Jflf \0 ofc COMETi- 



to imagination, we must look for some circumstances in the 

 phenomenon itself, and in antilogy, to limit our Held of in- 

 quiry. One circumstance in the phenomenon itself, which 

 tends to answer this purpose, is, that the tail of a comet is 

 always on the side farthest from the Sun : and analogous to 

 Aurora borea- this on our globe we find the luminous corrnscattons, thatfre- 

 queutly accompany the aurora borcalis, likewise take place 

 in our iright, that is opposite to the Sun. Farther* some- 

 times we see in the mass of atmospheric fluids, all of whieh 

 gravitate toward the Earth, a fluid sometimes formed, which, 

 from the prodigious rapidity of its motion in u right line, ex- 

 periences no perceptible deflection from the action of gravity ; 

 which thus, if it dart vertically upward, may soon get out of 

 the sphere of gravitation toward our globe, and which is tu- 

 Electric fluid, minous in its course, i mean the electric fluid', for this, 

 though little deflected by gravitation in its rapid course, is 

 subjected to another law, which retains it in the atmosphere. 

 This law is its strong tendency to unite with the particles 

 Lightning. of the air, which occasions the lightning to disappear in its 

 horizontal movements, after the zigzags it has been forced 

 to make by the resistance of the air; zigzags which are not 

 perceptible in the corruscations of the aurora borealis. Sun- 

 Tails of comets pose then, that a fluid in all other respects similar to the 

 a similar fluid, electric is formed on that side of a comet which is turned 

 from the Sun ; a fluid so rapid in its rectilinear motion, 

 that it soon gets out of the sphere of gravity of that body, 

 without being detained by its atmosphere, and whieh is ren- 

 dered luminous by its decomposition on its course: thus we 

 shall have the phenomenon of the tail, with a circumstance 

 characteristic of these causes, namely the curvature of these 

 tails, the convexity of which is turned toward the side to 

 which they are moving ; the cause of which appears to me 

 to be as follows. 

 Cauii of their The particles of the fluid of the tail, as they are detached 

 curtature. from the comet, possess the same projectile movement with 

 it, and in the same direction. Accordingly they must con- 

 tinue to follow it. But if they-extend very far, that is to 

 say, if ihe tail become very long; the particles that proceed 

 the farthest, continuing to move with the s'ftmevelocity but 

 in-a'rarger orbit, must have a less angular movement. Thi» 



fe *3T must 



