[ M3 j 
XXVI. Notice respecting a Volcanic Appearance in ithe Moon, 
in a Letter addressed to the President. By Captain HENRY 
Karer, F.R.S.* 
3 London, Feb. 8, 1821. 
Dzar Sir,— IL, may perhaps be interesting to the Royal Society 
to be informed, that on Sunday evening, the 4th instant, I ob- 
served a luminous spot in the dark part of the moon, which I 
Was inclined to ascribe to the eruption of a voleano. 
The telescope used was an excellent Newtonian of 64 inches 
aperture, with a power of 74. The moon was exactly two days 
old, and the evening so clear, that I was able to discern the ge- 
neral outlines in the dark part of her disk. Her western azimuth 
was about 70°, and her altitude about 10 degrees. 
In this position at 6 hours 30 minutes, the volcano was situ- 
ated (estimating by the eye) as in the accompanying sketch [di- 
stant from the northern limb of the moon about one-tenth of 
her diameter]. Its appearance was that of a small nebula sub- 
tending an angle of about three or four seconds. 
Its brightness was very variable; a luminous point, like a small 
star of the 6th or 7th magnitude, would suddenly appear in its 
centre, and as suddenly disappear, and these changes would some- 
times take place in the course of a few seconds. 
On the evening of the 5th, having an engagement which pre- 
vented my observing it myself, 1 arranged the telescope for two 
friends, who remarked the same phenomena as the night before, 
but in an inferior degree, partly perhaps in consequence of the 
evening not being so favourable. 
On the 6th, I again observed it; it had certainly hecome more 
faint, and the star-like appearance less frequent. 1 could see it 
very distinctly with a power of 40. As the moon approached 
the horizon, it was visible only at intervals when the star-like 
appearance took place. On the same evening I had the pleasure 
of showing it to Mr. Henry Browne, F.R.S. 
1 regret that 1 had no micrometer adapted to my telescope ; 
but I have reason to believe the distance of the volcano from the 
edge of the moon was about one-tenth of her diameter, and the 
angle it formed this evening with a line joining the cusps was 
about 50°, 
I remarked near the edge of the moon, a well known dark 
_ spot, from which the volcano was distant, as nearly as I could 
estimate, three times its distance from the edge of the moon. 
In a map of the moon published by Dr. Kitchener (and which 
is the best small map with which I am acquainted), there is a 
mountain sufficiently near the situation of the volcano, to autho- 
rize the supposition that they may be identical. 
* From the Transactions of the Royal Society for 182), Part I.’ 
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Vol, 58, No, 280, Aug. 1821, I On 
