I learn from an abolition prisoner, captured on the night in question, that the enemy moved upon Camp Finegan, with the following force: One hundred and fifteenth New York, Colonel Sammon (this regiment was captured at Harper's Ferry); Forty-eighth New York, Colonel Barton; Forty-seventh New York, colonel's name not known; Fifty-sixth New York, colonel's name not known; Pennsylvania Negro regiment; Fortieth Massachusetts Mounted Infantry, and 10 pieces of artillery. This forces, after dispersing the small command at Camp Finegan, moved rapidly on to Baldwin and occupied that place on Tuesday morning, the 9th instant. On Wednesday they moved as far as the Little Saint Mary's, when they were met by Major Harrison with two companies of cavalry, who were marching from Camp Cooper (near Fernandina) to this place, and being unaware of the force of the enemy, gave them battle at a strong position. The enemy's loss at this point, as reported by a woman, whom they have permitted to come through their lines, was 15 killed and 30 wounded. We lost 2 killed and 2 wounded, the latter in the enemy's hands. The enemy's wounded are at a hospital at Barber's place, which they have established for their reception. This is near the Little Saint Mary's. The enemy advanced immediately and occupied Sanderson, our troops burning 1,500 bushels of corn, the only stores left at that place. There had been a large amount of commissary and quartermaster stores at that point, but they had all been removed, with the exception of the corn, on the preceding day. The enemy remained one night at Sanderson, and on the following morning moved upon this place, arriving at my hastily constructed works at 10 a.m., evidently confident of entering the place without much difficulty. They advanced only their mounted infantry and four pieces of artillery to attack this place, leaving their infantry at Baldwin and on the Little Saint Mary's. Citizens who live upon the road over which they passed agree in representing the force that marched on this place at 1,400 mounted men and four pieces of artillery. Their artillery was not used.

As I suggested in my letter of the 10th instant, the plan of the enemy was to advance so rapidly as to prevent the concentration of my scattered command or such re-enforcements as might reach me. Fortunately, however, a part of the troops from Middle Florida arrived, and with one or two companies collected together, amounting in all to 491 infantry and 110 cavalry, with two pieces of artillery, the further progress of the enemy was prevented.

I have thus given you a hasty sketch of operations since the enemy made his landing at Jacksonville. He has captured no stores, taken but few prisoners, and we have lost but 2 men killed and 2 wounded. We lost at Camp Finegan 2 pieces of artillery of Captain Dunham's battery, and 3 pieces of Captain Abell's battery.

This expedition is really formidable, and, organized as it is with so large a force of cavalry or mounted infantry, threatens disastrous results, unless checked at once by a sufficient force. The enemy is fortifying Baldwin, and also, I understand, a position on the Little Saint Mary's. I should have more cavalry to prevent their superior mounted force from making raids into the rich counties of Alachua and Marion, and not only running off the negroes by the Saint John's River, but destroying the large amounts of sugar and syrup which has not yet been sent to market. The supply of beef from the peninsula will of course be suspended until the enemy is driven out.