432 LECTURE XLV. 



Our countryman Horrox* was the first that particular!}' attended to the 

 phenomena of a transit of Venus over the sun's disc : Dr. Halley,t when 

 he observed a transit of Mercury at St. Helena, thought that he could 

 ascertain the times of immersion and emersion without an error of a single 

 second ; and hence he concluded, that by means of a transit of Venus, the 

 sun's distance might be determined within a five hundredth part. The 

 most advantageous places for the experiment being such as differ most in 

 longitude, and are most remote from each other, Captain Cook was sent 

 by the British government to the South Seas, in the years 1761 and 1769, 

 in order to observe the transits of Venus in the island of Otaheite. 

 These observations were compared with those which were made at Ward- 

 huys, in Danish Lapland ; the difference of the times occupied by the 

 transit at these places was found to be 23 minutes 10 seconds, and from 

 this comparison, corrected by a number of collateral observations, the sun's 

 mean parallax was found to be 8 seconds and two thirds, or perhaps 8f ; 

 for it does not appear that we are sure of having avoided even an error of 

 one fortieth part of the whole ; although Mr. Laplace's determination of 

 the sun's distance, from the lunar motions, agrees very well with that 

 which is usually considered as the result of the observations of the transit 

 of Venus. J 



The comparative densities of the sun, and of such planets as have satel- 

 lites, may be calculated from the periods and distances of the bodies revolv- 

 ing round them ; the densities of the other planets have sometimes been 

 assigned from conjecture only, but of late years the mathematical theory 

 of the planetary perturbations has been rendered so perfect, that some 

 dependence may perhaps be placed on the density assigned to them from 

 calculations of this kind. It was formerly supposed that the densities of 

 the planets were regularly greater as they were nearer to the sun ; but it 

 is now certain that the Georgian planet is more dense than Saturn, and it 

 is probable that Venus is somewhat less dense than the earth. The mass 

 of the moon is deduced from a comparison of the effects of her attraction on 

 the earth and sea with those of the sun's attraction. 



The artificial globe serves as a useful instrument for determining, in a 

 rough manner, without calculation, the affections of the heavenly bodies at 

 particular times ; their places being first ascertained from tables, or, in the 

 case of the sun, merely from a scale on the globe's horizon, or on its surface. 

 We have only to adjust the elevation of the pole of the globe in such a 

 manner, that its axis may form the same angle with its horizon as the axis 

 of the earth does with the real horizon of the place ; then finding a point 

 on its surface corresponding to the place of the sun or planet, we may 

 represent its apparent motion by the motion of this point, and the time 

 occupied by that motion will be shewn by the index of the globe ; thus we 

 may find the length of the day and night, and the time and place of rising 



* See Hevelius, Mercurius in Sole visus Gedani An. 1661, cuiannexa est Venus 

 in Sole visa An. 1639, Liverpolise, a J. Horroxio, fol. Ged. 1662. 



t De Parallax! ope Veneris Determinanda, Ph. Tr. 1716, p. 454. 



I Euler on the Sun's Parallax, computed by Lexell, Ph. Tr. 1772, p. 69, makes 

 it 8"-55 ; Laplace, from the moon's motion, makes it 8"-6. 



