COLLECTIONS AND RECOLLECTIONS. 
157 
The Moon.— In the annual circuit of the earth about the sun, it 
is constantly attended by its satellite the moon, which revolves round 
it, or rather both round their common centre of gravity : while this 
centre, strictly speaking, and not either of the two bodies thus con¬ 
nected, moves in an elliptic orbit, undisturbed by their mutual action, 
just as the centre of gravity of a larger and smaller stone tied toge¬ 
ther and flung into the air describes a parabola as if it were a real 
material substance under the earth’s attraction, while the stones cir- 
* . 
culate round it or round each other, as we choose to conceive the 
matter. 
If we trace, therefore, the real curve actually described by either 
the moon’s or earth’s centres, in virtue of this compound motion, it 
stantly circulating in the sun’s immediate neighbourhood, or traversing the pla¬ 
netary spaces, and exciting in the upper regions of its atmosphere, these phe¬ 
nomena of which, on however diminutive a scale, we have yet an unequivocal 
manifestation in our aurora borealis. The possible analogy of the solar light 
to that of the aurora, has been distinctly insisted on by my father, in his paper 
already cited. It would be a highly curious subject of experimental enquiry 
how far a mere reduplication of sheets of flame, at a distance one behind the 
other (by which their light might be brought to any required intensity,) would 
communicate to the heat of the resulting compound ray the penetrating charac¬ 
ter which distinguishes the solar calorific' rays. We may also observe, that the 
tranquillity of the sun’s polar, as compared with its equatorial regions (if its 
spots be really atmospheric,) cannot be accounted for by its rotation on its axis 
only, but must arise from some cause external to the sun, as we see the belts of 
Jupiter and Saturn, and our trade-winds, arise from a cause, external to those 
planets, combining itself with their rotation, which alone can produce no mo¬ 
tions when once the form of equilibrium is attained. 
The prismatic analysis of the solar beam exhibits in the spectrum a series of 
“ fixed lines,” totally unlike those which belong to the light of any known ter¬ 
restrial flame. This may hereafter lead us to a clearer insight into its origin. 
But before we can draw any conclusions from such an indication, we must re¬ 
collect, that previous to reaching us, it has undergone the whole absorptive ac¬ 
tion of our atmosphere, as well as of the sun’s. Of the latter we know nothing, 
and may conjecture every thing ; but of the blue colour of the former, we are 
sure; and if this be an inherent (i. e. an absorptive) colour, the air must be ex 
pected to act on the spectrum after the analogy of other coloured media, which 
often (and especially light blue media) leave unabsorbed portions separated by 
dark intervals. It deserves enquiry, therefore, whether some or all the fixed 
lines observed by Wallaston and Fraunhofer may not have their origin in 
our own atmosphere. Experiments made on lofty mountains, or the cars of 
balloons, on the one hand, and on the other with reflected beams which have 
been made to traverse several miles of additional air near the surface, would de¬ 
cide this point. The absorptive effect of the sun’s atmosphere, and possibly 
also of the medium surrounding it (whatever it be,) which resists the motions of 
comets, cannot be thus eliminated.— Author. 
