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HISTORICAL VIEW OF TIIF

was proved to be the very same which affects the moon.Its identity was defined by the actual observed diminu-tion of its intensity as we ascend in the atmosphere, andits precise agreement with the diminished force withwhich it was proved to act on the moon. Does it, then,asked Newton, act between every two particles of mat-ter throughout the universe ? Is there (without anyphysical hypothesis as to the modus operandi) a realtendency to approach each other, with an intensity in-versely as the square of the distance actually existingand operating throughout the planetary system ? Tothis question the accumulating testimony of the observedphenomena, as examined by Newton and his successors,to the present day, has been giving the answer. Theforce of the argument depends upon the collective proofswhich all the facts, in their several ways, minister to thetruth of the great principle of gravitation.

In a system of bodies, connected by this sort of at-tractive influence, and revolving about their commoncentre of gravity, if .one body were considerably greaterthan the other, the common centre of gravity might fallwithin the mass of the greater body, and even be situ-ated sensibly in its centre. Thus, the centre of theearth was seen to be, St once situated in the centre of forceof the moon’s orbit, and, the actual source of the physicalforce of gravitation. In the same way it followed thatJupiter and Saturn were, in like manner, actual sourcesof gravitating influence to their satellites, as well assituated in the centres of force of their orbits. In likemanner, therefore, the centre of the sun, by virtue otthe universal observance of the same law, of the inversesquares of the distances, was the source of gravitating in-fluence to all the planets, as well as the point in spaceto which the equable description of areas, in all theirorbits, pointed as the centre of force. Among thevarious remarkable applications which Newton made ofthis truth, none are, perhaps, at first sight, more striking,or even seem more incredible, to a person unacquaintedwith the subject, than the determination of the actual