The Isthmus of Panama.

--We subjoin a copy of the letter from Baron Humboldt to M. Salomon, read by M. Guizot:--"August, 1842.

Sir--I learn with regret that you are not more advanced in your interesting enterprise than you were when I last had the pleasure of seeing you at Paris.

It is twentyfive years since a project for a communication between the two oceans, either by the Isthmus of Panama, by the Lake Nicaragua, or by the Isthmus of Capica, has been proposed and topographically discussed, and yet nothing has been yet commenced.

I had believed that the English embassy would have found the means of inspiring confidence, by sending an engineer to survey the valley which separates the two seas, and across which a canal might be cut.

Be assured that the parties who have made use of my name in saying that the two seas have not the same level, have done so for the purpose of an excuse for not entering on the undertaking."

We likewise subjoin the extract from the document addressed to the Academy of Sciences on the subject, by an American gentleman, Mr. Warden:--"The cutting necessary to unite the two seas by means of the three rivers, Vinotinto, Bernardino, and Farren, is only 121/2 miles in length.

The canal in all would be 29 miles in length.

It might be made navigable for vessels of from 1,000 to 1,200 tons burden, and the water may be kept at its proper level by means of only two locks.

All the materials necessary for the construction of the canal are found in abundance on the spot.

The total expense is estimated at 2,778,615 dollars (14,821,800f.), including the expense of four steam boats and two iron bridges to open for the passage of ships."