It had been the intention of the British commander to surprise the Americans, and so severe were the precautions taken for this purpose, that the officers employed in the expedition were only informed of it on the preceding day. Information of a meditated attack had been, however, for some time in possession of the Americans; the first intimation having been given by a patriotic lady of Boston, the wife of a royalist officer. A most vigilant observation was, in consequence, maintained upon the movements of the British; and, in this operation, great advantage was derived from the services of an association, composed chiefly of Boston mechanics, which had been formed in the autumn of the preceding year. The late Col. Paul Revere was an active member of this society, and was employed by Dr. Warren, on this occasion, as his principal confidential messenger. Some preparatory movements took place among the British troops on the 15th of April, which attracted the attention of Warren. It was known that the principal object of the contemplated expedition was to seize the stores at Concord. Presuming that the movement would now be made without delay, the committee of safety took measures for securing the stores by distributing a part of them among the neighboring towns. John Hancock and Samuel Adams were then at the house of the Rev. Mr. Clark, in Lexington, and Colonel Revere was dispatched as a special messenger to inform them of the probable designs of General Gage. On his return to Boston, he made an agreement with his friends in Charlestown, that, if the expedition proceeded by water, two lights should be displayed on the steeple of the North Church, if it moved over the neck, through Roxbury, only one. The British commander finally fixed upon the 19th for the intended attempt; and, on the evening of the 18th he sent for the officers whom he had designated for this service, and communicated to them, for the first time, the nature of the expedition upon which they were to be employed. So strict had been the secrecy observed by the governor in regard to this matter. The same discretion had not been maintained in other quarters, for Lord Percy, who was to command the reserve, on his way home to his lodgings, heard the expedition talked of, by a group of citizens, at the corner of one of the streets.