SIR JOSEPH BANKS. 341 



of much more rare occurrence, and which he could not 

 hope to see, as it was calculated to happen next in the 

 year 1761. He had shown 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. 

 Maskelyne, to St. Helena. The French Government 

 at the same time sent Le Gentil to Pondicherry, in the 

 East Indies, and La Chappe to Tobolsk, in Siberia, and 

 Pingre to Rodrigues, near the Mauritius. But the 

 weather proved so unfavourable that no certain con- 

 clusion could be derived from their observations ; for 

 though Pingre and Mason's observations proved after- 

 wards to be correct, they differed so widely from the 

 others that the whole subject remained in great un- 

 certainty. 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 dif- 

 fering from a right angle, that is, as unity to the sine 

 of the horizontal parallax. Henco the distance is equal 

 to the radius of the earth divided by the sine of that 

 very small angle. The distances from the sun of the 

 other planets are easily found, because we know their 

 relative distances ; and the real diameters of the sun 



