I think it has been said that the Mississippi, at low water, is many feet lower opposite New Orleans than Lake Pontchartrain. But the fact is impossible, being in contradiction to the laws of nature; two beds of dead water connected with the same ocean, in vicinity to one another, must each be in the level of that ocean, and consequently of one another. Although Pontchartrain receives the Amite and some other small streams, they probably do little more than supply its evaporation. No doubt, however, that the lake must receive the small ebb and flow of the sea. The Mississippi, on the contrary, even at its lowest tide, always flows downwards to and beyond its mouth; it must, then, at New Orleans, be one, two, or three feet higher than the sea, and consequently than Pontchartrain. If a simple canal were cut from that of Carondelet to the Mississippi without lock or gate, there would be two risks. 1. That in high water of the Mississippi the current would be too strong for a gun-boat to ascend or descend. This might perhaps be remedied by the draught of horses. 2. The force of such a current, (unless the whole canal were lined with brick or masonry,) might convert the canal into a bay, one of an unknown size, and involve New Orleans in it. On the whole, I suspect our plan is pretty obvious: suppose we want six feet water; make a canal of that depth below the lowest ebb of Pontchartrain from the lake to where the lock is to be placed,--then bring a canal from the river to the lock, the depth of which shall be six feet below the lowest water of the Mississippi ever known; at the back there will be a descent, suppose of one, two or three feet, or any other number. The lock remedies that. If the lock were near the lake it would lessen the work by giving nearly the whole length to the shallowest canal, and it would probably be in a more tranquil and safe situation. But it might be inconvenient, perhaps unsafe, to the sides of the Mississippi canal, to permit such a depth of water as would be in it, through its whole length, at the time of the high water of that river. Of the best position, therefore, of the lock, the superintendent must judge on the spot, as he must indeed of the correctness of all the preceding conjectures, formed without a knowledge of the localities. They are hazarded merely to give us some fixed notions of the nature of the enterprize, and are submitted to your consideration. I salute you with affectionate respect. TO THE SECRETARY OF THE TREASURY. MONTICELLO, May 15, 1808.