THE COMPOSITION OF MIND 



scious operations, whether intellectual or emo- 

 tional, consist. 



Beginning, then, with a somewhat compli- 

 cated operation of intelligence, let us consider 

 the process by which an astronomer, knowing 

 the dimensions of the earth, is enabled to cal- 

 culate therefrom the distance of the moon. He 

 must, in the first place, assimilate in thought the 

 case of the moon to like cases in which the dis- 

 tances 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 measure- 

 ment. Upon his own side of the river he first 

 measures the distance between two points suffi- 

 ciently removed from each other, and this dis- 

 tance he calls a base line. From each end of 

 the base line he now takes a sight at the inac- 

 cessible tower, and, with the proper instruments, 

 measures the difference between its direction and 

 the direction 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 pro- 

 cess, assumes as a base line the known distance 

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