2 5 o THE POPULAR SCIENCE MONTHLY. 



"When the hase-line has been established and measured, and its termi- 

 nals have been so marked with permanent material as to be practically 

 indestructible, the base has to be developed — that is, a complete set of 

 observations has to be made for the purpose of connecting the base 

 with the stations of the net. The base being, as a rule, much shorter 

 than the sides of the triangles of the actual net, it can not be connected 

 directly with these large sides, as the triangles thus formed would 

 have very small angles. A special net of triangles, the sides of which 

 grow larger by degrees until they reach the large sides of the actual 

 net, is established. This small net is given the form of a polygon for 

 the purpose of increasing the accuracy. Longer bases are sometimes 

 divided into two halves, and, besides the two terminals, a central sta- 

 tion is established in the middle of the base, and thus three base sta- 

 tions are obtained instead of two. All the angular observations at 

 the base stations, and at those which may be called stations of develop- 

 ment, are made in the same number and with the same accuracy as in 

 those of the net proper. They are all considered as first-class stations. 



All geodetical points at which angular observations are made can 

 be divided into four classes. In the first class are included all the 

 base stations, the developing stations, and the actual stations of the 

 geodetical net. The second class includes those stations which are of 

 secondary importance geodetically, and which do not belong to the 

 net propei*. The observations at these stations are not so exhaustive 

 as in first-class stations, although they are used also for controlling 

 the observations of the others. Third- and fourth-class stations have 

 more importance as topographical points, as they are used by the 

 topographical operators as starting-points when mapping out the coun- 

 try. For scientific purposes, only the data collected at first-class sta- 

 tions are used, all others being rejected. 



For ordinary topographical purposes, the number of angular ob- 

 servations at each station is not so large as when these have to be used 

 for scientific purposes. 



Except for the measurement of bases, geodetical triangulation con- 

 sists almost exclusively in angular observations. In fact, it can be 

 called essentially a measurement by angles, the work to be done, and 

 on which many years may be spent before even a small net can be 

 called complete, being an uninterrupted series of measurements of 

 angles. Very delicate instruments are used in these measurements. 

 The best part of an observer's outfit consists of a good theodolite. 

 Although simple in principle, the theodolite is a very complicated 

 instrument, and a good deal of practice is necessary to enable an 

 observer to become efficient in handling this delicate machine. It 

 consols chiefly in a good field-glass, which can be turned in every 

 direction, s°> as to enable the observer to see the exact spots in the 

 distance which are the stations of the net, and of a circle on which a 

 scale, carefully divided, enables the observer to read the angle between 



