186 CELESTIAL MEASURINGS AND WEIGHINGS. 



did.* Thus the bar has been moved forward by its exact 

 length in the air, as it were, without touching anything. 

 Arrived at the end of the base, a dot is made and ad- 

 justed under the terminal microscope on a gold or 

 platina plate let into a solid block of stone already pre- 

 pared the starting point having been a similar one 

 similarly fixed at the other end. 



(n.) The base measured, the " Trianguiation" com- 

 mences. This is founded on the universally known fact 

 that when two angles of a triangle are known, a know- 

 ledge of the length of the side between them leads by 

 exact rules of calculation to that of the other two ; ac- 

 cordingly, at the two extremities of the base, and cen- 

 trally over the dots which mark them, are placed deli- 

 cately divided instruments called theodolites, competent 

 to the measurement of angles to an extreme nicety. The 

 telescopes of these being pointed so as to look down the 

 throats of each other, it is clear that both must be 

 directed along the base line, and if then turned on some 

 one object at a distance considerably greater from either 

 than they are from each other, that object becomes the 

 summit of a triangle, the inclinations of whose sides to 

 the base is measured. Its distance from either end of 

 the base then can be calculated. Thenceforward either 

 of those sides becomes available as a new and longer 

 base. And thus the survey may go on, throwing out 



* In actual practice the procedure is a little more complex, but 

 the principle is the same; and it is only intended here to convey to 

 the uninitiated a general notion of the sort of niceties which have 

 to be attended to. 



