SIR:  Agreeably to your instructions, I have examined the explosive machine sent to this arsenal yesterday.  It is a small miniature case containing twelve copper cartridges, such as are used in a Smith & Wesson pocket pistol, a bundle of sensitive friction matches, a strip of sand-paper, and some fulminating powder.  The cartridges and matches are imbedded in common glue to keep them in place.  The strip of sand-paper lies upon the heads of the matches.  One end has been thrown back, forming a loop, through which a bit of thread evidently passed to attach it to the lid of the case.  This thread may be seen near the clasp of the lid, broken in two.  There are two wire staples, under which the strip of sand-paper was intended to pass to produce the necessary pressure on the matches.  The thread is so fixed that the strip of sand-paper could be secured to the lid after it was closed.

The whole affair is so arranged that the opening of the lid would necessarily ignite the matches, were it not that the lower end of the strip has become imbedded in the glue, which prevents it from moving.  That the burning of the matches may explode the cartridges, there is a hole in each case, and all are covered with mealed powder.

One of the cartridges has been examined and found to contain ordinary grain powder.  Two of the cartridges were exploded in a closed box sent herewith.  The effect of the explosion was an indentation on one side of the box.

    Very respectfully, your obedient servant,
    J.G.  BENTON,
    Major of Ord. and Bvt.  Col.  Comdg.

Between the outside covering and the box there were two or three folds of tissue-paper—­placed there, no doubt, to prevent the possibility of an explosion from the stamping at the post office, or the striking against other packages during the voyage from San Francisco to New York.

On the inside of the lid was pasted a slip cut from a San Francisco paper, dated October 31st, 1864, stating that on the day previous I had decided the case of the City against the United States, involving its claim to four square leagues of land, and giving the opening lines of my opinion.

The Secretary of War, Mr. Stanton, immediately telegraphed in cypher to General Halleck, then in command in San Francisco, to take active measures to find out, if possible, the person who made and sent the infernal machine.  General Halleck put the detectives of his department on the search.  Others employed detectives of the San Francisco police—­but all in vain.  Suspicions were excited as to the complicity of different parties, but they were never sustained by sufficient evidence to justify the arrest of any one.  The instrument, after remaining in the hands of the detectives in San Francisco for nearly two years, was returned to me and it is now in my possession.[5]