246 Dr Lehmann's Inquiries regarding 



cleus and the particle towards the sun. Thus, in all cases, the 

 particle will remove from the nucleus with an increasing rapi- 

 dity on the side opposite the sun. In conformity with this, we 

 often see the tail attaining an astonishing length in a short time, 

 while at first sight it might seem that the diiference which ori- 

 ginally exists between the distance of the nucleus and that of 

 the particle from the sun, compared with this very distance, is 

 too small a quantity for the diminution that must result from it 

 in the gravitation ever having an appreciable influence, or being 

 capable of determining the formation of a tail. But this forma- 

 tion depends essentially upon the circumstance that, according 

 to my supposition, the comet always presents the same side of 

 the sun. Its extension to a length of several millions of miles, 

 will then be possible, because all the causes which tend to aug- 

 ment this length, act without interruption in the same direction. 

 If, on the contrary, the comet should turn on its axis like the 

 planets, the enormous increase of its tail from the causes which 

 I have just exposed, could not take place. 



How does it now happen that the tail decreases after the pas- 

 sage to the perihelion ? It is not difficult to reply to this ques- 

 tion, by means of the principles already laid down. When the 

 comet approaches the sun, two causes concur to retard the 

 growth of its tail. The first resides in the law of aerostatics, 

 which determines that the power of expansion diminishes with 

 the density. The tail will not therefore increase so quickly as 

 it would have done, had the power of expansion remained the 

 same. The other cause is connected with the circumstance 

 that we cannot see objects which transmit too little light to our 

 eyes. There results from this, that we can no longer distinguish 

 the extreme parts of the tail, when it is much extended and con- 

 sequently highly rarified, because a diminution of lustre is ne- 

 cessarily connected with this rarefaction of an atmosphere lumin- 

 ous in itself. For these two reasons, the tail will seem to us 

 to be shorter, and to increase more slowly. It is very difficult 

 to distinguish the limits of the atmosphere of a comet, or its 

 last layers : They escape our view, on account of their extreme 

 rarity, the passage to vacuity being effected in a manner entire- 

 ly imperceptible. There follows from this that the tail of a co- 

 met may appear so much the shorter that we employ a greater 



