THE EARTH. 3 



greatest body in our system, is, of consequence, possessed of muca 

 the greatest share of this attracting power ; and all the planets, 

 af which our earth is one, are, of course, entirely subject to its 

 juperior influence. Were this power, therefore, left uncontrol- 



their oommon centre of attraction ; and he says, that the tides of the ocean 

 are the effects of the moon's attraction, heaping up the waters immediately 

 under her. Thee, adopting the opinion of Dr Gilbert, that the earth is a 

 great magnet, he explains how this mutual attraction will produce a deflec. 

 tioQ into a curvilineal path. Dr Hooke appears to have had very accurate 

 general notions of tlie nature of the mutual attraction of the celestial bodies ; 

 for, at a meeting- of the Royal Society in the year 1666, he expressed himself 

 as follows : " I will explain a system of the world very different from any 

 yet received, and it is founded on the three following positions. 1. That all the 

 heavenly bodies have not only a gravitation of their parts towards their own 

 proper centre, but that they mutually attract each other within their spheres 

 of action. 2. That all bodies having a simple motion, wiU continue to move 

 in a straight line, unless continually deflected from it by some extraneous 

 force, causing them to describe a circle, an ellipse, or some other curve. 

 3. That tins attraction is so much the greater as tlie bodies are nearer. Aa 

 to the proportion in which those forces diminish by an increase of distance, 

 I own," said he, " I have not discovered it, although I have made some ex- 

 periments to this purpose : I leave this to others, who have time and know- 

 ledge sufficient for this task." Previous to this period, Dr Hooke had ex- 

 hibited to the Society an experiment, with a \ iuw to show how a motion in a 

 curve might be produced in consequence of a tendency in a body towards a 

 centre. A ball suspended by a thread from the ceiling was made to revolve 

 about another ball laid on a table immediately below the point of suspension. 

 When the push given to the pendulous ball was properly adjusted to its devia- 

 tion from the perpendicular, it described a perfect circle round the ball on the 

 table, but when the push was very great, or very smaU, it described an 

 ellipse, having the other ball in its centre. Dr Hooke showed that this was 

 the operation of a deflecting force directly proportional to the distance from 

 the other ball; but he added, that although this illustrated the planetary 

 motions in some degree, yet it was not suitable to their cause ; for the 

 planets describe ellipses, having the sun not in the centre, but in one of their 

 foci ; therefore they are not retained by a force proportional to their distance 

 from the sun. In these rertiarks, we have a clear and modest account of a 

 rational theory ; and it must be inferred from them, that Dr Hooke had an. 

 ticipated Newto. in describing the general nature of the planetary motions, 

 although it is solely to the latter that we owe the discovery of the precise 

 law of the force by which the very motions we observe are produced. 



To this extent the true theory of the motions of the heavenly bodies hnd 

 been discovered, or rather conjectured, when Sir Isaac Xewton turned his at- 

 t«ntion to the subject. The circumstances under which he discovered tlie 

 true theory of the planetary motions, are stated by Dr Pemberton, in his 

 preface to this T'iew of Sir Isaac Newton's Philosophy. They are in sub. 

 stance as follows: He had retired from Cambridge to his country house in 

 the year 1666, on account of the plague ; and one day as he sat alone in his car- 

 din, reflecting on the power by which all terrestrial bodies gravitate toward* 



