ISAAC BROCK, Major-Gen, and President. Head Quarters, Fort George, July 22, 1812. By order of his honor the president. J.B. GLEGG, Captain and Aide-de-Camp. _Major-General Brock to Sir George Prevost_. FORT GEORGE, July 20, 1812. My last to your excellency was dated the 12th instant, since which nothing extraordinary has occurred on this communication. The enemy has evidently diminished his force, and appears to have no intention of making an immediate attack. I have herewith the honor of enclosing the copy of two letters which I have received from Lieut.-Colonel St. George, together with some interesting documents found on board a schooner, which the boats of the Hunter captured on her voyage from the Miami to Detroit. From the accompanying official correspondence between General Hull and the secretary at war, it appears that the collected force which has arrived at Detroit amounts to about 2,000 men. I have requested Colonel Proctor to proceed to Amherstburg, and ascertain accurately the state of things in that quarter. I had every inclination to go there myself, but the meeting of the legislature on the 27th instant renders it impossible. I receive this moment a dispatch dated the 15th instant, from Lieut.-Colonel St. George, giving an account of the enemy having landed on the 12th and immediately after occupied the village of Sandwich. It is strange that three days should be allowed to elapse before sending to acquaint me of this important fact. I had no idea, until I received Lieut.-Colonel St. George's letter a few days ago that General Hull was advancing with so large a force. The militia, from every account, behaved very ill. The officers appear the most in fault. Colonel Proctor will probably reach Amherstburg in the course of to-morrow. I have great dependence in that officer's decision, but fear he will arrive too late to be of much service. The enemy was not likely to delay attacking a force that had allowed him to cross the river in open day without firing a shot. The position which Lieut.-Colonel St. George occupied is very good, and infinitely more formidable than the fort itself. Should he therefore be compelled to retire, I know of no other alternative than his embarking in the king's vessels and proceeding to Fort Erie. Were it possible to animate the militia to a proper sense of their duty, something might yet be done--but I almost despair. Your excellency will readily perceive the critical situation in which the reduction of Amherstburg will place me.