62 LIGHT SCIENCE FOR LEISURE HOURS. 



one set of instruments. Suppose, however, that from 

 one end of the two-feet line an observer using one set 

 of instruments took the bearing of the steeple; and 

 that, half a year after, another observer brought another 

 set of instruments and took the bearing of the steeple 

 from the other end of the two-feet line, is it not obvious 

 how enormously the uncertainty of the result would 

 be increased by such an arrangement as this? One 

 observer would have his own peculiar powers of obser- 

 vation, his own peculiar weaknesses ; the other would 

 have different peculiarities. One set of instruments 

 would be characterised by its own faults or merits, so 

 would the other. One series of observations would be 

 made in summer, with all the disturbing effects due to 

 heat ; the other would be made in winter, with all the 

 disturbing effects due to cold. 



The observation of the sun is characterised by all 

 these difficulties. Limited to the base-lines he can 

 measure on earth, the astronomer must set one observer 

 in one hemisphere, another in the other. Each observer 

 must have his own set of instruments ; and every ob- 

 servation which one has made in summer will have to 

 be compared with an observation which the other has 

 made in winter. 



Thus we can understand that astronomers should 

 have failed totally when they attempted to determine 

 the sun's distance without aid from the other celestial 

 bodies. 



It may seem at first sight as though nothing the other 

 celestial bodies could tell the astronomer would be of 



