COMETS. 751 



of that amount, which is but little more than sufificient to overcome the attraction 

 of gravitation. 



It will be seen that it is equally erroneous to suppose any great amount of 

 material wasted in the formation of the tail, when one reflects upon the tran- 

 scendent lightness of its structure. 



HOW COMETS AFFECT THE EARTH. 



§ 6. The influence of comets upon the earth is in all probability quite in- 

 significant. They may, like the sun, affect the earth's magnetic condition, and 

 and thus to some extent, possibly, its meteorology. No such effect has ever been 

 perceived. In spite of some chance coincidences between the apparitions of great 

 comets and remarkable public events, no well-informed person now believes that 

 there is any real connection between them. By a liberal and credulous interpre- 

 tation of apy frequently occurring celestial phenomenon, similar coincidences 

 could be shown. 



When a comet is converted into meteoric bodies, which impinge upon the 

 earth's mtmosphere, there is some direct though probably minute effect. Some 

 have thought that a sensible portion of the heat which the earth receives is gener- 

 ated in this way; but the weight of scientific opinion seems to be against that hy- 

 pothesis. The impact of meteors upon our atmosphere must add some matter to 

 it, and this is probably in the form of dust. This may be the origin of the so- 

 called cosmic dust, which has been collected at sea in recent times. The finer 

 particles of it may have some influence on cloud formations, and other meteoro- 

 logical phenomena, but all this is merely conjecture. 



A more remote effect may be sought in the possible fall of meteors and 

 comets upon the surface of the sun. Owing to his vast bulk, the sun would at- 

 tract an immense number of these bodies; but it is quite certain that their effect 

 upon the sun's heat is insignificant. It is now generally admitted that we must 

 look for the origin of the sun's heat in a constant, though to us, imperceptible 

 shrinkage of his vast bulk. 



Some connection between the frequency of sun-spots and comets has been 

 rather vaguely suspected. Were the search for comets systematically pursued 

 with equal persistence for a long period, we might have some data for the forma- 

 tion of a sound opinion. Yet it would still be an open question, whether comets 

 cause the spots, or whether greater activity of the sun tends in some way to rend- 

 er comets brighter, so that more will be visible — with probability in favor of the 

 latter supposition. 



Finally, it may be said with all due respect to scientific decorum, that the 

 appearance of a great comet does exert one most happy influence on the earth, in 

 that it stimulates the curiosity of mankind and directs their thoughts to the more 

 particular contemplation of the glorious universe which surrounds them. 



V— 47 



