28 HISTORY OP ASTRONOMICAL DISCOVERY. 



travel with greater velocity from a to b, than through the other parts of its course. 

 This is the SECOND of Keppler's laws, technically expressed by saying that equal areas 

 are described in equal times, to accomplish which the velocity of the planet cannot be 

 uniform. Previous observation discovered the want of uniformity ; but Keppler found 

 the rule which governed the phenomena, and thus did his master mind bring into 

 conformity to law a case of apparent deviation from it. The THIRD of his laws refers to 

 the distances of the planets from the sun. His mathematical mind caught the idea 

 that these bodies must bear some mysterious relation to each other, and after a long 

 and harassing pilgrimage in search of a clue to their affinity after roaming in the wilds 

 of conjecture, and setting the planets to music in imitation of a Pythagorean phantasy 

 he groped his way to the perception of a physical fact, which unfolded before him 

 the unity of the solar system. This was the discovery, that the squares of the periodic 

 times of any two planets are to one another as the cubes of their distances from the sun. 

 His joy was unbounded at this discovery, as it transpired in an unexpected manner. He 

 could hardly persuade himself that he had not been dreaming ; and it may be ranked 

 among the most important and glorious physical truths ever reached by the intelligence 

 of mankind. Popularly expressed, the fact arrived at is, that the distances of two planets 

 from the sun being known, and the period or year of one of them, the period of the other 

 may be ascertained by the above simple proportion ; and clearly does this fact involve the 

 idea of the bodies to which it applies not being thrown at random into their places not 

 being independent but members of one great system. As an example, Mars is about 

 four times the distance of Mercury from the sun ; his period of revolution is about eight 

 times that of the latter ; and the cube of four, or sixty four, is equal to the square of eight. 

 Such are the laws of Keppler. They obviously confirm while they correct Copernicus ; 

 and their author became a zealous Copernican. They served afterwards as a founda 

 tion already prepared for the splendid superstructure of the Newtonian philosophy. 

 They were framed out of materials gathered by Tycho. Hoene, in the Baltic, has thus 

 played a conspicuous part in the world's history, looking to the causes of things. The 

 castle of the heavens was not built in vain ! 



The phenomenon of a new star was again presented to the gaze of mankind in the year 

 1604, thirty-two years after the former appearance. It suddenly shone forth from the 

 constellation of Serpentarius, continued visible upwards of a year, and gradually waned 

 in its lustre before its final disappearance. When at the brightest it surpassed the fixed 

 stars of the first magnitude and rivalled Venus, changed in colour from a yellow to a 

 purple and fiery hue, and presented no sensible parallax. Keppler was one of its 

 observers, and adopted the hypothesis that it proceeded from some vast combustion. But 

 what these stellar apparitions portend is one of the mysteries of the universe. We may 

 conjecture we may theorise but we cannot know. If they are bodies beyond tele 

 scopic range, that start into temporary visibility under the action of fervent heat, their 

 diminishing lustre and ultimate disappearance being the decline and termination of 

 mighty conflagrations, " it is impossible," says Mrs. Somerville, " to imagine anything 

 more tremendous than a conflagration that could be visible at such a distance." In 1607 

 the comet, afterwards known as Halley's, engaged the attention of Keppler, and again in 

 1618 he had the opportunity of observing a similar object. It is somewhat remarkable 

 that he did not grasp the analogy between the orbits of these bodies and those of the 

 planets. He attempted to determine the path of Halley's on the supposition of its motion 

 being rectilinear, conceiving comets to be evanescent bodies, appearing and vanishing for 

 ever. But he fulfilled a ministry in the great temple of nature sufficiently glorious as it 

 is, and required not the anticipation of the triumphs of others to give him a hold upon 

 the admiration of posterity. It is melancholy to reflect, that he lived in misery, owing 



