[ 2 I ] 
whence there will refnlt two values of the radius, 
which remains unknown, either of which will pro- 
duce an aberration equal to that of the other lens. 
V. ExtraS of a Fetter from the Abbe De la 
Caille, of the Royal Academy of Sciences 
at Paris, and F. R. S. to William Watfon, 
M. D. F. R. S. recommending to the Rev. 
Mr. Nevil Maikelyne, F. R. S. to make 
at St. Helena a Series of Obfervations for 
dif covering the Parallax of the Moon. 
Lincoln’s Inn-Fields, 8 Jan. 1761. 
Read Jan. 8 , 1 \R. Watfon lately received a letter 
1761 ' _LJ from the Abbe De la Caille at Paris, 
in which he takes notice, “ That although the pa- 
“ rallax of the moon feems fufliciently well deter- 
“ mined, by the obfervations made in 1751, in 
“ Europe and at the Cape of Good Hope j never- 
lc thelefs, an element of this importance cannot be 
“ too well afcertained. He is of opinion, that Mr. 
<c Mafkelyne’s continuance in St. Helena may be 
advantageoufly employed in making new obferva- 
<c tions ; fince the bafe, upon which thefe parallaxes 
“ fhould be calculated, fhould exceed the earth’s 
“ radius. 
“ That if the Royal Society does approve of his 
<c proportion, and recommend to Mr. Mafkelyne 
“ the execution of the fcheme of correfpondence, 
“ which 
