THE COMING TRANSIT OF VENUS. 23 



stream, or exactly midway between the house and the 

 barge. Then a moment's consideration will show that 

 the boat traverses 12 yards between the moments when 

 the spectators at a and b severally see it towards A. 

 Now suppose that the observer at a indicates by a call 

 or other signal the moment when the flag is thus seen 

 by him, and that the observer at 6, provided with a 

 stop-watch, notes that two seconds elapse before he sees 

 the flag towards A. This, then, is the time occupied 

 by the boat in traversing 1 2 yards ; so that she is mov- 

 ing at the rate of six yards per second. Similar remarks 

 apply to the apparent transit of the flag past B as seen 

 from a and 6. * In like manner, the astronomer can 

 gather from observations by Delisle's method the rate 

 at which Venus is moving in her orbit, that is, the 

 exact number of miles over which she moves per minute. 

 So that, since he knows exactly how long she is in com- 

 pleting the circuit of her orbit, he learns, in fact, the 

 exact circumference of her orbit in miles, whence its 

 radius (or her distance from the sun) follows at once. 



It is manifest that Delisle's method can be applied 

 with equal advantage either to the ingress or to the 

 egress of Venus. The comparison of two observations 

 in one of which her ingress happens as early as possible, 

 while in the other it happens as late as possible is 

 quite sufficient to determine the sun's distance. So also 

 the comparison of two observations of egress (most 

 accelerated and most retarded) is separately sufficient 

 to determine the sun's distance. This is an important 

 advantage of the method. Because while, as in Halley's 



