24 LIGHT SCIENCE FOR LEISURE HOURS. 



method, two stations are absolutely necessary, there is 

 but a single observation to be made at each, whereas in 

 Halley's the beginning and end of the transit must be 

 observed at both stations. This introduces a double 

 difficulty. For first, there is the necessity for a longer 

 continuance of clear sky, since the transit may last 

 several hours ; and, secondly, there is the difficulty of 

 securing a station where the sun is well placed on the 

 sky, both at the beginning and end of the transit. It 

 will not suffice, in applying Halley's method, to have 

 the sun well above the horizon at the moment of ingress 

 if he is low down at the moment of egress, or to have 

 the sun high at egress if he is low at ingress. Accord- 

 ingly, the condition has to be secured that at stations 

 where the day is short (that is, in December, at north- 

 erly stations) the middle of the transit shall occur 

 nearly at mid-day. This limits the choice for northern 

 stations considerably. 



On the other hand, Delisle's method has this disad- 

 vantage, that the exact moment at which ingress or 

 egress occurs must be known. A mistake, even of a 

 second or two, would be of serious moment. So that 

 the clocks made use of at each station where this 

 method is applied, must not only have good rates, but 

 must show absolutely true time at the moment of the 

 observed phenomenon. Moreover, the latitude and 

 longitude of the place of observation must be known, 

 the latter (the only difficult point) with especial 

 accuracy, since on its determination depends the change 

 of local time into (say) Greenwich time; and this 



