t « ] 
•of the fun's limb, and A B C D a fe&ion of the 
fpot, SL the fun’s (emidiameter, LG the ob- 
lerved diftance from the limb, when the fide of 
the umbra changes, then will the plane <of the um- 
bra, CD, coincide with the line, EDO, drawn 
perpendicular toSL, at the point G. Let F H be 
a tangent to the limb, at the point D, and join S D. 
Since GL, the verfed fine of the angle LSD, is 
given by obfervation, that angle is given; which, by 
the figure, is equal to F D E, or G D H; which 
angle is therefore given, and is the angle of incli- 
nation of the plane of the umbra to the fun's fphe- 
rical furface. In the fmall triangle therefore C M D, 
which may be confidered as rectangular, the angle 
MDC is given, and the fide DC, equal to A B, is 
f iven nearly, by obfervation; therefore the fide 
1C is given, which may be regarded as the depth 
of the nucleus, without any material error. 
I had not an opportunity, in the courfe of the 
foregoing obfervations, to meafure the diftance G L, 
not having feen the fpOt, at the time when either 
of the fides of the umbra changed. It is however 
certain, that, when the fpot came upon the difc for 
the fecond time, thi-s change happened fometime 
in the night between the nth and 12th of De- 
cember; and 1 judge that the diftance of the plane 
of the umbra, when in a line with the eye, muft 
have been about i' 35 / from the fun's eaftern limb; 
from which we may fafely conclude, that the nu- 
cleus of the fpot was, at that time, not lefs than a 
:femidiameter of the earth, below the level of the fun’s 
Ipherical furface, and made the bottom of an ama- 
’ C 3 ; zing 
