350 THE POPULAR SCIENCE MONTHLY. 



wlio have studied the astounding configuration of comets. Olbers be- 

 lieved in a rej)ulsive action ; Bessel in a polar force ; Bond, recently, 

 on the occasion of Donati's comet, so deeply studied in America, de- 

 duced from it a simply repulsive force, and M. Roche, of Montpellier, 

 adopted the same idea. 



At first sight, it seems very strange to find the same body pro- 

 ducing at once two opposite actions, an attraction and a repulsion. 

 Nevertheless, if these two forces act according to difierent laws, they 

 may coexist without being confounded in one single result, and may 

 produce perfectly distinct effects. It is thus that the Newtonian at- 

 traction, which subsists between the molecules of every individual 

 body, is by no means confounded or incorporated with the electrical 

 or magnetic phenomena of which that same body may be the seat, or 

 with the repulsive actions due to heat. 



Now, although the most delicate observation of the celestial move- 

 ments (planets and satellites) has hitherto revealed attraction alone, 

 it is impossible at the present day to deny that the striking phenom- 

 ena displayed by comets betray the existence of a quite difierent 

 fox'ce, capable of driving to a distance, with incredible velocity, the 

 most loosely attached and most attenuated particles of the matter 

 composing them. 



When a comet, arriving from the depths of the firmament, ap- 

 proaches the Sun, describing round him an immense ellipse almost 

 parabolic in form, it appears to us as a spherical nebulosity more or 

 less condensed toward its centre that is, in the shape assumed by a 

 body whose particles have freely taken their places under the sole in- 

 fluence of their mutual attractions. The sun's attraction (which at 

 that great distance is virtually equal for all those particles), does no 

 more than draw the comet toward it, as a whole, and in a lump as it 

 were, without afifecting its shape. But, when the distance diminishes, 

 the parts of this sphere nearest the sun are drawn with greater force 

 than the more distant parts diametrically opposite, and the primitive 

 sphei-ical figure can no longer subsist. The comet tends to grow 

 longer and longer in the direction of the ideal line which connects it 

 with tlie sun, absolutely in the same way as our globe, in its liquid 

 portion, is drawn out into two opposite swellings familiarly known as 

 ocean-tides. Nay, more : if the bond of mutual attraction which holds 

 the particles of the comet together is not sufficiently powerful, it will 

 give way ; under the sun's attractive action, the comet will be decom- 

 posed, scattering its materials along its orbit, gradually transform- 

 ing itself into a sort of very elongated ring of dust, like those which 

 Schiaperelli's discovery shows to be the cause of shooting-stars when 

 the earth happens to traverse them. 



This is all that can result from the sun's attraction. But matters 

 do not end here ; and comets which have resisted for ages the de- 

 structive agency of attraction, now present quite difierent phenomena, 



