190 KANSAS CITY REVIEW OF SCIENCE. 



almost in a direct line to the sun, passing nearer to that luminary than any comet 

 before known. It passed its perihelion on December i8th, and, sweeping round 

 a large arc, went back in a direction not very different from that from which it 

 came. The observations have been calculated and the orbit investigated by many 

 astronomers, beginning with Newton ; but the results show no certain deviation 

 from a paraboHc orbit. Hence, if the comet ever returns, it is only at very long 

 intervals. Halley, however, suspected, with some plausibility, that the period ' 

 might be 575 years, from the fact that great comets had been recorded as appear- 

 ing at that interval. The "first of these appearances was in the month of Septem- 

 ber, after Julius Csesar was killed; the second, in the year 531; the third, in 

 February, 1106; while that of 1680 made the fourth. If, as seems not impossi- 

 ble, these were four returns of one and the same comet, a fifth return will be 

 seen by our posterity about the year 2255. Until that time the exact period 

 must remain doubtful, because observations made two centuries ago do not possess 

 the exactitude which will decide so delicate a point. 



Halley' s Comet. — Two years after the comet last described, one appeared 

 which has since become the most celebrated of modern times. It was first seen 

 on August 19th, 1682, and observed about a month, when it disappeared. Hal- 

 ley computed the position of the orbit, and, comparing it with previous orbits, 

 found that it coincided so exactly with that of comet observed by Kepler in 

 1607, that there could be no doubt of the identity of the two orbits. So close 

 were they together that, if drawn on the heavens, the naked eye would almost 

 see them join into a single line. The chances against two separate comets mov- 

 ing in the same orbit were so great that Halley could no doubt that the comet of 

 1682 was the same that appeared in 1607, and that it therefore revolved in a 

 very elliptic orbit, returning about every seventy-five years. His conclusion was 

 confirmed by the fact that a comet was observed in 153 1, which moved in appar- 

 . ently the same orbit. Again subtracting the period of seventy-five years, it was 

 found that the comet had appeared in 1456, when it spread such terror through- 

 out Christendom that Pope Claxitus ordered prayers to be offered for protection 

 against the Turks and the comet. This is supposed to be the circumstance 

 which gave rise to the popular myth of the Pope's Bull against the Comet. 



This is the earliest occasion on which observations of the course of the 

 comet were made with such accuracy that its orbit could be determined. If we 

 keep subtracting 75)^ years, we shall find that we sometimes fall on dates when 

 the apparition of a comet was recorded; but without any knowledge of the orbits 

 of these bodies, it cannot be said with certainty that they are identical. How- 

 ever, in the returns of 1456, 1531, 1607, and 1682, at nearly equal intervals, 

 Halley had good reason for predicting that the comet would return again about 

 1758. 



De Damoiseau announced that it would reach its perihelion on November 

 4th, 1835 ; while De Pontecoulant, after revising his computations with more 

 exact determinations r f the masses of the planets, assigned November 13th, at 



