[Inclosure.]

Ohio and Mississippi Rivers, transportation steamers, gun-boats, distances, &c.

[Memoranda for the use of Lieutenant-General Scott.]

JUNE 3, 1861.

After several conversations with intelligent and reliable persons on the subjects embraced in the following memoranda, I have the honor to submit the same as containing information of importance:

Passenger steamers.-Most of them now unemployed; 250 steamers on the Ohio, rather more than less, that may carry an everage of 300 men - 75,000 men; 150 steamers at Saint Louis, and thereabouts, that may carry as many each, 45,000 men - 120,000.

Freight barges on the Ohio, 100 in number, that will carry 2,500 barrels each on the average - 250,000 barrels. At and about Saint Louis as many besides.

Coal barges on the Ohio, 200 in number, will carry an average load of coal of 10,000 bushels - 2,000,000 bushels. At and about Saint Louis there are as many others.

One barge will carry coal enough for the largest steamer all the way to New Orleans and more than enough (two barges to three large steamers).

A steam-tug (of which there are many) would take down in two barges enough to supply a great many steamers.

Each passenger staemer would herself carry a large quartity of coal in addition to troops, their baggage, and provisions.

It is an important question whether the "June rise" may not advantageously be availed of to send a good - supply of coal to Cairo. The nearest considerable coal supply above Cairo is at Caseyville, about 120 miles up the Ohio.

Pittsburg coal is the best. Pomeroy coal nearly if not quite as good. (Pomeroy is half way between Pittsburg and Cincinnati.)

Draft of water in the river.-Four feet draft, with some certainty after middle of October; five feet draft 1st of November; six feet draft, with great certainty after 15th of November. This, in ordinary seasons, a very dry summer and dry early autumn, will give less water in October, but the middle of November will very surely give from five to six feet draft.

Building-yards for boats.-The principal boat - building places on the Ohio River are: (1) Pittsburg, Pa.; (2) Wheeling, Va.; (3) Cincinnati, Ohio; (4) Madison, Ind.; (5) New Albany, Ind.; (6) Mound City, Ill.

Steam-engine building.-At the first five places named above all kinds of engines are supplied and all other fittings. Steam gun -boats may be perfectly fitted to receive armament, provisions, and men at each of those places.

Time.-Three months will suffice to get the boats ready (including engines and all else) from the time of closing contract, if the matter be pressed; but this assumes great energy on the part of the agents of the Government and the contractors.

Gun-boats.-There is atteched to this paper a draught made by John Lenthall, esq., Chief of the Bureau of Construction, Equipment, and Repairs of the Navy Department, of a gun-boat deemed to be well adapted to operations on the Ohio and Mississippi River. Such a boat, besides crew, provisions, ammunition, &c., will carry about four 8-inch guns or other armament of about equal weight. The boat is planned to draw not over five feet when full and to have the requisite speed.