Anniversary Address. xxvii. 



Dr. Halley, of comet renown, who died in 1742, left to his 

 countrymen, not only the famous prediction of the return of his 

 comet, which has since been twice verified at the appointed times, but 

 also the most earnest recommendation to observe the transits of 

 Venus of 1761 and 1769. Halley' s injunction was well obeyed, 

 especially in 1769, when the observations of Captain Cook's expedi- 

 tion at Otaheite, combined with observations at other stations in 

 various parts of the world, resulted in the conclusion, until recently 

 relied on as correct, that 95,000,000 miles is the true distance of the sun. 



The mention of the honoured name of Cook recalls to mind how 

 close the association is between the observation of the transit of Venus 

 in 1769 and the rediscovery and settlement of these southern lands. 



The principle of the observation is that followed by the surveyor 

 in ascertaining the distance of an inaccessible object. A line is 

 measured on the ground, also the two -angles which it forms with the 

 inaccessible point ; the third angle of the triangle is then inferred, 

 and the computation of the distance required is one of the simplest in 

 plane trigonometry. But the distance to the inaccessible sun is so 

 immeasurably great that any base line which the surveyor could mark 

 off on the earth's surface would be as useless for the purpose as a 

 mathematical point. Even if we could stay the sun in his course, and 

 grant other impossible conditions, the most delicate instrument would 

 fail to show any convergence of the sides of the wished-for triangle. 

 In other words, there would be no parallax. 



The solution of the problem must be tried in some other way, 

 and the most obvious thing to do, in the first instance, is to increase 

 the length of the base. The longest possible base on the earth is, oi 

 course, the diameter of the earth itself. By placing observers suitably, 

 in widely separated parts of the globe, the longest practicable base will 

 be obtained, but still the problem is insoluble, unless we can have some 

 intermediate body of known rate of circular motion coming in line 

 between the observers and the sun. In the problem before us, Venus 

 is that body, and, as she is, at transit, nearer the earth than the sun 

 in the ratio of about 2 to 5, it will be seen that, to observers widely 

 apart, Venus must necessarily come in line with the edge of the sun 

 at different times to the two observers ; just as would be the case were 

 two observers, standing apart on the bank of a river, each to signal 

 as a passing boat came in line with a tree on the opposite bank. It 

 would be seen that an interval of time elapsed between the two signals. 

 This interval, in the case of Venus, gives the measure of the angle 

 subtended at the sun by the base line joining the stations of the 

 observers. For the rate of the motion of Venus relative to that of 

 the earth being known, the interval observed is convertible into 



