ASTRONOMY AND METEOROLOGY. 403 



matter. Luminous phenomena precisely similar to thosfe here supposed 

 take place in the upper atmosphere of the earth, to which we have given the 

 name of Aurora Borealis and Aurora Australis ; and probably from the same 

 cause. They are almost uninterrupted at the pole during the long polar 

 winter, and only at intervals display their coruscations in the skies of the 

 temperate latitudes, where the changes of temperature are less, and the 

 vaporous columns assume a more oblique position. On the other hand, 

 while a comet is approaching the sun its temperature rises, and at the same 

 time its atmospheric electricity increases; condensations of aqueous vapor 

 and their attendant electric discharges are now much less frequent. It thus 

 happens that the evolution of vaporous matter to form the head and tail is 

 much less copious before than after the perihelion passage, and increases in 

 quantity for a certain interval of time after it. While these auroral phe- 

 nomena, as they may be styled, are thus subject to great fluctuations, and 

 to sudden interruptions, and are most prevalent in the polar regions * of the 

 nucleus, there Avould seem also to be an uninterrupted electric discharge from 

 all points of the nucleus turned toward the sun, continually detaching parti- 

 cles of aqueous vapor. This should be most abundant at the regions to 

 which the sun is vertical, and where the electric excitement produced by it 

 is the greatest, and may give rise to the hemispherical form of envelope. 



The phenomenon of separate concentric envelopes, or rings, often noticed, 

 shows that the vaporous matter set free at any time is not all expelled to the 

 same distance from the nucleus. This would be the case if we were looking 

 down upon the polar regions of a comet whose axis was perpendicular to 

 the plane of its orbit, and the matter was detached in zones from different 

 latitudes. It would seem, also, that different intensities of electrical discharge 

 should be attended with different velocities of projection. Upon the theo- 

 retical views I have formed, these electric variations should also give rise to 

 different intensities of repulsive action, as exerted by the nucleus. Again, if 

 all the particles set free should not be of the same size, the smaller ones 

 would experience the greater repulsive acceleration, provided the material 

 repulsion is of the nature of an impulsive action against the surface of the 



particle. 



If the speculative notions just presented be correct, the question arises 

 whether the earth may not be regarded, from our present point of view, as a 

 comet; and if so, why do we not see its luminous train. The proper answer to 

 this inquiry would seem to be, that the earth is actually, in a certain sense, a 

 comet, and that its -luminous train is seen by us in the zodiacal light. The 

 nebulous earth-ring contended for by the Rev. Mr. Jones, in explanation of 

 his admirable observations upon the zodiacal light, would seem then, in a 

 modified sense, to have a real existence; instead of being in a condition of 

 statical equilibrium, as supposed, it is in a dynamical condition of perpetual 

 dispersion and renewal. 2 



Nor is the expulsion of vaporous matter into the surrounding regions of 

 space confined to the nuclei of comets and the earth. It occurs at the sur- 

 face of the sun, and perhaps of all the heavenly bodies. It is beautifully 



1 It is to be observed that the motion of the nucleus in its orbit occasions a vir- 

 tual rotation around an axis perpendicular to the plane of the orbit, so far as 

 exposure to the sun is concerned. 



2 The vaporous matter which is incessantly streaming off from the sun into remote 

 space should enhance the brightness of the zodiacal light. 



