VENUS ON THE StJN'S FACE. 77 



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 favorable posi- 

 tion at the beginning of the transit to an unfavorable 

 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 

 others, the astronomer may inquire how much earlier 

 it begins or ends in some places than in others. 



Here is another artifice, extremely simple in prin- 

 ciple, though not altogether so simple in its applica- 

 tion. Our readers must bear with us while we briefly 

 describe the qualities of this second method, because in 

 reality the whole question of the transit and all the 

 points which have to be attended to in the equipment 

 and placing of the various observing parties depend 

 on these preliminary matters. Without attending to 

 them or at least to such primary points as we shall 



