160 SEVENTEENTH CENTURY. PT. in. 



men go to places 7,200 miles apart, and each observes Venus 

 at a particular moment upon the sun's face. Just as you, 

 from two different positions, saw the globe cover two 



FIG. 26. 



Venus as seen upon the sun by two observers, one at E' and one at E. (Proctor.) 



s. The sun. v v', Appearance of Venus on the sun's face. Venus is travelling in the 



direction of the arrow. 



different points of the wall, so these men will see Venus 

 against different points in the sun, as in Fig. 26 ; and since the 

 distance between Venus and the sun is z\ times her distance 

 from the earth, the two points will be 2\ times 7,200 miles, 

 that is 18,000 miles apart. Here, then, we have a certain 

 number of miles measured off on the sun's face. But how 

 are we to tell accurately what proportion this interval be- 

 tween the spots bears to the whole diameter of the sun ? 



By Halley's method the whole time that Venus takes in 

 crossing the sun is used as the means of measurement. 

 The observer at each of the two stations notes exactly the 

 time when Venus begins to cross the face of the sun, and 

 the moment when she passes off it again, and so reckons 

 exactly how long she has taken in making the whole transit. 



It was already known, from the rate at which Venus 

 moves, exactly how long she would take in crossing the 

 centre or widest part of the sun. We will call this time 6 

 hours, so as to use whole numbers. But it is clear that in 

 crossing a narrower part of the disc she will take less time. 

 Suppose, therefore, that one man says she was exactly 5 



