66 LIGHT SCIENCE FOR LEISURE HOURS. 



noticed that what is required for the successful applica- 

 tion 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 pos- 

 sibly 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 very 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 unfavour- 

 able one at the end. 



Without entering on all the points of difficulty in- 

 volved by such considerations as these, we may simply 

 indicate the fact that the astronomer has a problem of 

 considerable complexity to solve in applying Halley's 

 mode of observation to a transit of Venus. 



It was long since pointed out by the French as- 

 tronomer 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 



