The explanation, also drawn up by Mr. Lenthall, that accompanies the drawing gives all requisite specifications for the construction, with the understanding, however, that the design is to be regarded as a basis on which modifications, probably slight in general, may be molded, according to modes of construction determined by experience to be best for the Western waters; all being fixed clearly and explicitly in the specifications of the contract.

It is for the Government to determine how many boats of this kind will be needed and when. Ten may be regarded as a minimun number, twenty as a maximum. There may be margin enough, as to time, between this and the - for this point to be settled, without delaying general results.

A certain mofe of causing this particular preparation to be made in due time and proper manner will be to send a naval officer of some rank and experience, say a commander, assisted by a lieutenant or two, to the Ohio, with full authority to the former to make all necessary contracts, distributing the work along the river as may be found best regarding timely execution and due competition; this officer having also authority, on consultation with boat and engine builders on that river and also with Naval Constructor Samuel M. Pook, who is understood now to be on naval duty there, to make any necessary modifications of the drawing and descriptions prepared by Mr. Lenthall. This and other preliminary arrangements not to be avoided make it indispensable that such authority be given and such officer be dispatched fortwith.

The cost of each boat complete may be taken at $20,000; making the cost for the $200,000, and for twenty $400,000.

These contracts may be called for by advertisement; but it is to be considered whether such a public announcement of this kind of preparation will not be exposing designs injuriously and unnecessarily, at least as to time and extent and particulars.

As it is understood that these boats are each to be put under the commnad of a naval officer, the request of the commanding general might refer to the advantage of a detail of these at an early day. Placing them severally in charge thus early will be perhaps the very best means of securing a timely and complete outfit, as well as a faithful execution of contract.

I hand in which this memorandum a table, giving all the landing places for steamers between Cincinnati and Cairo and also between Cairo and New Orleans, derived from Captain J. S. Neal, of Madison, Ind., who has been many years navigating those waters in command of a steamer. I have no doubt of the accuracy of the list according to river distances.

It may be safely assumed, I suppose, that troo at every one of these places, although at some - perhaps a good many on the portion of the Mississippi next below Cairo - there may be a pinched space and steep banks. Troops can nevertheless get up. On the other hand, it is known that, for long ranges of river, landings are not restricted to the points mentioned in the list, but are easy and good everywhere.

Every gun - boat and passenger steamer must carry a good river pilot. In addition, each commander of a considerable division of force should be attended by a person who, besides being a first - rate pilot, should be a practical engineer and possess a good general knowledge of the resources of the bordering country. Such a person I believe Captain Neal to be. He will be found at Madison, Ind., where he and his partners have a large building establishment, and will be likely to engage in building some of the gun - boats.

JOS. G. TOTTEN,

Brevet Brigadier-General and Colonel of Engineers.