[ i.SO ] 
XI. Notice respecting a volcanic appearance in the Moon , in a 
Letter addressed to the President. By Captain Henry Kater 
F. R. S. 
Read February 8, 1821. 
Dear Sir, 
London , February Stb, 1821. 
It may perhaps be interesting to the Royal Society to be 
informed, that on Sunday evening, the 4th instant, I observed 
a luminous spot in the dark part of the moon, which I was 
inclined to ascribe to the eruption of a volcano. 
The telescope used was an excellent Newtonian of 6 ^ 
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 general 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 
situated (estimating by the eye) as in the accompanying 
sketch. [See Plate X.] Its appearance was that of a small 
nebula subtending an angle of about 3 or 4 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 sometimes take place in the course of a few 
seconds. 
On the evening of the 5th, having an engagement which 
prevented my observing it myself, I arranged the telescope 
