VENUS ON THE SUN'S FACE. 67 



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 application. 

 Our readers must bear with us while we briefly de- 

 scribe 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 

 select it would be impossible to form a clear con- 

 ception of the circumstances with which astronomers 

 are about to deal. There is, however, no real dif- 

 ficulty about this part of the subject, and we shall 

 only ask of the reader to give his attention to it for a 

 very brief space of time. 



Suppose the whole of that hemisphere of the earth 

 on which the sun is shining when the transit is about 

 to begin were covered with observers waiting for the 

 event. As Venus sweeps rapidly onwards to the 

 critical part of her path, it is clear that some of these 

 observers will get an earlier view of the commencement 

 of the transit than others will ; just as at a boat-race, 

 persons variously placed round a projecting corner of 

 the course see the leading boat come into view at 

 different times. Some one observer on the outer rim 

 of the hemisphere would be absolutely the first to see 

 the transit begin. Then rapidly other observers would 

 see the phenomenon ; and in the course of a few 



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