not half that length. I send you a copy of that part of the map, which may be useful to you on other occasions, and is more to be depended on for minutia, probably, than any other existing. I have marked on that the conjectured route of the canal, to wit, from the bridge on Lynhaven river to King's landing, on the eastern branch. The exact draught of water into Lynhaven river you have in the Navy office. I think it is over four feet.