_The Horizontal Pendulum._--This delicate instrument is [Page 272] represented in Fig. 82. It consists of an upright standard, strongly braced; a weight, _m_, suspended by the hair-spring of a watch, B D, and held in a horizontal position by another watch-spring, A C. The weight is deflected from side to side by the slightest influence. The least change in the level of a base thirty-nine inches long that could be detected by a spirit-level is 0".1 of an arc--equal to raising one end 1/2068 of an inch. But the pendulum detects a raising of one end 1/36000000 of an inch. To observe the movements of the pendulum, it is kept in a dark room, and a ray of light is directed to the mirror, _m_, and thence reflected upon a screen. Thus the least movement may be enormously magnified, and read and measured by the moving spot on the screen. It has been discovered that when the sun rises it has sufficient attraction to incline this instrument to the east; when it sets, to incline it to the west. The same is true of the moon. When either is exactly overhead or underfoot, of course there is no deflection. The mean deflection caused by the moon at rising or setting is 0".0174; by the sun, 0".008. Great results are expected from this instrument hardly known as yet: among others, whether gravitation acts instantly or consumes time in coming from the sun. This will be shown by the time of the change of the pendulum from east to west when the sun reaches the zenith, and _vice versa_ when it crosses the nadir. The sun will be best studied without light, in the quiet and darkness of some deep mine. [Page 273] _Light of Unseen Stars._--From careful examination, it appears that three-fourths of the light on a fine starlight night comes from stars that cannot be discerned by the naked eye. The whole amount of star light is about one-eightieth of that of the full moon. _Lateral Movements of Stars_, page 226-28.