INTRODUCTION TO ASTRONOMY. xxxix 



whose apparent dimensions are diminished by distance." If the heavenly 

 bodies revolved round our earth in twenty-four hours, the centrifugal force 

 implied in so rapid a motion would be quite destructive, and no power 

 can be assigned which would be sufficient to balance it ; grindstones 

 driven by machinery in manufactories have been known to fly in pieces 

 from their great velocity. Why then should these enormous globes tra- 

 verse such an immensity of space, merely to prevent the necessity of our 

 earth revolving on its axis ? The motion produced by the revolution of 

 the earth on its axis is about thirteen miles and a half a minute to an 

 inhabitant of London. A person at the equator moves much quicker; 

 and one situated near the poles much slower, since they each perform 

 a revolution in twenty-four hours. But in performing its revolution 

 round the sun, every part of the earth moves with an equal velocity, and 

 this velocity is no less than a thousand miles a minute. 



In ancient times, the earth was supposed to occupy the centre of the 

 system ; and the sun, moon, and stars to revolve round it. This was the 

 system of Ptolemy ; but so long ago as the beginning of the sixteenth 

 century it was discarded, and the solar system, such as we have shown, 

 was established by the celebrated astronomer Copernicus and his fol- 

 lowers, and is hence called the Copernican system. But the theory of 

 gravitation, the discovery of the source whence this beautiful and har- 

 monious arrangement flows, we owe to the powerful genius of Sir Isaac 

 Newton, who lived at a much later period. 



It is far less difficult to trace by observation the motion of the planets, 

 than to divine by what power they are impelled and guided. The idea of 

 gravitation, it is said, was first suggested to Sir Isaac Newton by a cir- 

 cumstance from which one should little have expected so grand a theory 

 to have arisen. During the prevalence of the plague in the year 1665, 

 Newton retired into the country to avoid the contagion. When sitting 

 one day in his orchard, he observed an apple fall from a tree, which led to 

 a train of thought, whence his grand theory of universal gravitation 

 was ultimately developed. His first reflection was, whether the apple 

 would fall to the earth if removed to a great distance from it ; then, how 

 far it would be required to be removed from the earth, before it would 

 cease to be attracted ; would it retain its tendency to fall at the distance 

 of a thousand miles, or ten thousand, or to the distance of the moon ? 

 and here the idea occurred to him that it was not impossible that the moon 

 herself might have a similar tendency, and gravitate to the earth in the 

 same manner as the bodies on or near its surface, and that this gravity 

 might possibly be the power which balanced the centrifugal force implied 

 in her motion in her orbit. It was then natural to extend this idea to the 

 other planets, and consider them as gravitating towards the sun, in the 

 same manner as the moon gravitates towards the earth. He followed up 

 this beautiful hypothesis by a series of calculations and demonstrations, 

 unparalleled for their originality, and the industry and judgment with 

 which they were conducted, until he established the stupendous doctrine 

 of universal gravitation ! Who would imagine that the simple circum- 

 stance of the full of an apple would have led to such magnificent results? 

 It is the mark of superior genius to find matter for observation and re- 

 search in circumstances which, to the ordinary mind, appear trivial, 

 because they are common, and with which they are satisfied because they 

 are natural, without reflecting that Nature is our grand field of observa- 

 tion that within it is contained our whole store of knowledge : in a word, 

 that to study the works of Nature, is to learn to appreciate and admire. 



