TdL. LII.J PHILOSOPHICAL TRANSACTIONS. . SIQ^ 



y. Extract of a Letter from the Ahhe De la Caille, of Paris, and F. R. S. to 

 William Watson, M. /)., F. R. S., recommending to the Rev. Nevil Maskelyne, 

 F.R.S., to make at St. Helena a Seriet of Observations for discovering the 

 Parallax of the Moon. p. 21. 



The Abbe takes notice, " That though the parallax of the moon seems suffi- 

 ciently well determined, by the observations made in 1751, in Europe and at the 

 Cape of Good Hope; yet an element of this importance cannot be too well as- 

 certained. He is of opinion that Mr. Maskelyne's continuance in St. Helena 

 may be advantageously employed in making new observations ; since the base on 

 which these parallaxes should be calculated, ought to exceed the earth's radius. 

 That if the e. s. approve of his proposition, and recommend to Mr. Maskelyne 

 the execution of the scheme of correspondence which he has drawn up, he 

 promises to comply with it punctually on his part. 



The Abbe has accordingly sent Dr. Watson a series of observations, which he 

 recommends to Mr. >laske1yne to make, from the 13th of June 1761, a few 

 days after the transit of Venus, till the Qth of May 1762. This paper Mr. 

 Maskelyne has transcribed, and proposes to make these observations in concert 

 with the Abbe de la Caille. And if a copy of this paper, which Dr. Watson 

 proposes to lay before the Society at their next meeting was put into the hands 

 of Dr. Bradley, that gentleman might likewise make correspondent observations. 



The Abbe likewise adds, ' that he has supposed that the sector, which Mr. 

 Maskelyne takes with him to St. Helena, would take in 54- degrees on each side 

 the zenith ; and that his clock would be regulated by sydereal time.' This sector 

 extends much beyond the Abbe's expectation, as it takes in 8-^ degrees on each 

 side of the zenith. 



