60 



THE POPULAK EDUCATOR. 



fused to look through the instrument which made such un- 

 heard-of revelations. The followers of Copernicus, on the 

 other hand, welcomed the discovery as presenting a miniature 

 model of the solar system, and thus upholding their theory. 



The telescope soon made other discoveriea. By its aid 

 Galileo found that Venus presented the same phases as the 

 moon, appearing at times as a narrow crescent, and then 

 gradually becoming more and more illuminated, till at last it 

 shone with an almost circular disc. It could not, however, be 

 een with a complete disc, as at such a time the earth must be 

 in the part of its orbit exactly opposite to Venus, which would 

 therefore appear in conjunction with the sun and be lost in its 

 brightness. 



This discovery was a very important one, as it afforded a 

 strong confirmation of the truth of the Copernican system. In 

 fact, an objection had been raised to this system on the ground 

 (that these phases were not seen, as they ought to be if the 

 theory were true. The telescope, however, soon settled this 

 difficulty and silenced these objections. 



Another discovery was made when the planet Saturn was 

 examined. Instead of appearing with a circular disc, like the 

 other heavenly bodies, it was found to be elongated, as if ears 

 were affixed to each side of it. Owing to imperfections in the 

 onstr action of his telescope, Galileo failed to discover that 

 this appearance was caused by a large ring which completely 

 encircled it, and imagined that the planet was in reality composed 

 of three smaller ones. Both these discoveries were, according 

 to the practice of scientific men in those days, made known in 

 anagrams only intelligible to those who possessed the key. 



We see even thus early what an important instrument the tele- 

 ecope proved, and we shall shortly find that almost all discoveries 

 since this time have been made by its use, and that now nearly 

 all our astronomical instruments consist, wholly or in part, of a 

 telescope. We see thus to what important results the accident 

 of a child playing with two spectacle-glasses has led. 



The whole career of Galileo was a splendid one ; it was, how- 

 ever, somewhat marred near its close. The prominent position 

 he had taken as an upholder and defender of the new doctrines 

 had attracted the attention of the papal authorities, who re- 

 garded hia views as heretical, and demanded of him r. public 

 recantation of his belief in the motion of the earth. This he 

 reluctantly gave, though he is related to have said immediately 

 afterwards, " It moves for all that." It seems probable, how- 

 ever, that he considered this act as one which he was called 

 upon to perform by the Church, and that therefore it was his 

 duty to obey ; still, it was, in several ways, a sad scene. Not 

 very long after this, in 1642, he died. In the same year there 

 was born Newton, a man even more celebrated than Galileo or 

 Kepler. 



From this time onward we come across the names of so many 

 great astronomers that we can but refer to a few of the more 

 distinguished. Huyghens discovered that the appendage to 

 Saturn was in reality a ring surrounding it, and further, he 

 found one of the satellites of that planet. Napier had some 

 forty years before this invented logarithms, and thus reduced 

 the work of weeks to days or even hours ; and a little later, re- 

 flecting telescopes were introduced by Gregory. 



The name of Newton, however, stands foremost amidst all 

 these names as the discoverer of the one great law on which all 

 those of Kepler depend. Kepler seems to have suspected that 

 some such general law did exist, but failed to discover it ; he 

 seems likewise to have been aware of the fact that the tides 

 were in some way influenced by the moon, and that the other 

 heavenly bodies were in some way connected so as to influence 

 one another ; but he could not find what this mysterious bond 

 of union was, and therefore with him it was a mere conjecture. 



Newton, however, applied himself to clear up this difficulty. 

