62 Dr. Herschel on the Nature and Construction 
tions; and that faculas are, on the contrary, more copious 
mixtures of such fluids as decompose each other. The pe- 
numbra 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 thereto 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 consider what 
would be the brilliancy of a space two or three thousand miles 
deep, filled with such corruscations as we see now and then in 
our atmosphere, their apparent intensity, when viewed at the 
distance of the sun, might not be much inferior 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 
upon 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 diversified with mountains and vallies. 
