Being satisfied from the information thus acquired, that there was danger ahead for the small settlements of whites in the Northern portion of the State, he immediately addressed a letter to Gov. Reynolds, of Illinois, from which we take the following: FORT ARMSTRONG, April 13, 1832. DEAR SIR:--The band of Sacs, under Black Hawk, joined by about one hundred Kickapoos and a few Pottowattomies, amounting in all to about five hundred men, have assumed a hostile attitude. They crossed the river at the Yellow, Banks on the sixth inst., and are now moving up on the east side of Rock river, towards the Prophet's village. "The regular force under my command is too small to justify me in pursuing the hostile party. To make an unsuccessful attempt to coerce them would only irritate them to acts of hostility on the frontier sooner than they probably contemplate. "Your own knowledge of the character of these Indians, with the information herewith submitted, will enable you to judge of the course proper to pursue. I think the frontier is in great danger, and will use all the means at my disposal to co-operate with you in its protection and defense. With great respect, Your most obedient servant, H. ATKINSON, Brigadier General of the U. S. Army, His Excellency, Gov. Reynolds, Belleville, Ills." On receipt of Gen. Atkinson's letter, Gov. Reynolds issued his proclamation, calling out a strong detachment of militia to rendezvous at Beardstown on the 22d of April. In obedience to this command a large number of citizens assembled and offered their services. They were met by Gov. Reynolds, and after bring organized into a brigade, he appointed Brig. Gen. Samuel Whitesides commander. His brigade embraced 1600 horsemen and two hundred footmen--being four regiments and an odd spy battalion. First regiment, Col. Dewitt; second, Col. Fry; third, Col. Thomas; fourth, Col. Thompson; Col. James D. Henry, commanded the spy battalion. The troops took up their line of march at once, under command of Gen. Whitesides, accompanied by the Commander-in-Chief, Gov. Reynolds. For the purpose of laying in provisions for the campaign they went to Yellow Banks, on the Mississippi river, where Major S. S. Phelps, who had been appointed quarter master, supplied them. They arrived on the 3d of May, and left for Rock river on the 7th. THE BLACK HAWK WAR.