60 LIGHT SCIENCE FOR LEISURE HOURS. 



nomer 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. 

 My readers must bear with me while I 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 I shall 

 select it would be impossible to form a clear concep- 

 tion of the circumstances with which astronomers 

 have to deal. There is, however, no real difficulty 

 about this part of the subject, and I 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 minutes 



