62 LIGHT SCIENCE FOR LEISURE HOURS. 



The two observers who saw the transit end earliest 

 and latest could do the like. 



Speaking generally, and neglecting all the com- 

 plexities which delight the soul of the astronomer, this 

 is Delisle's method of utilising a transit. It has ob- 

 viously one serious disadvantage as compared with the 

 other. An observer at one side of the earth has to 

 bring his observations into comparison with those made 

 by an observer at the other side of the earth. Each 

 uses the local time of the place at which he observes, 

 and it has been calculated that for the result to be of 

 value there must not be an error of a single second in 

 their estimates of local time. Now, does the reader 

 appreciate the full force of this proviso ? Each observer 

 must know so certainly in what exact longitude he is, 

 that his estimate of the time when true noon occurs 

 shall not be one second wrong ! This is all satisfactory 

 enough in places where there are regular observatories. 

 But matters are changed when we are dealing with such 

 places as Woahoo, Kerguelen Land, Chatham Island, 

 and the wilds of Siberia. 



In the transit 1 of 1874 there are many such 

 difficulties to be encountered. In fact, it is almost im- 

 possible to conceive a transit the circumstances of which 

 are more inconvenient. On the other hand, however, 

 the transit is of such a nature that if once the pre- 



1 The reader will remember the time at which the essay appeared. 

 For several reasons it seems well to leave the essay unaltered. In 

 the second series of Light Science a later stage is presented, and 

 the account is carried up to the present date in my work on Tlte 

 Transits of Venus. 



