60 LIGHT SCIENCE FOR LEISURE HOURS. 



rock, or tree, or the like. We shall see presently that 

 the ingenuity of astronomers has, in fact, suggested 

 some other indirect methods. But clearly the most 

 satisfactory estimate we can have of the sun's distance 

 is one founded on such simple notions and involving in 

 the main such processes of calculation as we have to 

 deal with in ordinary surveying. 



There is, in this respect, no mystery about the solu- 

 tion of the famous problem. Unfortunately, there is 

 enormous difficulty. 



When a surveyor has to determine the distance of 

 an inaccessible object, he proceeds in the following 

 manner: He first very carefully measures a base-line 

 of convenient length. Then from either end of the 

 base-line he takes the bearings of the inaccessible 

 object that is, he observes the direction in which it 

 lies. It is clear that if he were now to draw a figure 

 on paper, laying down the base-line to some convenient 

 scale, and drawing lines from its ends in directions 

 corresponding to the bearings of the observed object, 

 these lines would indicate, by their intersection, the 

 true relative position of the object. In practice, the 

 mathematician does not trust to so rough a method 

 as construction, but applies processes of calculation. 



Now it is clear that in this plan everything depends 

 on the base-line. It must not be too short in com- 

 parison with the distance of the inaccessible object ; for 

 then, if we make the least error in observing the bear- 

 ings of the object, we get an important error in the 

 resulting determination of the distances. The reader 



