WASHINGTON, April 15, 1865.

To J. C. DERBY, United States Dispatch Agent, New York:

Send a copy of the following to Mr. Adams at London by the steamer of to-day, if in time:--

CHARLES FRANCIS ADAMS, &c., &c.:

The sad duty devolves upon me to announce the assassination of the President, at Ford's Theatre, last night, by a pistol-shot from a person who entered his box for the purpose. The assassin escaped, but it is supposed has since been arrested.

The President died at half-past seven o'clock this morning.

Vice-President Johnson has assumed the functions of President, having been sworn in by the Chief-Justice.

About the same time an attempt was made by, it is believed, a different person, to assassinate Mr. Seward; but the murderer only succeeded in inflicting painful and severe wounds, principally upon his face.

Mr. F. W. Seward was beaten over the head with a heavy weapon in the hands of the person who attacked his father, and is grievously hurt. His brother was also wounded by the dagger of the assassin, as was Mr. Hansell, a messenger of the department, who was with the Secretary, and the male nurse in attendance.

WILLIAM HUNTER, Acting Secretary of State.

[The above telegraphic dispatch was sent off by the Portland steamer at three P. M. on Saturday, April 15.]