70 LIGHT SCIENCE FOR LEISURE HOURS. 



determining the distance of an inaccessible castle, or 

 rock, or tree, or the like. We snail 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 so- 

 lution 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 ob- 

 ject 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 every thing de- 

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

 comparison with the distance of the inaccessible object ; 



