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LIX. On a possible Cause of the Formation of Comets' Tails. 

 By A. S. Davis, M.A* 



IT is well known that the phenomena observed during the 

 formation of a comet's tail point to the conclusion that 

 the material which forms that appendage is being continually 

 emitted from the head of the comet with great velocity by 

 some force acting in a direction directly away from the sun. 

 The material appears in most cases to be first ejected from that 

 side of the comet's nucleus which is turned towards the sun, 

 and afterwards, under the influence of this force, to be turned 

 backwards to form a tail. It is the object of this paper to 

 suggest an explanation of this force. 



The remarkable identity which has been found to exist be- 

 tween the orbits of certain comets and the orbits of certain 

 meteoric clouds renders it little short of certain that comets 

 are themselves masses of solid or liquid bodies separated from 

 one another by great intervals, except perhaps at the nucleus, 

 where they may be closer together, and may even contain a 

 solid core. The spectroscope indicates the presence of gas in 

 a state of incandescence in the comet's head and nucleus ; but 

 how this incandescent gas is produced is not known. 



The violent action which is observed to take place as a comet 

 approaches the sun, on that side of its nucleus which is 

 exposed to the solar radiation, appears to indicate that the 

 comet consists largely of matter which is rapidly volatilized 

 under the influence of the sun's rays. 



Let us assume that such is the case, and let us consider 

 what will be the effect of evaporation on the motion of one of 

 the bodies undergoing it. 



In the first place the mass of a comet is so small, that the 

 force of gravitation towards the centre on any of the bodies at 

 some distance from the nucleus must be so small that it may 

 be left out of consideration. We know that a molecule of 

 matter in the gaseous condition has at ordinary and high tem- 

 peratures a very quick motion of translation. A molecule, as 

 it evaporates from the surface of one of the bodies composing 

 the comet, must acquire a velocity relative to the body of 

 several hundred yards per second. The body must in conse- 

 quence suffer a recoil in an opposite direction to that in which 

 the molecule escapes. Now since the evaporation is caused 

 by the sun's heat, it must take place chiefly on that side of the 

 body which is exposed to the sun's rays. The resultant effect 

 of all the small recoils due to the evaporation of the different 

 molecules will therefore be to drive the body in a direction 

 away from the sun. If the body has a motion of rotation, the 

 * Communicated by the Author. 



