But while the solar rays traverse the earth's at mosphere, they suffer another change from the re sisting medium which they encounter. When the sun, or any of the heavenly bodies, arc considerably elevated above the horizon, their light is transmit ted to the earth without any perceptible change ; but when these bodies are near the horizon, their light must pass through a long tract of• air, and is considerably modified before it reaches the eye of the observer. • The momentum of the red, or greatest refrangible rays, being greater than the mo mentum of the violet, or least refrangible rays, the former will force their way through the resisting medium, while the latter will be either reflected or absorbed. A white beam of light, therefore, will be deprived of a portion of its blue rays by its hori zontal passage through the atmosphere, and the re sulting colour will be either orange or red, accord ing to the quantity of the least refrangible rays that have been stunt in their course. Hence the rich and brilliant hue with which nature is gilded by the set ting sun ; hence the glowing red which tinges the morning and evening clouds ; and hence the sober purple of twilight which they assume when their ruddy glare is tempered by the reflected azure of the sky.
We have already seen, that the red rays penetrate through the atmosphere, while the blue rays, less able to surmount the resistance which they meet, are reflected or absorbed in their passage. It is to this cause that we must ascribe the colour of the sky, and the bright azure which tinges the mountains of the distant landscape. As we ascend in the atmo sphere, the deepness of the blue tinge gradually dies away ; and to the aeronaut who has soared above the denser strata, or to the traveller who has ascended the Alps or the Andes, the sky appears of a deep black, while the blue rays find a ready passage through the attenuated strata of the atmosphere. It is owing to the same cause that the diver, at the bottom of the sea, is surrounded with the red light which has pierced through the superincumbent fluid, and that the blue rays are reflected from the surface of the ocean. Were it not for the reflecting; power of the air, and of the clouds which float in the lower regions of the atmosphire, we should be involved in total darkness by the setting of the sun, and by eve ry cloud that passes over his disc. It is to the mul tiplied reflections which the light of the sun suffers in the atmosphere, that we are indebted for the light of day, when the earth is enveloped with impenetrable clouds. From the same cause arises the sober hue of the morning and evening twilight, which increases as • we recede from the equator, till it blesses with per petual day the inhabitants of the polar regions.
If the earth were at rest, and not influenced by any other body of the system, its own figure, and , that of its atmosphere, would be exactly spherical. s In consequence of the diurnal motion of the earth, however, the figure of its atmosphere must be sphc roidal like itself. " All the atmospheric strata," says La Place, " should take after a time the rota tory motion, common to the body which they sur round. For the friction of these strata against each other, and against the surface of the body, should ac celerate the slowest motions, and retard the most rapid, till a perfect equality is established among them. In these changes, and generally in all those which the atmosphere undergoes, the sum of the pro ducts of the particles of the body, and of its atmo sphere, multiplid respectively by the area which their radii vectores projected on the plane of the equa tor describe round, their common centre of gravity, are always equal in equal times.
Supposing then, that by any cause whatever, the atmosphere should contract itself, or that a part should condense itself on the surface of the body, the rotatory motion of the body, and of its atmosphere, would be accelerated, because the radii vectores of the area, described by the particles of the primitive atmosphere becoming smaller, the sum of the pro duct of all the particles, by the corresponding area, could not remain the same, unless the velocity of ro tation augments.
At its surface the atmosphere is only retained by its weight, and the form of this surface is such, that the force which results from the centrifugal and at tractive forces of the body, is perpendicular to it. The atmosphere is flattened towards the poles, and distended at its equator ; but this ellipticity has li mits, and in the case where it is the greatest, the proportion of the axis of the pole and the.equator is as two to three.
The atmosphere can only extend itself at the equa tor, to that point where the centrifugal force exactly balances the force of gravity ; for it is evident, that beyond this limit the fluid would dissipate itself. Relative to the sun, this point is distant from its centre by the length of the radius of the orbit of a planet, the period of whose revolution is equal to that of the sun's rotation.
The sun's atmosphere then does not extend so far as Mercury, and consequently does not produce the zodiacal light, which appears to extend even to the terrestrial orbit. Besides, this atmosphere, the axis of whose poles should be at least two-thirds of that of the equator, is very far from having the lenticular form which observation assigns to the zodiacal light.