YORK, July 29, 1812.

I have the honor to transmit herewith a dispatch this instant received from Captain Roberts, announcing the surrender by capitulation, on the 17th instant, of Fort Michilimakinack.

The conduct of this officer since his appointment to the command of that distant post, has been distinguished by much zeal and judgment, and his recent eminent display of those qualities your excellency will find has been attended with the most happy effect.

The militia stationed here volunteered this morning their services to any part of the province without the least hesitation. I have selected 100, whom I have directed to proceed without delay to Long Point, where I purpose collecting a force for the relief of Amherstburg. This example, I hope, will be followed by as many as may be required. By the militia law, a mail refusing to march may be fined £5, or confined three months; and although I have assembled the legislature for the express purpose of amending the act, I much fear nothing material will be done. Your excellency will scarcely believe, that this infatuated house of assembly have refused, by a majority of two, to suspend for a limited time the habeas corpus.

The capture of Michilimakinack may produce great changes to the westward. The actual invasion of the province justifies every act of hostility on the American territory.

It was not till this morning that I was honored with your excellency's dispatches, dated the 7th and 10th instant. Their contents, I beg to assure your excellency, have relieved my mind considerably. I doubt whether General Hull had instructions to cross to this side of the river; I rather suspect he was compelled by a want of provisions. I embark immediately in the Prince Regent for Fort George. I return here the day after to-morrow, and shall probably dissolve the legislature.

Sir George Prevost to Major-General Brock.

QUEBEC, July 31, 1812.

I have received your letter of the 20th instant, accompanied by the copy of two letters from Lieut.-Colonel St. George, who is in command at Amherstburg, and some interesting documents found on board a schooner, which had been taken by the boats of the Hunter.

In consequence of your having desired Colonel Proctor to proceed to Amherstburg, and of your presence being necessary at the seat of government to meet the legislature of Upper Canada, I have taken upon myself to place Major-General Sheaffe on the staff, to enable me to send him to assist you in the arduous task you have to perform, in the able execution of which I have great confidence. He has been accordingly directed to proceed without delay to Upper Canada, there to place himself under your command.