Previous reconnaissance, made first by Brigadier General W. F. Smith, chief engineer, and afterward by Thomas, Sherman, and myself, in company with him, of the country opposite Chattanooga and north of the Tennessee River, extending as far east as the mouth of the North Chickamauga, and also of the mouth of the South Chickamauga and the north end of Missionary Ridge, so far as the same could be made from the north bank of the river without exciting suspicion on the part of the enemy, showed good roads from Brown's Ferry up the river and back of the first range of hill opposite Chattanooga, and out of view of the enemy's positions. Troops crossing the bridge at Brown's Ferry could be seen and their numbers estimated by the enemy, but not seeing anything further of them as they passed up in rear of these hills, he would necessarily be at a loss to know whether they were moving to Knoxville or held on the north side of the river for future operations at Chattanooga. It also showed that the north end of Missionary Ridge was imperfectly guarded, and that the banks of the river from the mouth of South Chickamauga Creek westward to his main line in front of Chattanooga was watched only by a small cavalry picket. This determined the plan of operations indicated in my dispatch of the 14th to Burnside. Upon further consideration (the great object being to mass all the force possible against one given point, namely. Missionary Ridge, converging toward the north end of it) it was deemed best to change the original plan, so far as it contemplated Hooker's attack on Lookout Mountain, which would give us Howard's corps of his command to aid in this purpose, and on the 18th the following instructions were given Thomas:

Major General GEORGE H. THOMAS:

All preparations should be made for attacking the enemy's position on Missionary Ridge by Saturday at daylight. Not being provided with a map giving names of roads, spurs of the mountains, and other places, such definite instructions cannot be given as might be desirable. However, the general plan, you understand, is for Sherman, with the force brought with him, strengthened by a division from your command, to effect a crossing of the Tennessee River just below the mouth of Chickamauga, his crossing to be protected by artillery from the heights on the north bank of the river (to be located by your chief of artillery); and to secure the heights from the northern extremity to about the railroad tunnel before the enemy can concentrate against him. You will co-operate with Sherman. The troops in Chattanooga Valley should be well concentrated on your left flank, leaving only the necessary force to defend fortifications on the right and center, and a movable column of one division in readiness to move whenever ordered. This division should show itself as threateningly as possible on the most practicable line for making an attack up the valley. Your effort then will be to form a junction with Sherman, making your advance well toward the northern end of Missionary Ridge, and moving as near simultaneously with him as possible. The juncture once formed, and the ridge carried, communications will be at once established between the two armies by roads on the south bank of the river. Farther movements will then depend on those of the enemy.

Lookout Valley, I think will be easily held by Geary's division and what troops you may still have there belonging to the old Army of the Cumberland. Howard's corps can then be held in readiness to act either with you at Chattanooga or with Sherman. It should be marched on Friday night to a position on the north side of the river, not lower down than the first pontoon bridge, and there held in readiness for such orders as may become necessary. All these troops will be provided with two days' cooked rations in haversacks and 100 rounds of ammunition on the person of each infantry soldier. Special care should be taken by all officers to see that ammunition is not wasted or unnecessarily fired away. You will call on the engineer department for such preparations as you may deem necessary for carrying your infantry and artillery over the creek.

U. S. GRANT,

Major-General.