22 LIGHT SCIENCE FOR LEISURE HOURS. 



the barge (coming first between the ends A, a). And 

 for simplicity of description let us confine our remarks 

 to the little flag carried at the bow of the boat. It is 

 manifest that an observer at a will see the little flag 

 cross his line of vision towards A before an observer at 

 b sees the like. And the observer at a will in like 

 manner see the light blue flag (I beg pardon, I should 

 say the blue flag simply) crossing his line of vision 

 towards B before an observer at b sees the like. The 

 flag will traverse the range A B as seen both from a 

 and from 6, but both its ingress on this range and its 

 egress from it will be earlier as seen from a than as 

 seen from b. Now our earth may be compared to the 

 barge; the sun to the building A B.; and Venus to the 

 boat. There is one spot on the earth at which Venus 

 will seem to enter earliest on the sun's face, and an- 

 other spot (on the opposite side, just as b is farthest 

 away from a) where Venus will seem to enter latest ; 

 and in like manner there is one spot at which Venus 

 will seem to leave the sun's face earliest, and another 

 (on the opposite side) at which Venus will seem to leave 

 the sun's face latest. 



And as our illustrative case explains the nature of 

 Delisle's method, so also it illustrates the rationale of the 

 method. Of course, the two cases are not exactly similar ; 

 but they are sufficiently so to make the illustration in- 

 structive. Suppose that the length of the barge a b is 

 known (as the dimensions of the earth are known) ; 

 thus, say that it is 24 yards in length. Now suppose 

 that the course of the boat is known to be in mid- 



