RESULTS OF BRITISH TRANSIT EXPEDITIONS. 63 



circle, it follows that the diameter of the bird's path is about 

 382 feet, and his distance from the centre of the globe 191 

 feet. So that the distance of the globe from the window, 

 known to be half as great again, is about 2 8 6^ feet. 



If we regard the globe as representing the sun ; the 

 window of known size as representing our earth of known 

 dimensions ; the bird travelling round in a known period 

 and at a distance whose proportion to the window's distance 

 is known, as representing Venus travelling in a known period 

 round the sun and at a distance bearing a known proportion 

 to the earth's ; this way of determining the distance of a 

 remote globe illustrates what is called Delisle's method of 

 determining the sun's distance. It requires that the two 

 observers, A and B, should each make exact note of the 

 moment when the bird seemed to begin to cross the disc of 

 the remote globe ; and in like manner Delisle's method re- 

 quires that two observers, widely separated on the earth in a 

 direction nearly parallel to that in which Venus is travelling, 

 should make the most exact note of the moment when Venus 

 begins to cross the sun's face. Also, as all I have said 

 about the bird's beginning to cross the face of the distant 

 globe would apply equally well if said about the end of his 

 seeming passage across that disc, so two observers, widely 

 separated on the earth, can determine the sun's distance by 

 noting the end of her transit instead of the beginning, if they 

 are suitably placed for the purpose. The window of our 

 illustration remains unchanged during the bird's imagined 

 flight, but as the face of the earth turned sunwards (which 

 corresponds to that window) is all the time changing with 

 the earth's rotation, a different pair of stations would have 

 to be selected for observing the end of transit, than would 

 be suitable for observing the beginning. 



So much for the method called Delisle's. The other is 

 in principle equally simple. In the imaginary experiment 

 just described we supposed the two observers at the right 

 and left sides of the circular window. Imagine them now 

 to watch the bird from the top and bottom of the window, 3 



