514 Mr, herschel’s JJlronomical Obfervations 
more aperture of the objecft fpeculum than four inches ; 
and, I believe, that for diftin£tnefs of vifion this inftru- 
ment is perhaps equal to any that was ever made. 
OBSERVATIONS. 
November 30, 1779, fix o’clock in the morning, a 
rock, fituated near what hevelius calls Lacus niger 
major., was meafured to projedt 41", 56. To reduce this 
quantity into miles, put r for the femi-diameter of the 
Moon in feconds, as given by the Nautical Almanac at 
the time of obfervation, and 9. for the obferved quantity, 
alfo in feconds and centefimals ; then it will be in general 
r: 1090 :: q_: on, in miles. Thus it is found 
that 41", 56 is 46,79 miles. This diftance of the Sun, 
and Moon at the fame time was, by the Nautical Alma- 
nac, about 93 0 57'|. The fine of which to the radius 1 
is .9985, 8tc. and in this cafe, is lm = 46,85 miles. 
Then, by hevelius’s method the perpendicular height 
of the rock is found to be about one mile. 
The fame morning, a great many rocks, fituated 
about the middle of the dilk, projected from 25^,93 to 
26" 56. This gives on about 29,3 miles, and thefe rocks 
are all lefs than half a mile high. 
January 
