28 LIGHT SCIENCE FOR LEISURE HOURS. 

 advantage, but even more advantageously than De- 



lowered, and therefore manifestly will be lengthened. From southern 

 stations, the line will be raised, and therefore shortened. We therefore 

 set an observer at as northerly a station as we can, to get as great a 

 lengthening as we can, and that is one point gained. We set an observer 

 at as southerly a station as we can, and so get as great a shortening as 

 possible, and that is a second point gained. But it is easily shown (we 

 do not trouble our readers with the proof) that our northerly observer 

 is so shifted by the earth s rotation while the transit is in progress that 

 Venus is seemingly hastened on her course in transit. This shortens 

 the time of transit at the northern station, and is discordant with the 

 lengthening obtained by setting an observer as far north as possible. 

 Here, then, is one point against us. Lastly, the southern station can 

 be taken so as to give either a hastening or a retarding of Venus s 

 motion, simply because the transit occurs in the southern summer, when 

 places far south have no night, so that we can set the observer either 

 where he will have the sun moving from east to west during the transit, 

 or where he will have the sun moving from west to east. We set our 

 observer so that Venus is hastened (which is secured by taking a station 

 where, during the transit, the sun moves from east to west). This 

 hastening is manifestly accordant with the shortening of her path at 

 southern stations, and thus we get a third point in our favour. We 

 have, then, three points in our favour and one against us, or * balance 

 of only two favourable points. 



Now, in 1882, Venus crosses the lower part of the sun s face, or some 

 what as from figure VII to figure IV on a clock-face. In this case, the 

 northern station gives the lowest or shortest course, while the southern 

 gives the highest or longest course. As before, we get two points in our 

 favour by setting an observer far to the north and another far to the 

 south. As before, the northern observer sees Venus hastened on her 

 course ; but now this is a favourable point, since it manifestly accords 

 with the shortening of the northern line of transit. This makes point 

 three in our favour. And again, as before, we can set our southern 

 observer where the motion of Venus can be hastened or retarded as we 

 please. We assign him a station where she will be retarded (which is 

 secured when the sun moves from west to east during the transit) : this 

 manifestly accords with the lengthening of her path. Thus we have 

 four favourable points in all in 1882; whereas in 1874 we can secure 

 only three or (one being unfavourable), a majority of only two favour 

 able points. 



It seems manifest, then, that the transit of 1882 is twice as favour- 



