Kaxsas Academv of SIciexce. 15 



and preceded tliera in going away, attaining their maximum soon after i)ass- 

 ing })erilK'li()U. Tliis it is tliat huti so puzzled astronomers. The nucleus 

 evidently obeys the law of gravity, but in passing perihelion the tail sweeps 

 around contrary to that law, and with a vivacious energy that Avould laugh 

 gravity to scorn. 



It has been suggested that the matter composing the comets' tails differs 

 essentially from that of the nucleus, being of a nebulous character, or at any 

 rate in a nebulous condition; though the exact nature of this character or 

 condition of matter no one could define, but it was thought to be mistv-like, 

 or vaporous and extremely rarefied. It was further thought that some 

 repulsive force, emanating in the sun, acted on the nebulous fog of the tail, 

 while it did not act on the nucleus, thus giving to the sun a double-dealing- 

 character, blowing hot and blowing cold at the same time, quite at variance 

 with his usual character for consistency. Bessel computed the form and 

 motion of the comets' tails subject to such a repulsive force in the sun ; and 

 after him. Pierce. Bessel supposed that the material elements of the tail 

 were developed from the nucleus, floAving away from it and soon lost to iff 

 that is, became separated from it so far that the attraction of the nucleus no 

 longer had an appreciable eflect upon them, while the repulsive force of the 

 sun carried them off still further and dissipated them entirely from view. 

 But this hypothesis involves the difficulty of supposing that the same matter 

 which originally formed a part of the nucleus, and as such moved subject to 

 the attractive force of the sun, afterwards becoming evolved from the nucleus, 

 was then repelled by the force of the same sun with an increased energy. 



Now a change in the molecular condition of the elements of a body may 

 often develop repulsion among those elements, as for instance, the ignition 

 of powder, or even an increase of temperature, and so we may understand 

 the development of a repelling force whose center is situated within the 

 nucleus; but in Bessel's hypothesis the center of the repellant force is 

 supposed to be in the sun, millions of miles away. 



Though Bessel's hypothesis may have given away in the light of more 

 recent investigations, yet a statement of it in this place may help to present 

 in a clearer light the real difficulties in the problem before us. And in 

 looking for all the facts that may have a bearing on the question, let us not 

 forget that the space between the siui and planets, and even that between our 

 solar system and other systems, cannot be absolutely void, for light, whether 

 it come from our sun as the bright harbinger of day, or whether it come in 

 the faint twinkling that reaches us after a tireless flight of centuries, in either 

 case travels along the waves of celestial ether of unbroken continuity. 

 Where the terrestrial atmosphere fades away, or at what limits the solar 

 atmosphere loses itself in the etherial depths, we know not, yet it has been 

 clearly established that flames of meteors become visible in the neighborhood 

 the earth at a much greater distance from the earth than the supposed 

 limits of our atmosphere, these flames apparently resulting from the resist- 

 ance of the medium through which the meteors pass ; and the zodiacal lights 



