344 SIR JOSEPH BANKS. 



was calculated to happen next in the year 1761. He 

 had shewn how complete a measure that phenomenon 

 would afford of the sun's parallax, or the angle subtended 

 by the earth's radius at the surface of the sun. This 

 angle could be with great accuracy best ascertained by 

 different contemporaneous observations at distant points 

 of the arc which the planet described in its passage, the 

 planet affording, as it were, an object between the sun 

 and the earth, a kind of signal-post, by means of which 

 the angle sought might be measured. 



Accordingly, in 1761 the British Government sent one 

 observer, Mr. Mason, to the Cape, and another, Dr. Maske- 

 lyne, to St. Helena. The French Government at the 

 same time sent Le Gentil to Pondicherry, in the East 

 Indies, and La Ohappe to Tobolsk, in Siberia, and Pingre 

 to Rodrigues, near the Mauritius. But the weather proved 

 so unfavourable that no certain conclusion could be 

 derived from their observations ; for though Pingre and 

 Mason's observations proved afterwards to be correct, 

 they differed so widely from the others that the whole 

 subject remained in great uncertainty. A second transit 

 was expected in 1769, and the British Government now 

 sent an astronomer (Mr. Green) again to make those 

 important observations. 



The great value of the object in view is manifest. If 

 we can ascertain the parallax, we have, by an easy process, 

 the exact distance of the sun from the earth; for, as in 

 every triangle the sides are as the sines of the opposite 

 angles, the distance of the sun must be to the earth's 

 radius as the sine of an angle not sensibly differing from 

 a right angle, that is, as unity to the sine of the paral- 

 lax. Hence the distance is equal to the radius of 



