PHYSICAL SCIENCE. 



XXI 11 



(he centre of the world, is an absurd proposition 

 false in philosophy, heretical in religion, and 

 contrary to the testimony of Scripture. That it 

 is equally absurd and false in philosophy to assert, 

 that the earth is not unmovable in the centre ol 

 the world, and, considered theologically, equally 

 erroneous and heretical.'' 



Here was an example, among many others that 

 might, be given, of a number of men who con- 

 ceived that power was able to subdue truth. But 

 the puny efforts of popes and cardinals are totally 

 unable to stop the steady flow of human know- 

 ledge. The decree remains still in force among 

 -the infallible oracles of the Roman Catholic 

 church. But who pays any regard to it ? * A 

 promise was extorted from Galileo that he would 

 not teach the doctrine of the earth's motion, 

 either by speaking or writing. But this promise 

 he did not keep. His third dialogue, published 

 long afterwards, contains a full display of the 

 beauty of the new system, and such an exposure 

 of the inconsistencies of Ptolemy and Tycho, as 

 completed the triumph of the new system. 



In 1663, when he was seventy years of age, 

 he was again brought before the inquisition, and 

 forced solemnly to disavow his belief in the 

 earth's motion. He was condemned to perpetual 

 imprisonment; though the sentence was after, 

 wards mitigated, and he was allowed to return to 

 Florence. This sentence sank heavily into his 

 mind ; he never after either talked or wrote on 

 the subject of astronomy. But no decisions of 

 popes or inquisitors could stop the progress of 

 truth. The opinion that the earth is one of the 

 planets, and that, like the others, it moves round 

 the sun in an elliptical orbit, became more and 

 more prevalent, and was soon universally em- 

 braced. 



The time was now come when the motions of 

 the heavenly bodies were to be compared with 

 the laws of motion, as known on the earth. For 

 this comparison, and for the developement of a 

 new science, the sublimest that has hitherto been 

 exposed to the eyes of mankind, the world is 

 indebted to Newton. In the year 1666, when he 

 was a very young man, he was driven from Cam- 

 bridge by the plague. As he sat one day in his 

 garden, musing on the nature of the mysterious 

 force by which the phenomena at the earth's sur- 

 face are so much regulated, he observed an apple 



In the edition of Newton's Principia Mathematica by 

 the Jesuits, Le Seur and Jacquier, printed at Rome in the 

 year 1742, there occurs at the beginning of the third book Die 

 following declaration, by the reverend editors : " Newton 

 in this third book has assumed the hypothesis of the motion 

 of the earth. The propositions of the author could not be 

 explained without adopting that hypothesis. We were 

 therefore forced, in consequence, to adopt the opinion of 

 another person. But we acknowledge ourselves obedient 

 in the decrees of the pope against the motion of the earth/' 



fall from a tree. The thought occurred to him, 

 since gravity is a tendency not confined to bodies 

 on the very surface of the earth ; but since it 

 reaches to the tops of trees, and to the summits 

 of the highest mountains, without its intensity 

 suffering any sensible change, why may it not 

 reach to a much greater distance, and even to 

 the moon itself? And if so, may not the moon be 

 retained in her orbit by gravity, and forced to 

 describe a curve, like a projectile, on the surface 

 of the earth ? Here another consideration very 

 naturally occurred. Though gravity be not 

 sensibly weakened at small distances from the 

 earth, yet it may be weakened at greater dis- 

 tances, and at the moon may be much less. To 

 estimate the quantity of diminution, Newton 

 seems to have reasoned thus : If the moon be re- 

 tained in her orbit by the gravitation of the 

 earth, it is probable that the planets likewise are 

 carried round the sun, in consequence of a 

 similar power directed to the centre of that 

 luminary. He proceeded, therefore, to inquire 

 by what law the gravitation of the planets to the 

 sun must diminish, in order to correspond with 

 Kepler's third law. 



Such an investigation would have been beyond 

 the power of most mathematicians of that age ; 

 but Newton soon discovered that Kepler's law 

 would require the force of gravity to diminish 

 exactly as the square of the distance increased. 

 The moon, therefore, being distant from the 

 earth about sixty semi-diameters of the earth, 

 the force of gravity at that distance must be re- 

 duced to the 3600th part of what it is at the 

 earth's surface. Was the deflexion of the moon, 

 then, from the tangent of her orbit in a second 

 of time, just the 3600th part of the distance 

 which a heavy body falls in a second at the 

 earth's surface ? This question could be precisely 

 answered, supposing the moon's distance in feet 

 known, and her angular velocity, or the time of 

 her revolution in her orbit also known. 



Being at a distance from books, he took the 

 common estimate of the earth's circumference 

 then in use; according to which, a degree was 

 held equal to sixty miles. This being an erro- 

 neous supposition, the result of the calculation 

 did not represent the force as adequate to pro- 

 duce the effect. Hence Newton concluded that 

 some other cause than gravity must act on the 

 moon, and laid aside, in consequence, all further 

 peculations on the subject for the time. 



Some years after, his attention was again 

 called to the subject by a letter from Dr Hooke, 

 proposing, as a question, To determine the line 

 in which a body let fall from a height descends to 

 the ground, taking into consideration the motion 

 of the earth on its axis. This led him to resume 

 the subject of the moon's motion, and the mer.- 



