HEADQUARTERS ARMY OF THE POTOMAC, December 25, 1863.

Colonel TAYLOR,
Commanding Second Division Cavalry:

Dispatch just received states that Mosby has made great preparation to have a frolic, with his principal officers, at the house of Dr. Bishpa and Mrs. Murray, in Salem, to-night. Dr. Bispham's is the second house as you go in the village from Warrenton, an Mrs. Murray lvies about the middle of the street, in a large white house. The major-general commanding directs that you send a party from the brigade which is at Warrenton, under the command of a smart and competent officer, to capture them.

E. B. PARSONS,

Captain, Acting Assistant Adjutant-General.

HEADQUARTERS CHIEF ENGINEER OF DEFENSES, Washington, December 26, 1863.

Brigadier General JOSEPH G. TOTTEN,
Chief Engineer, &c., Washington, D. C.:

SIR: By letter of the Secretary of War to you, of the 2nd instant, you are instructed to detail me-

To make an examination of the shore of Lake Erie, and designate at what points defensive works can be advantageously erected to guard the States of New York, Pennsylvania, and Ohio against hostile raids from Canada.

In compliance with the tenor of the above, I have, after first visiting and consulting the Governor of Ohio, at Columbus, visited the following points on the shore of Lake Erie, viz, Sandusky, Toledo, Cleveland, Erie, Dunkirk, and Buffalo.

I understood my instructions to refer solely to the actual condition of things, and not to refer to another state of things in which war shall exist betwixt Canada itself (and the Government to which she belongs) and the United States. I have of course made it a point, so far as I was able in a brief and at such a season, to form in my mind some idea of the positions and character of works required for the latter supposition, but report only in reference to the former.

There seems to me to be two objects, and only two, for which "hostile raids" might be made by parties of the numerous sucessionsits in Canada. One of these objects would be the rescure of the prisoners on Johnson's Island; the other, to destroy shipping, railroad depots, grain elevators, machine shops, &c., which are situated contiguous to the harbors of the places I have mentioned. Plunder might perhaps be considered an object, but the risks would be great of penetrating into the interior of such places as Buffalo, Cleveland, &c., where alone there is anything to be carried off, and the means of guarding against it are identical with those to be used against destruction.

First, in reference to the rescue of prisoners. There are now 2,800 of them, all officers, confined on Johnson's Island. This island is almost a mile long and one-half mile broad (at its broadest part). Its nearest distance to the mainland is over a half mile. The permanent guard is a battalion of four companies of Ohio Volunteers (to be, as I understood speedily increased to a regiment) and a company of heavy artillery. Besides this, there was stationed on