5. _Light_, though extremely minute, _is supposed to have a certain degree of force or momentum_. In order to prove this, the late ingenious Mr. Mitchell contrived the following experiment. He constructed a small vane in the form of a common weather-cock, of a _very thin_ plate of copper, about an inch square, and attached to one of the finest harpsicord wires, about ten inches long, and nicely balanced at the other end of the wire, by a grain of very small shot. The instrument had also fixed to it in the middle, at right angles to the length of the wire, and in an horizontal direction, a small bit of a very slender sewing needle, about half an inch long, which was made magnetical. In this state the whole instrument might weigh about ten grains. The vane was supported in the manner of the needle in the mariner’s compass, so that it could turn with the greatest ease; and to prevent its being affected by the vibrations of the air, it was enclosed in a glass case or box. The rays of the sun were then thrown upon the broad part of the vane or copper plate, from a concave mirror of about two feet diameter, which, passing through the front glass of the box, were collected into the focus of the mirror upon the copper plate. In consequence of this the plate began to move with a slow motion of about an inch in a second of time, till it had moved through a space of about two inches and a half, when it struck against the back of the box. The mirror being removed, the instrument returned to its former situation, and the rays of the sun being again thrown upon it, it again began to move, and struck against the back of the box as before. This was repeated three or four times with the same success.