POST-OFFICE DEPARTMENT, October 16, 1861.

Honorable WILLIAM H. SEWARD, Secretary of State.

MY DEAR SIR: I inclose the letter of C. E. Detmold, esq., to L. J. Brengle, esq., of Frederick, Md., containing information valuable to the Government. * * * I shall be obliged to you to have the letter returned after use has been made of it to me.

Very respectfully, your obedient servant,

JOHN A. KASSON,

First Assistant Postmaster-General.

[Inclosure.]

NEW YORK, October 11, 1861.

L. J. BRENGLE, Esq.

MY DEAR SIR: The result of the election in Baltimore proves the wisdom of the action of the Government in having the prominent traitors arrested. Even the secessionists in Western Maryland are reconciled and even approve it for they dread civil war within the State.

At the same time, however, I learn from a very reliable source in Allegany County that a secret movement is on foot by the peace party, i. e., secessionists in disguise to nominate an opposition ticket; and for the purpose of defeating the Union ticket the commissioners, nearly all secessionists, have lately had a meeting and appointed the rankest secessionists as judges of election. I mention the name of one so appointed for Cumberland, W. O. Sprigg, well known as a rabid secessionist, having a son in the rebel army. Amongst the opponents of the Government the foremost in Allegany County are Judge Perry and Doctor Fitzpatrick. The former appointed young Brien, now an officer in the rebel ranks, foreman of the grand jury and permitted him to come into court with a large secession badge on his breast. I mention this fact as a glaring instance of his proclivities. He and his Confederates Doctor Fitzpatrick, W. O. Sprigg (who I believe has also a son in the rebel ranks) and if I mistake not Devecmon, the lawyer, are the head and front of the secret movement now going on. They are in constant communication with the rebels in Virginia and are doing all the mischief they can. Now it seems to me these people should for a while be placed where they can do no harm. If the Government could be made aware of the state of things I think they should give these gentlemen free quarters at Fort McHenry or Fort Lafayette from now until after the election. The quiet and safety of the State of Maryland would be promoted by such a proceeding and an election result obtained which could not but have a most beneficial effect upon the whole country.

Cannot you, my dear sir,rmation before the Government in such manner as to command their attention? I am sure that any suggestions coming from you would receive the promptest consideration.

After much impatience the public here is becoming reconciled to the inaction of the Union forces along the line of the Potomac. A large naval expedition is preparing which is nearly ready and destined in all probability to take Mobile or New Orleans. About the same time an advance movement will probably be made by the army at and near Washington, and most devoutly do I hope and pray that both may be completely successful. The best spirit prevails here. The Government can have whatever they ask for. No sacrifice or effort is too great not to be made promptly and cheerfully. All we ask is that the Government shall respond with energy to the wishes of the people.