TO PLEASANT WA YS IN SCIENCE. 



at any rate) in the case of the nearer members of the solar 

 family. Yet the method is one which, unlike others, will have 

 an accumulative accuracy, the discrepancies which are to test 

 the result growing larger as time proceeds. The method has 

 already been to some extent successful It was, in fact, by 

 observing that the motions of Mercury are not such as can 

 be satisfactorily explained by the perturbations of the earth 

 and Venus according to the estimate of relative masses 

 deducible from the lately discarded value of the sun's dis- 

 tance, that Leverrier first set astronomers on the track of 

 the error affecting that value. He was certainly justified in 

 entertaining a strong hope that hereafter this method will 

 be exceedingly effective. 



We come next to a method which promises to be more 

 quickly if not more effectively available. 



Venus and Mars approach the orbit of our earth more 

 closely than any other* planets, Venus being our nearest 

 neighbour on the one side, and Mars on the other. Looking 

 beyond Venus, we find only Mercury (and the mythical Vul- 

 can), and Mercury can give no useful information respecting 

 the sun's distance. He could scarcely do so even if we 

 could measure his position among the stars when he is at 

 his nearest, as we can that of Mars ; but as he can only then 

 be fairly seen when he transits the sun's face, and as the sun 

 is nearly as much displaced as Mercury by change in the 

 observer's station, the difference between the two displace- 

 ments is utterly insufficient for accurate measurement But, 

 when we look beyond the orbit of Mars, we find certain 

 bodies which are well worth considering in connection with 

 the problem of determining the sun's distance. I refer to 

 the asteroids, the ring of small planets travelling between 

 the paths of Mars and Jupiter, but nearer (on the whole *) 

 to the path of Mars than to that of Jupiter. 



* Only very recently an asteroid, Hilda (I53rd in order of detection), 

 has been discovered which travels very much nearer to the path of 

 Jupiter than to that of Mars a solitary instance in that respect Its 

 distance (the earth's distance being represented by unity), is 3*95, 



