ON" PnACTICAL ASTRONOMY. 566 



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

 the times of immersion and emersion Avithout an error of a single second; 

 and hence he concluded, that by means of a transit of Venus, the sun's dis- 

 tance might be determined within a five hundredth part. The most advan- 

 tageous places for the experiment being such as diifer most in longitude, 

 and are most remote from each other. Captain Cook was se^nt by the British 

 government to the South Seas, in tne years I76I and 1/69, in order to ob- 

 serve the transits of Venus in the island of Otaheite. These observations 

 were compared with those which were made at Wardhuys,iu 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 num- 

 "ber of collateral observations, the sun's mean parallax was found to be 8 se- 

 conds and two thirds, or perhaps 85; for it does not appear that we are 

 sure of having avoided even an error of one fortieth part of the whole; al- 

 though Mr. Laplace's determination of the sun's distance, from the lunar mo- 

 tions, agrees very well with that which is usually considered as the result of 

 the observations of the transit of Venus. 



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

 may be calculated from the periods and distances of the bodies revolving 

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

 from conjecture only, but of late years the mathematical theory of the planet- 

 ary 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 re- 

 gularly 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 par- 

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

 of the sun, m.erely 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, 



VOL. I. 4 a 



