SCIENTIFIC SUMMARY. 
79 
matter of extreme tenuity. Hence tlie immense comets’ tails of 30, 40, and 
60 millions of leagues, "haying a direction away from the sun. These rarefied 
matters have a yery high velocity, as if solicited by a force twelve or fifteen 
times greater than that of gravity. Spectral analysis shows that comets 
have two hinds of light — one from solar illumination, the other proper to 
them, and characterised hy bright lines of a discontinuous spectrum, indi- 
cating incandescence of gaseous parts. The earth, too, viewed from afar, 
would present two spectra — that of solar light, and in the obscure part near 
the poles the discontinuous spectrum of its auroras, boreal and austral. The 
author does not think the feeble incandescence of cometary matter is caused 
hy solar heat, for the same rays do not produce such effects with us. If a 
screen were placed across the tail, the particles striking it would likely 
become suddenly incandescent. Now the nucleus is just such a screen, 
against which the anterior molecules of the nebulosity strike, producing 
heat and light ; while, on the other hand, molecules not thus arrested pass 
rapidly behind and from the tail. Thus there is a double effect. On our 
globe only the extreme and very rare layers of atmosphere have some 
analogy to these cosmic nebulosities, but they may give rise to some of 
these phenomena ; not tails indeed, for the greater attraction of the globe 
holds in the matter about it. But they might produce some feeble light- 
effects similar to those of comets if the repulsive force communicated to 
them in certain regions a considerable velocity, transferring them to other 
regions of our globe. The limits of our atmosphere are unknown, hut the 
true limit will be where our air, having become more rare than the vacuum 
in our best pneumatic machines, has been reduced to a medium doubtless 
comparable in density to the cometary nebulosities, on which the repulsive 
force of the sun acts. Consider this limit j it is not likely spherical. The 
lower layers of our atmosphere show hy the barometer a well-marked 
minimum of pressure at the poles, and maxima which do not coincide with 
the equator. Temperature and radiations produce great irregularities in 
them ; and it must he the same with the extreme layers. They probably 
experience on the side next the sun, the side on which they attain highest 
elevation, a repulsive force appearing in a slight pressure centrally, and 
movement at the edges. This limiting layer is thus conceived as having a 
less curvature, though a higher elevation, than that of the opposite side ; 
and, as in the inferior layers (though in greater degree), presenting a depres- 
sion near each pole on the right side, where the ground and inferior layers 
radiate least to the heavens. Then as to the edges of the hemisphere 
turned towards the sun. The superficial parts, reduced to extreme rarity, 
obey the repulsive force and are driven tangentially, acquiring considerable 
velocity in an hour or two. Beaching the depression near the poles, they 
enter the vacuum and rush across it. The earth’s attraction produces a 
strong curvature in their trajectories, and they meet the limiting surface 
of the atmosphere beyond the depression; and if their velocity may have 
reached several hundred metres per second, the incessant shock of these 
mobile particles against the fixed will give rise to light. The slight illu- 
mination which will be visible to us in a limited part of the heavens will 
have the characters of gaseous incandescence. This phenomenon will not 
occur equally all round the globe. In regions a littleTremoved from the 
