VOL. LXXXV.J PHILOSOPHICAL TRANSACTIONS. 487 



from which, and the little agreement that can be found between the elements of 

 the orbits of all the comets that have been observed, it appears clearly that they 

 may be directed to carry their salutary influence to any part of the heavens. 



My hypothesis, however, as before observed, does not lay me under any obli- 

 gation to explain how the sun can sustain the waste of light, nor to show that it 

 will sustain it for ever; and I should also remark that, as in the analogy of gene- 

 rating clouds, I merely allude to their production as owing to a decomposition of 

 some of the elastic fluids of our atmosphere, that analogy, which firmly rests on 

 the fact, will not be less to my purpose to whatever cause these clouds may owe 

 their origin. It is the same with the lucid clouds, if I may so call them, of the 

 sun. They plainly exist, because we see them; the manner of their being gene- 

 rated may remain an hypothesis; and mine, till a better can be proposed, may 

 stand good; but whether it does or not, the consequences I am going to draw 

 from what has been said, will not be affected by it. 



Before I proceed, I shall only point out, that according to the above theory, a 

 dark spot in the sun is a place in its atmosphere which happens to be free from 

 luminous decompositions; and that faculae are, on the contrary, more copious 

 mixtures of such fluids as decompose each other. The penumbra which attends 

 the spots, being generally depressed more or less to about half way between the 

 solid body of the sun and the upper part of those regions in which luminous de- 

 compositions take place, must of course be fainter than other parts. No spot 

 favourable for taking measures having lately been on the sun, I can only judge, 

 from former appearances, that the regions in which the luminous solar clouds are 

 formed, adding also the elevation of the faculae, cannot be less than 1843, nor 

 much more than 2765 miles in depth. It is true that in our atmosphere the 

 extent of the clouds is limited to a very narrow compass; but we ought rather 

 to compare the solar ones to the luminous decompositions which take place in 

 our aurora borealis, or luminous arches, which extend much farther than the 

 cloudy regions. The density of the luminous solar clouds, though very great, 

 may not be exceedingly more so than that of our aurora borealis. For if we con- 

 sider what would be the brilliancy of a space 2 or 3 thousand miles deep, filled 

 with such coruscations as v/e see now and then in our atmosphere, their ap- 

 parent intensity, when viewed at the distance of the sun, might not be much in- 

 ferior to that of the lucid solar fluid. 



From the luminous atmosphere of the sun I proceed to its opaque body, 

 which by calculation from the power it exerts on the planets we know to be of 

 great solidity; and from the phenomena of the dark spots, many of which, 

 probably on account of their high situations, have been repeatedly seen, and 

 otherwise denote inequalities in their level, we surmise that its surface is diver- 

 sified with mountains and valleys. 



