292 On a luminous Appearance in the Moon. 
the appeasance, and find it about as distinct as it was about ten 
-minutes before I discontinued observing on the night of the 4th. 
The spot is certainly Aristarchus; but it is now much more 
difficult to observe on account of the moon throwing much more 
light down the tube of the telescope, and the luminous advancing 
edge being much nearer the spot, and my telescope having a 
large aperture: but I should imagine, if my 42-inch tube were in- 
closed in one which projected five or six feet beyond the object- 
glass, that the spot might be seen one night at least longer. 
When I first examined on the 4th the proportion of light thrown 
on the moon by the earth, and consequently on Aristarchus, was 
1-777 out of 2000 ; to-night it was only 1-422, a diminution of 
*355 5 consequently exactly one-fifth less light is reflected by 
the spot, to say nothing of the inconvenience arising from the 
addition of one-fifth to the light of the moon. Hevelius de- 
scribes Aristarchus under the name Mons Porphyrites, as aut 
ex rupe rubra, aut sabulo (this, by the by, is impossible; for the 
moon’s attraction of gravity to its centre would not admit of a 
cavity of sand (loose sand) similar to Aristarchus) sive ¢errd ru- 
bicunda constare, aut prorsus ardere, sive perpetuo igne exun- 
dare. Its colour must therefore have greatly changed since 
1644, for it is singularly white when illustrated by the sun; and 
when the other parts of the moon are yellow, or faintly red, this 
preserves its predominant whiteness; and its appearance on the 
4th and 6th instants was similar to the light of the glow-worm. 
Could any light, such as we read is occasionally seen on the 
mountains of Asia Minor, or the phosphoric fire near Derbend, 
be peculiar to this cavity of the moon ? and if so, has it changed, 
and does it change the colour of its flame ? 
Your polite attention to me when in town has occasioned my 
taking the liberty of troubling you with these hasty observations, 
which I would have put into a more regular form, but I am go- 
ing to the philosophical lectures.at Birmingham this evening ; 
and, in order to save this day’s post, I must now conclude with 
begging you to accept my esteem and thanks. 
I am, dear sir, 
Yours very sincerely, 
Tamworth, May 4, 1821. Micuarnt Warp, 
LXVIIL. No- 
