THE COMING TRANSIT OF VENUS. 25 



change must be accurately effected before two observa- 

 tions made in different longitudes can be compared as 

 respects the absolute time of their occurrence. On the 

 contraiy, Halley's method, while only requiring a rela- 

 tively rough determination of the longitude, can be 

 satisfactorily applied when the clocks employed are 

 simply well rated ; for it depends only on the duration 

 of the transit as seen at different stations. A clock 

 must be badly rated indeed utterly unfit, in fact, for 

 any astronomical use whatever which should lose a 

 single second in four or five hours. 



But the most important point to be noticed is, that 

 both methods ought to be employed, if possible, apart 

 from all nice considerations of their relative value. It is 

 certain that astronomers will place much more confidence 

 in closely concordant results obtained by the application 

 of these two methods, differing wholly as they do in 

 principle, than in as many and equally concordant 

 results all obtained by one method. A third method is 

 indeed to be applied, viz., a method based on the 

 ingenious use of photography. But as yet too little is 

 known respecting the chances of success by this method 

 to warrant too implicit reliance upon it. 



Let us inquire what preparations are being made by 

 astronomers, and especially by the astronomers of Eng- 

 land, to make adequate use of the opportunities pre- 

 sented by the coming transit. 



It has first, unfortunately, to be noted, that, so far 

 as this country is concerned, no provision whatever has 

 been hitherto made for the employment of Halley's 



