HISTORY OF PHYSICAL ASTRONOMY.

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that, besides the waters of the ocean, the earth contains subterraneanhumours and spirits, which are drawn out by the attraction of the moon;and, when that body has retired, are then absorbed again into the bowelsof the earth. “ The moon,” says he, “ attracts not so much the sea asthe subterranean spirits and humours, and the interposed earth has nomore power of resistance than a table or any other dense body has tojresist the force of a magnet.”

The preceding remarks of Gilbert contain unquestionably one of theearliest traces which is to be found among the writings of modern authors,of the notion of an attractive force acting between the bodies of the solarsystem. The moon’s attractive influence upon the earth is naturallyenough suggested by the phenomenon of the tides; but the influence ofthe earth upon the moon is mixed up with a great deal of error and con-fusion. It appears to him to be indicated not by the revolution of themoon in a curvilinear orbit round the earth, but by her accompanying that. body in a continuous course round the sun. In fact the principle ofI terrestrial attraction is suggested by the notion of the earth dragging themoon along with her in her annual orbit. Finding himself utterly unableto account for the mutual attraction of the earth and moon, without thecontinual approach and ultimate union of the two bodies, he attempts toget rid of the difficulty by shifting his hypothesis, or, in other words, byasserting that the effects resulting from the mutual influence of the twobodies is not similar to the effects of magnetic attraction. AlthoughGilbert, therefore, deserves much credit for the sagacity with which herecognised, to a certain extent, the principle of gravitation, his ideas of itare so vague and inconsistent, that liis speculations cannot be said to riseabove the merit of mere conjectures.

Kepler , in the introduction to his “ Astronomia Nova, ” published in 1609,announces the mutual gravitation of matter in very remarkable terms.He asserts, as Copernicus had already done, that bodies do not tendtowards the centre of the earth, because it is the centre of the universe,but because it is the centre of a round body of the same nature withthemselves. If two stones were situated in space beyond the influence ofa third body, they would approach towards each other like two magneticneedles, and would meet in an intermediate point, each passing through aspace proportional to the comparative mass of the other. If the moonand earth were not retained by their animal force, or some other equiva-lent, the earth would mount to the moon by a fifty-fourth part of theirdistance, and the moon would fall to the earth through the other fifty-three parts, and they would there meet. If the earth should cease toattract the waters to itself, all the waters of the sea would be raised, andwould flow to the body of the moon.

These remarks are indeed very striking, and show how profoundly theirillustrious author could penetrate into the secrets of nature; but we shouldnot be justified in attaching to them all the importance due to a distinctrecognition of the principle of gravitation. In his ideas and reasoning hecoincides with Gilbert, except that he extends the principle of gravitationto the whole material universe. The difficulty which Gilbert experiencedin accounting for the constant separation of the moon and earth, notwith-standing their mutual attraction, occurs with its full force to Kepler . Thelatter, however, gets over it not as Gilbert had done, by assuming aprinciple inconsistent with his previous ideas on the subject, but bysupposing the terrestrial attraction to be neutralized by the animal. force

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