A plumb-line, under ordinary circumstances, hangs in a direction truly vertical; but if it be near a large mass of matter, as a mountain, it has been observed to be deflected from the true vertical, towards the mountain. This effect was observed by Dr. Maskeline near the mountain called Skehallien, in Scotland, and by French astronomers near Chimboraco. For particulars of these observations, see our treatise on GEODÆSY. Cavendish succeeded in exhibiting the effects of the mutual gravitation of metallic spheres. Two globes of lead A, B, each about a foot in diameter, were placed at a certain distance asunder. A light rod, to the ends of which were attached small metallic balls C, D, was suspended at its centre E from a fine wire, and the rod was placed as in _fig. 22._, so that the attractions of each of the leaden globes had a tendency to turn the rod round the centre E in the same direction. A manifest effect was produced upon the balls C, D, by the gravitation of the spheres. In this experiment, care must be taken that no magnetic substance is intermixed with the materials of the balls. Having so far stated the principles on which the law of gravitation is established, we shall dismiss this subject without further details, since it more properly belongs to the subject of PHYSICAL ASTRONOMY; to which we refer the reader for a complete demonstration of the law, and for the detailed development of its various and important consequences. CHAP. VII. TERRESTRIAL GRAVITY. (115.) GRAVITATION is the general name given to this attraction, by whatever masses of matter it may be manifested. As exhibited in the effects produced by the earth upon surrounding bodies, it is called “terrestrial gravity.”