VENUS ON THE SUN'S FACE. 53 



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 

 solution 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 con- 

 venient length. Then from either end of the base-line 

 he takes the bearing 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 draw- 

 ing 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 comparison 

 with the distance of the inaccessible object; for then, if 

 we make the least error in observing the bearings of the 

 object, we get an important error in the resulting de- 

 termination of the distances. The reader can easily 



