SE VENTEENTH CENTUR Y. 



FT. III. 



Therefore the one point to be learnt was, How many miles 



wide is the face of the sun ? 



Now suppose you place a globe or any other object upon 



the table in the- middle of the room, as at G, Fig. 26, and 



place yourself at the 

 point A. The globe will 

 then hide from you (or 

 eclipse) the point c on 

 the opposite wall. Move 

 your position to B, and 

 the globe will then hide 

 A B the point D. If the globe 



FIG. 26. / x i i i r 



Diagram showing how the distance between the 1S ( aS at G ) exactly half- 



points D c and d c can be known without waybetWCCn yOU and tllC 



measuring them, 



o, A globe half-way between DC and AB. g, A Wall, the tWO points D 



globe three times as far from u c as from A u. and Will be the Same 



distance apart as the points A and B. But if you move the 

 globe to g, which is three times as far from the opposite wall 

 as it is from you, then the points d and c will also be three 

 times as far apart as the points A and B. So that by know- 

 ing how much farther the globe is from the wall than it is 

 from you, you can tell accurately the distance between the 

 points hidden without measuring them. 



It is exactly in this way that Halley proposed to measure 

 off a certain number of miles upon the face of the sun. We 

 are able to learn accurately how many miles distant any two 

 places are upon our globe. Suppose, therefore, that two 

 men go to places 7200 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 

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

 against different points in the sun, as in Fig. 27 ; and since 

 the distance between Venus and the sun is 2- times her 



