252 LIGHT SCIENCE FOE LEISURE HOURS. 



observed with a theodolite the angles ABC and B A c, 

 we might lay these down on the chart with a protractor, 

 and so the position of c would be determined with an 

 accuracy proportioned to the care with which the ob- 

 servations were made, and the corresponding construc- 

 tions applied to the chart. But in ( plane-tabling ' a 

 more direct plan is adopted. A ruler bearing sights, 

 resembling those of a rifle, is so applied that the edge 

 passing through the point A on the chart (the observer 

 being situated at the real station A) passes through the 

 point B on the chart, the line of sight passing through 

 the real station B. The table being fixed in the position 

 thus obtained, the ruler is next directed so that its edge 

 passes through A, while the line of sight points to C. A 

 line is now ruled with a pencil through A towards c. In 

 a similar manner, the table having been removed to the 

 station B, a pencil line is drawn through the point B 

 on the chart towards c. The two lines thus drawn 

 determine by their intersection the place of c on the 

 chart. 



The above is only one instance of the modes in 

 which a plane-table can be applied ; there are several 

 others. Usually the magnetic compass is made use of 

 to fix the position of the table in accordance with the 

 true bearing of the cardinal points. Also the bearings 

 of several points are taken around each station; and 

 thus a variety of tests of the correctness of the work 

 become applicable. Into such points as these we need 

 not here enter. It is sufficient that our readers should 

 have been enabled to gather the simple principles 



