Sir,—As Mr Clerk Maxwell, in a paper lately published in the Transactions of this Society, has done me the honour to refer to me as having been the first to show how to combine in one diagram a system of lines representing the directions and magnitudes of all the forces acting in a given frame, I wish to put on record, in the Proceedings of the Society, the time and manner of my first publication of the method in question. It was in the year 1856, in a lithographed synopsis of lectures which I delivered in the University of Glasgow, entitled “Mechanical Laws, Formulaæ, and Tables.” Copies of that synopsis were distributed to the students of my class, and to a few men of science.