PROF. O. REYNOLDS ON COMETARY PHENOMENA. 193 



showing signs of the same action. Now although at first 

 sight there seems nothing improbable in the supposition 

 that the sun causes a great amount of evaporation on a 

 comet, yet before we can admit it as altogether satisfac- 

 tory it is necessary to show why the same action should 

 not go on to the same extent on the earth and on the 

 planets; for neither do the planets show this same ap- 

 pearance, nor are we aware of any action on the earth 

 which could give rise to these appearances. I do not 

 know that any attempts have as yet been made to explain 

 this ; but I think an explanation may be found in the dif- 

 ference between planets and comets in their size and the 

 shape of their orbits, in the fact that the planets are, so 

 to speak, large bodies moving in approximately circular 

 paths and so remaining at about the same distance from 

 the sun, while comets are small and move in eccentric 

 paths, continually altering their distance from the sun. 

 I have (in the subsequent part of this paper) endeavoured 

 to state these reasons, and, further, to show that the differ- 

 ence of evaporation on a comet and on a planet is a suffi- 

 cient cause for electrical phenomena on the former which 

 do not take place on the latter. 



I think that the reason why the materials of comets 

 should at times (i. e. when the comets are in certain posi- 

 tions) evaporate under the sun's heat in a greater degree 

 than those of planets will be rendered clear by con- 

 sidering the reasons why the heat of the sun does not 

 continually evaporate the materials of the earth — always 

 remembering : — 



ist, That comets move in eccentric, and planets in 

 nearly circular orbits ; 



2nd, That comets are very much smaller in mass or 

 weight than planets. 



Why, then, does not the heat of the sun evaporate 

 the materials of the earth? The heat which the sun is 



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