355 
on the Atmosphere of the Moon. 
But admitting the height of the atmosphere, which may affect 
the brightness of a fixed star, not to be less than 5376 feet, this 
will amount to an arc of only 0,94 ", or not quite one second ; 
and as the moon describes an arc of 1" in 2" of time, it follows 
that in general the fading of a star, which approaches to an 
occultation, cannot last quite 2" in time; that if theappulse be 
at a part of a limb of the moon w'here a ridge of mountains 
interferes, the gradual obscuration will last a still shorter time ; 
and that it may, under some circumstances of this nature, be 
even instantaneous. 
Supplement to the foregoing Observations on the Atmosphere of 
the Moon. 
The following observation of an occultation of Jupiter by 
the moon, when near its full, occurred to me by mere acci- 
dent, on the 7th of April, 1792 ; and I was the more gratified 
by it, as I had long wished for an opportunity to observe such 
a phenomenon, and had no expectation of seeing it at this 
time, no mention being made of it in the Ephemerides. 
The sky being very serene, and Jupiter uncommonly bright, 
I prepared my seven-feet reflector, magnifying 74 times, in 
hopes that the strong light and distinctness it afforded would 
enable me to compare the appearances of this phenomenon 
with the results which I had deduced from my late observations 
on the height and density of the atmosphere of the moon. 
Fig 1. Tab. IX. represents the situation of Jupiter’s four 
satellites, as they appeared, most distinctly, two of them to 
the westward, the second about one, and the first near two of 
Jupiter's diameters distant from its limb; and the two others 
