COMETS. 



110. From the 16th, on which it presented the appearance 

 shown in fig. 28, to the 23rd, when it assumed the appearance 

 shown in fig. 29, the magnitude went on increasing, while the 

 illumination became more and more faint, and this continued 

 until the comet's final disappearance ; the outline, after a short 

 time, became so faint as to be lost in the surrounding darkness, 

 leaving a bland nebulous blotch with a bright centre enveloping 

 the nucleus. 



111. According to Mr. Hind, the number of comets which have 

 appeared since the birth of Christ in each successive century is as 

 follows : first century, 22 ; second, 23 ; third, 44 ; fourth, 27 ; 

 fifth, 16 ; sixth, 25 ; seventh, 22 ; eighth, 16 ; ninth, 42 ; tenth, 

 26; eleventh, 36; twelfth, 26; thirteenth, 26; fourteenth, 29; 

 fifteenth, 27; sixteenth, 31; seventeenth, 25; eighteenth, 64; 

 nineteenth (first half), 80. Total, 607. 



112. Since comets are visible only near their perihelia, when 

 their velocity is greatest, the duration of their visibility at any 

 single perihelion passage is generally short. The longest appear- 

 ance on record is that of the great comet of 181 1 (No. 8, Table VI., 

 " Hand-Book of Astronomy," chap, xviii.), which continued to 

 be visible for 510 days. The comet of 1825 (No. 2, Table VI. 

 " Hand-Book of Astronomy," chap, xviii.) was visible for twelve 

 months, and others which appeared since have been seen for eight 

 months. In general, however, these bodies do not continue to be 

 seen for more than two or three months. 



113. Considering the vast number of comets which ha\e passed 

 through the system, such an incident as the collision of one of 

 them with a planet might seem no very improbable contingency. 

 Lexell's comet was supposed to have passed among the satellites 

 of Jupiter ; and, if that was the case, it is certain that the motions 

 of these bodies were not in the least affected by it. The nearest 

 approach to the earth ever made by a comet was that of the comet 

 of 1684 (No. 55, Table VIII., " Hand-Book of Astronomy," chap, 

 xviii.), which eame within 216 semidiameters of the earth, a 

 distance not so much as four times that of the moon. We are 

 not aware of any nearer approach than this being certainly 

 ascertained. 



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