250 LIGHT SCIENCE FOR 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 

 observations were made and the corresponding con- 

 structions 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 employed 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 appli- 

 cable. Into such details as these I need not here enter. 

 It is sufficient that my readers should have been 

 enabled to recognise the simple principles on which 



