Mr. E. V. Neale on the Tails of Comets. 293 



of gas which escape from the body of the comet and expand 

 when they leave it are more resisted by the sun's atmosphere 

 than its more solid head, which therefore gradually overtakes 

 them ; so that they seem to fall back, till they constitute an 

 envelope round it, and then spread in a conical form behind 

 the head, through the joint operation of their own lateral move- 

 ments, of the increasing expansive force of the sun's heat as 

 the comet approaches the sun, of the increasing resistance of 

 the sun's atmosphere (whence the head must continually gain 

 more and more on the parts of the tail at first thrown off), and 

 of the pressure of fresh envelopes continually forming round 

 the nucleus as it advances. The body of the comet is con- 

 stantly moving through the gases or vapours which it throws 

 off, and thus produces the appearance of a tail, by leaving 

 each successive part thrown off more and more behind it. 



The central line of these successive envelopes would ob- 

 viously tend to be a straight line from the sun's centre through 

 the head of the comet ; though the continual change of direc- 

 tion in this line, as the comet approaches its perihelion, must 

 be liable to produce an apparent curvature in the tail, because 

 the parts first emitted, and therefore most distant from the 

 body, if they retain luminosity enough to be visible, must fall 

 more and more behind the advance of this central line in its 

 sweep round the sun. 



The phenomena of divided tails, of bright streaks, &c, find 

 a ready explanation in the accidental variety of pressures to 

 be expected among jets of gas or vapour emitted under such 

 circumstances, and the effect of perspective, according as we 

 happen to look through the edges or across the more central 

 parts of the envelopes forming the comet's tail — possibly com- 

 bined with actual variations of pressure in the sun's invisible 

 atmosphere, arising out of the enormous changes which can 

 be observed in its visible atmosphere. But what as to the 

 change in the direction of the tail when a comet has passed its 

 perihelion? Why clo comets then carry their tails before instead 

 of behind their heads ? Because the direction of the pressures 

 which produce the tail has changed. Given an invisible solar 

 atmosphere, a comet moving towards the sun will be perpe- 

 tually passing from a rarer into a denser medium, while a 

 comet moving from the sun will be perpetually passing from 

 a denser into a rarer medium. At the same time the jets of 

 gases or vapours which it will continue to emit from the ex- 

 pansive force of the sun's heat will then consist of particles 

 moving from the sun. Thus the two tendencies — the move- 

 ment of these particles due to the action of gravity, and the 

 tendency of the expansive force to exert itself in the line of 



