NEW YORK, September 23, 1861.

Honorable WILLIAM H. SEWARD, Secretary of State, Washington.

DEAR SIR: Yours of the 21st instant inclosing documents in relation to the case of Edward B. Wilder, now confined in Fort Lafayette, is received. I have examined the papers transmitted and beg leave to report that the evidence contained in the interecepted letter from Mr. Wilder to Nathan R. Mendenhall, of Winchester, Va. (if unexplained), must be held as conclusive as to the animus of the writer. According to that he has thought, talked, written treason. He has offered to instruct the rebels how to construct a formidable implement of war to be used against his Government and directed them to whom to apply for information. This is offering aid to the enemy. It is a treasonable act, and his wife and family are all that prevent his actually taking up arms against the Government of the United States. This is quite sufficient to call for his arrest and confinement.

The testimony in his behalf by Messrs. Marshal, Grinnell, Sturgis and Dodge is clear and ample as to his general good character and integrity but it stops short of establishing his loyalty. As much can be said with equal truth for vast numbers who are now bearing arms against the United States Government. The only really pertinent testimony in behalf of Mr. Wilder is contained in the letter written by himself to Mr. Russell Sturgis. He says the interceptedletter was a ruse to prevent the confiscation of his property. He furnishes them valuable infomration how to arm themselves with a powerful means of destroying the armies of the Union as a ruse to cheat them into sparing his property. In plain English his motive for committing treason to his country was mercenary. The explanation may be true, which is doubtful, but it certainly is not a good defense. If instead of instructing them how to make a rifle battery he had actually sold and delivered over to the enemy and was on his trial for treason on that account the court would not allow him to prove in his defense that he made or saved money by the transaction. The man who instrucn the art of making rifle batteries which "work most admirably" to be used against the armies of the Union for the purpose of saving his Southern property will be likely to personally take up arms against his country for the same purpose unless prevented by the fear of greater penalties.

Finally the evidence of the disloyalty of Mr. Edward B. Wilder is such as to warrant and require his arrest and detention while the allegations in his defense are not established by any competent evidence and if established would not prove him to be innocent.

Very respectfully, yours,

S. C. HAWLEY.

I return herewith all the documents transmitted.

DEPARTMENT OF STATE, Washington, September 24, 1861.

RUSSELL STURGIS, Esq., New York.

SIR: I have examined the case of Mr. Edward B. Wilder and am of the opinion that he is clearly guilty of a treasonable correspondence and conspiracy to overthrow the Union. His discharge therefore would be incompatible with justice and the public safety.

I am, sir, your obedient servant,

WILLIAM H. SEWARD.