the Formation of the Tails of Cornets. 245 



forces into two others, and of which one is parallel to the radius 

 vector ; there are, then, six forces, of which we have only to con- 

 sider the three that act in the direction of the radius vector. We 

 shall consider the nucleus as a solid body, such as the atmosphere 

 which surrounds it cannot penetrate, and that the densest part of 

 this atmosphere rests upon its surface. 



If we consider the parts of the atmosphere situated on the side 

 opposite the sun, it is plain, that being somewhat more distant 

 from the sun than the nucleus, they will be attracted by the sun 

 with less power than the nucleus itself. The component of the 

 attraction exercised by the nucleus upon the atmosphere will 

 therefore be diminished by the attraction of the mass of the sun. 

 While this diminution will remain insensible, on account of the 

 too great distance of the comet from the sun, the gravitation of 

 the particles of the atmosphere toward the nucleus will be in 

 equilibrium with the power of expansion ; in the same manner 

 as the expansive power of the earth*'s atmosphere remains in equi- 

 librium with its weight, so long as the air is tranquil. But, in 

 proportion as the comet approaches the sun, this diminution will 

 become more and more sensible ; the component of the power of 

 expansion, directed according to the radius vector, will gain 

 more and more upon the component of the gravitation toward 

 the nucleus, in the same direction; it will therefore remove 

 from the nucleus the parts of the atmosphere, situated on the 

 side opposite the direction, and will form a tail. 



The tail which will be formed in this manner^ will always 

 elongate more, and will always increase with more rapidity, be- 

 cause several causes acting in the*same direction unite to accele- 

 rate this increase. In the first place, every motion once im- 

 pressed, will continue of itself in one direction, and with a con- 

 tinued celerity. Secondly, if the comet approach the sun, the 

 diminution of intensity of the gravitation toward the nucleus be- 

 comes on this very account always more sensible. In the third 

 place, when even a particle of the atmosphere is in motion, for 

 the purpose of removing from the nucleus, the difference which 

 there is between the distances of the sun from the nucleus and 

 the particle, goes on continually increasing, and consequently it 

 is the same with the difference between the gravitation of the nu^ 



