68 NOTES ON THE 



servable. Sir John Herschel says, " I can hardly 

 doubt that the comet was fairly evaporated in 

 perihelio by the heat and resolved into transparent 

 vapour, and is now in process of rapid condensation 

 and re-precipitation on the nucleus. During the 

 comet's retreat from the sun the tail began to be 

 developed. The nucleus became more bulky, hazy, 

 and ill defined, and its tail was strong, which after- 

 wards gradually and entirely disappeared." 



Henceforward the comet presented the appear- 

 ance of a round nebula, highly and very suddenly 

 condensed in the middle, which gradually died 

 away until finally lost. M. Boguslawski, professor 

 of astronomy, sixty days after the perihelion passage, 

 actually observed the cornet as a star of the sixth 

 magnitude a bright concentrated point. 



101. Is it not so, that the impact of a comet upon 

 our world is possible ; and w r hen we reflect upon 

 the number of comets which traverse the solar 

 system whose aphelion passage is beyond that of 

 our planet, but whose perihelion passage is within 

 the earth's orbit, is it not so, that the impact of a 

 comet upon our world is probable ? Moreover when 

 we consider the ages that have rolled on during the 

 past history of our world, and the countless num- 

 bers of comets which have traversed the solar 



