1 6 LIGHT SCIENCE FOR LEISURE HOURS. 



the determination of the distances and the magnitude of 

 the celestial bodies.* 



I propose here, after inquiring briefly into the 

 general question of the determination of the sun's 

 distance, to describe the nature of the opportunities 

 which will be afforded during the transit of 1874, and 

 to discuss the preparations which are being made by 

 this country to take her part in the work of observation. 

 It will be seen, as I proceed, that this discussion of the 



* I venture to quote here the appeal made by Halley (when Astro- 

 nomer "Roy*!") forty-five years before the transit of 1761, the earlier of 

 the pair of transits then looked forward to. It will show that, in dealing 

 with a transit 21 months before the date of its occurrence, I am not 

 looking forward so inordinately as might be supposed by those unfami- 

 liar with the nature of these inquiries. I should remark, however, that 

 since Halley's day other methods for determining the sun's distance have 

 been devised and employed. Six methods are described in my treatise 

 on the ' Sun,' and a seventh has, within the last few months, been 

 suggested by the great French astronomer Leverrier. Thus, then, wrote 

 Halley in 1716 : ' I could wish, indeed, that observations of the transit 

 should be undertaken by many persons in different places : first, because 

 of the greater confidence which could be placed in well-according obser- 

 vations ; and, secondly, lest a single observer should, by the interven- 

 tion of clouds, be deprived of that spectacle which, so far as I know, 

 will not be visible again to the men of this and the next century, and 

 on which depends the certain and sufficient solution of a most noble and 

 otherwise intractable problem. I therefore again and again urge upon 

 those inquiring observers of the celestial bodies, who, when I have de- 

 parted this life, will be reserved to observe these things, that, mindful 

 of my counsel, they should devote themselves strenuously and with all 

 their energies to conduct the observation ; I desire and pray that they 

 may be favoured in every way, and especially that they may not be 

 deprived of that most desirable spectacle by the inopportune darkness 

 of a clouded sky; and that, finally, the magnitudes of the celestial 

 bodies, forced into narrower limits (of exactness), may, as it were, make 

 submission to the glory and eternal fame of those observers.' 



These hopes were not fulfilled, so far as the transit of 1761 was con- 

 cerned; but the transit of 1769 was observed with great care at no less 

 than seventy-four stations, fifty of which, however, were in Europe. 



