THE EAUTH. 



9 



But what we have hitherto attempted to sketch, is but a smni! 

 part of that great fabric in which the Deity has thought proper 

 to manifest his wisdom and omnipotence. There are multitudes 

 of other bodies, dispersed over the face of the heavens, that lie 

 too remote for examination : these have no motion, such as the 

 planets are found to possess, and are, therefore, called fixed 



scarcely amount to any great convulsion ; but if the earth were actually to 

 receive a direct impulse from one of these bodies, the consequences irould 

 be awful. A new direction would be given to its rotatory motion, and the 

 globe would revolve round a new axis. The seas, forsaking their ancient 

 beds, would be hurried by their centrifugal force, to the new equatorial re- 

 gions ; islands and continents, the abodes of men and animals, would be 

 covered by the universal rush of the waters to the new equator, and every 

 vestige of human industry and genius at once destroyed. The chances 

 against such an event, however, are so very numerous, that there is no 

 dread of its occurrence. Various opinions have been entertained by astrono- 

 mers respecting the tails of comets. They were supposed by Appian, Cardan, 

 and Tycho Brahe, to be the light of the sun transmitted through the nucleus 

 of the comet, which they believed to be transparent like a lens. Kepler 

 thought, that the impulsion of the solar rays drove away the denser parts of 

 the comet's atmosphere, and thus formed the tail. Descartes ascribes the 

 tail to the refraction of light by the nucleus. Newton maintained, that it is 

 a thin vapour raised by the heat of the sun from the comet. Euler asserts, 

 that the tail is occasioned by the impulsion of the solar rays driving off the 

 atmosphere of the comet ; and that the curvature observed in the tail is the 

 joint effect of this impulsive force, and the grravitation of the atmospherical 

 particles to the solid nucleus. Mairan imagines that comets' tails are por. 

 tions of the sun's atmosphere. Dr Hamilton of Dublin supposes them to be 

 streams of electric matter ; and Biot supposes with Newton, that the tails 

 are vapours produced by the excessive heat of the sun ; and also, that the 

 comets are solid bodies before they reach their perihelion ; but that they are 

 afterwards either partly or totally converted into vapoiir by the intensity of 

 the solar heat Of all these theories, that of Euler seems to be most philo- 

 sophical. Since the comets are composed chiefly of nebulous matter, and 

 have very large atmospheres, the external atmospheric strata must be drawn 

 towards the comet by very slight powers of attraction, and will therefore 

 yield to the smallest impulse. From the great density of the planets, on the 

 contrary, and the small size of their atmospheres, the external strata are at- 

 tracted towards them with a very great force, and therefore cannot yield, 

 like those of the comets, to a slight impulse. Hence we see the reason why 

 the comets have tails, while none of the planetary bodies exhibit such a 

 phenomenon. Whatever opinion may be entertained of this explanation, it 

 must, at least, be admitted, that if light is a material substance, the atmo. 

 spherical particles of a comet may have their gravity diminished to such a 

 degree, either by their distance from its centre, or by the rarity of the nu- 

 deus, as to yield to the impulse of the solar rays, and be forced behind th» 

 nucleus, in the same maDner as smoke yields to the impulse of the geotiesk 

 hreezc. 



