CH. XIX. H ALLEY'S METHOD. ,59 



numbers) 2\ times as far from the sun as she is from the 

 earth. It was also known by the apparent size of the sun 

 that the sun's distance is about 108 times his diameter, or in 

 other words, if you could measure the number of miles 

 across the face of the sun and multiply that number by 

 108, it would give you the sun's distance from the earth. 

 Therefore you see the one point to be learnt was, How 

 many miles wide is the face of the sun 1 



Now suppose you place a globe or any other object upon 

 the table in the middle of the room, as at g. Fig. 25, 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 posi- 

 tion to B, and the globe 

 will then hide the point 

 D. If the globe is (as 

 at g) exactly half-way 



Diagram showing how the distance between the 

 between you and the points p c and d c can be known without 



measuring them, 

 wall, the two points D c, a globe half-way between d c and A b. g, K 



globe three times as far from d c as from a b. 



and c 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 th7'ee 

 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 