 It is said that his attention was first directed to the subject by 

 observing an apple fall one day while he was sitting in a summer- 

 house in his garden. There was nothing remarkable in the circum- 

 stance in itself, for it was an event that might be seen any day ; 

 it set him thinking, however, and he began to inquire why the 

 apple should fall downwards or towards the ground, instead of 

 upwards or to one side. To most men such a question would 

 have appeared utterly vain and frivolous ; to him, however, it 

 appeared an important step towards great and vast results, and 

 such in reality it became. After careful inquiry, he found that 

 all bodies were attracted towards the centre of the earth, and 



this attraction he called gravitation. The question then arose, 

 whether this action was confined to the surface of the earth, or 

 whether distant bodies were attracted in a similar way. The 

 intensity of this force was also believed to diminish with the 

 square of the distance ; but the difficulty arose, how was this to 

 be tested. Even if a body could have been raised several miles 

 from the earth's surface, this amount would have been so slight, 

 when compared with the radius of the earth, that no appreciable 

 difference would have been manifested. 



No way, therefore, appeared practicable of putting this theory 

 to the test, till at last the thought occurred to him, why not use 

 the moon as the falling body, and ascertain the distance through 

 which it falls in any given space of time say, for instance, in 

 one minute. This idea at first sight appears absurd, but tho 

 annexed figure will enable us 

 to understand it. 



We know that the moon re- 

 volves around the earth in an 

 orbit almost circular, as M B N. 

 Now, suppose the moon to be 

 at M, its tendency at that mo- 

 ment is to move along in the 

 direction of the tangent M c, 

 and in this direction it would 

 certainly move did not some "Fig. 3. 



other force bend it out of its 



course. This force Newton supposed to be the attraction of the 

 earth, and determined to calculate whether or not the amount it 

 deviated from a straight line was that which would arise from the 

 earth's attraction. We can easily see that when the moon has 

 moved into the position B, the distance which it has deviated from 

 its true path is equal to A B. He accordingly calculated how great 

 this distance would become after the lapse of one minute that 

 is, how far the moon would fall towards the earth in that time ; 

 he next computed the space through which a body removed 

 to the distance of the moon ought to fall in the same period 

 under the action of the earth's gravitation, and compared these 

 results together. 



Though this calculation seems to be simple enough, it was in 

 reality the work of many years ; and when at length it was 

 completed, he found a considerable resemblance between the 

 amounts, but not a sufficiently close one to satisfy him, and 

 his work was therefore, for a time, laid aside. After some time, 

 however, he heard that a more accurate determination of the 

 earth's diameter had been effected, and he accordingly repeated 

 his calculations, substituting the new figures, and when at 

 length the bewildering task was completed, the results were 

 found to agree most accurately. 



In order to fully satisfy himself of the accuracy of his theory, 

 the same calculations were gone through with reference to other 

 planets, and with the same results, and Notion then announced 

 this general law : 



Every particle of matter in the universe attracts every other 

 particle with a force proportional to the quantity of matter in each, 

 and decreasing inversely as the squares of their distances. 



The motion of the planets is thus seen to bo compounded 

 of two the one, the original motion or impetus given them 

 by the Creator, and remaining unchecked by any counteracting 

 force ; the other, the attraction of the body round which they 

 rotate. 



Having attained this result, Newton set himself one more 

 task, and that was to ascertain on mathematical principles the 

 curve in which the planets ought under these conditions to move. 

 This was a calculation requiring the utmost amount of mathe- 

 matical skill. Newton, however, possessed this, and set about 

 the work, fully expecting to find that the curve must be an 

 ellipse. He found, however, when the result was complete, one 

 which represented not only this, but any of the " conic sections," 

 that is, of the curves which may be obtained by cutting a cone. 

 These are the circle, which is the curve obtained when it is cut 

 parallel to tho base ; the ellipse, when it is cut a little inclined 

 to this ; the parabola, when the line passes parallel to one side 

 of the cone ; and the hyperbola, when it is parallel to the axis. 

 In any one of these curves, then, a planet may move under the 

 influence of these general laws ; and, as we shall find, tho 

 satellites of Saturn move in the first, the planets generally in 

 the second, while the comets career onwards in parabolas or 

 hyperbolas. 



