At intervals during the engagement we fired canister into the woods on our right and left, assisting the First Regiment on our right and the Fifth on our left in repelling the enemy, who were engaging theses regiments at short musketry-range.

The rifled battery continued to engage us until 8 o'clock p. m., when we ceased firing, having fired 800 or 900 rounds of shot, shell, and canister.

Lieutenant Fullerton, in command of the left section, reports having engaged the same time, repulsing them with case-shot and canister. Four desperate efforts were made to cross the bridge near the mill, but as often were they repulsed and forced to fall back.

Respectfully, your humble servant,

J. H. COOPER,

Captain, Pennsylvania Artillery, Commanding Battery B.

Captain JAMES C. CLARK,

Assistant Adjutant-General.

Having retired from Mechanicsville as rear guard of the division to Gaines' Hill at 3 p. m., we formed in line of battle with General Reynolds' brigade withdrew the infantry of his brigade, leaving for our support one regiment of New York troops. Remaining in this position until 5.30 p. m. the battery on our right retired, the enemy occupying their position, when we opened fire upon them and held them in check until 8 p. m., when, our support falling back, we retired in its rear to prevent the capture of our battery by a column of the enemy who were charging upon us. The effect of our shot on the enemy was destructive.

Respectfully, your obedient servant,

J. H. COOPER,

Captain, Pennsylvania Artillery, Commanding Battery B.

Captain JAMES C. CLARK,

Assistant Adjutant-General.

The battery having been ordered by General McCall to take position in line of battle near the above-named place at 2 o'clock p. m., where we remained until 4.30 p. m., when one of the enemy's batteries, concealed in the woods to our front, opened fire upon us, to which we replied with marked effect, as son but one gun replied to us. During the engagement parties of infantry attempted to cross the field in our front, but with the assistance of Battery G, First Pennsylvania Artillery, we compelled them to retire in disorder and confusion. This continued about one and a half hour, when Battery G was compelled to retire for want of ammunition, and our infantry support, with the exception of three companies, retired. The enemy, availing themselves of this opportunity, advanced a regiment from a point of woods in our front, which our canister failed to check, although it did marked execution.

The remaining infantry falling back, we were compelled to retire from our guns. The charge being so sudden and overpowering it was impossible to remove them, many of the horses being killed by the enemy's fire.