CH. XV.] THE COMPOSITION OF MIND. S9 



the first place, assimilate in thought the case of the moon to 

 like cases in which the distances of inaccessible objects upon 

 the earth are indirectly measured. When a land-surveyor 

 wishes to ascertain the distance of a church-tower situated 

 on the farther side of a river, he has recourse to an indirect 

 method of measurement. Upon his own side of the river he 

 first measures the distance between two points sufficiently 

 removed from each other, and this distance he calls a base- 

 line. From each end of the base-line he now takes a sight 

 at the inaccessible tower, and, with the proper instruments, 

 measures the difference between its direction and the direc- 

 tion of the base-line. In this way he obtains an ideal triangle, 

 of which the tower is the apex ; and, knowing the length of 

 the base-line, and the value of the two angles at the ends of 

 the base-line he calculates by trigonometry the length of the 

 two sides which express the distance of the tower from the 

 ends of the base-line. Now, the astronomer, imitating this 

 process, assumes as a base-line the known distance between 

 two remote points on the earth's surface, as for example 

 London and Cape Town ; and tlien from each of these points 

 he proceeds to take the bearings of the moon. The process, 

 indeed, is here complicated by the fact that, owing to the 

 long distance, the inequalities of the earth's surface, and its 

 curvature, the observer at Cape Town cannot see the position 

 of London, and vice versd. It is necessary, therefore, again 

 to resort to an indirect method, and, having measured the 

 meridional bearings of the moon from the north-pole at 

 London and from the south -pole at Cape Town, to compare 

 these bearings with the knowledge that the bearing of the 

 one pole from the other is 180 degrees or two right angles. 

 A further correction must be made for the fact that London 

 and Cape Town are not on the same meridian. But disre- 

 garding these steps in the process, as unnecessarily com- 

 plicating our case, we have to note that, when the astronomer 

 has thus indirectly measured the angles which ideal lines 



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