OUTLINES OF NATURAL PHILOSOPHY. 



This construction supposes the observers Q and O' to 

 be either exactly, or nearly in the plane of the or- 



.;- bit of Venus, but it may be extended to cases in 

 which that condition does not take place. Jt re- 

 quires, too, that the longitude of the places of ob- 

 servation should be accurately known. To avoid 

 the necessity of this determination, the durations 

 of the transit, as seen from different stations, have 

 been preferred, for ascertaining the parallax. If 

 we suppose observers, situated in respect of one 

 another, so that the line which Venus is seen to de- 

 scribe on the Sun's disk, is longer at the one sta- 

 tion than the other-, the duration of the transit 

 will be proportionally greater, and the difference 

 will evidently depend on the distance of the obser- 

 vers from one another, estimated in the direction 

 perpendicular to the lines which Venus traces out 

 on the surface of the Sun. The differences of du- 

 ration, therefore, depend on the parallax of the 

 3un, or on a. function of it; and therefore when 

 that function is known, the parallax may be infer- 

 red, from the comparison of one value of it with 

 another. See WOODHOUSE, Astron. p. 378, &c. 



The transit of 1769 was observed at Wardhus or 

 the North Cape, and also at Otaheite in the South 

 Sea, and was found to be longer at the former than 

 at the latter by 23'. 10". This difference, suppo- 

 sing the parallax to have been 8".83, should have 

 amounted to 23'.26 /; .95 ; and hence the parallax is 

 deduced = 8".72. VINCE, Ast. i. 622. Dr MAS- 



KELYNE'g 



