134 THE BOOK OF NATURE STUDY 



at this stage be applicable to the rough measurement of areas 

 upon the plans, and thus will further elucidate the meaning 

 and use of scales. 



By this time it should have become evident that the sketch 

 method of plan-making has inherent defects, and some more 

 accurate and definite method will be called for, which shall not 

 require the perpetual correcting and patching requisite in a sketch- 

 plan. This is the stage at which the plane-table may be intro- 

 duced, the simplest and most readily applicable of all surveying 

 instruments. In principle it is merely the application of the 

 sighting method already advocated for use in the schoolroom, and 

 in its least elaborate form it is merely a drawing-board mounted 

 on a tripod and furnished with a ruler (Fig. 35). A base-line 

 is chosen, measured, and marked in the ground by a stretched 

 string. The plane table is then placed at one end of this line, and 

 from the point A, immediately above one end of it, a line is drawn ' 

 upon the paper fixed to the board, exactly in the direction of the 

 base-line. On the line so drawn the position of the other end, 

 B, of the base-line is marked, on any scale which may be chosen. 

 Sighting lines are then taken along the ruler, or strings stretched 

 from the view-point, A, to a number of objects, and the lines 

 accurately drawn from the corresponding point A on the plan. 

 Each line is marked in some way to indicate the object to which 

 it refers. The board is then moved to the other end, B, of the 

 base-line, and so placed that the ruled line is exactly in the same 

 direction as the base line, and the point B of the plan above the 

 corresponding point on the ground. A second series of lines is 

 then drawn to the same series of objects, and the position- of 

 each object marked at the intersection of its two sight-lines 

 (Fig. 36). 



The correctness of the plan so obtained is tested in various 

 ways. First, by the measured distance of objects from the ends 

 of the line ; second, by their shortest distance from any part of 

 the line ; and third, by their distance from one another and their 

 relative position. It will be useful to choose amongst the objects 

 one at least which is eclipsed by one of the others from one of 

 the view-points. During this exercise it will also be well for the 

 students to pace distances, in order to ascertain the length of 



