VENUS ON THE SUN'S FACE. 59 



that what is required for the successful application of 

 the method is that one set of observers should be as far 

 to the north as possible, another as far to the south, 

 so that the path of Venus may be shifted as much as- 

 possible. Clearly the northern observers will see her 

 path shifted as much to the south as it can possibly be,, 

 while the southern observers will see the path shifted as 

 far as possible towards the north. 



One thing, however, is to be remembered. A transit 

 lasts several hours, and our observers must be so 

 placed that the sun will not set during these hours* 

 This consideration sometimes involves a difficulty. 

 For our earth does not supply observing room all 

 over her surface, and the region where observation 

 would be most serviceable may be covered by a widely- 

 extended ocean. Then again, the observing parties 

 are being rapidly swayed round by the rotating earth ;. 

 and it is often difficult to fix on a spot which may not, 

 through this cause, be shifted from a favourable position 

 at the beginning of the transit to an unfavourable one 

 at the end. 



Without entering on all the points of difficulty in- 

 volved by such considerations as these, I may simply 

 indicate the fact that the astronomer has a problem of 

 considerable complexity to solve in applying Halley's 

 method of observation to a transit of Venus. 



It was long since pointed out by the French astro- 

 nomer Delisle that the subject may be attacked another 

 way that, in fact, instead of noticing how much longer 

 the transit lasts in some places than in others, the astro- 



