CHAP. X.] TOPOGRAPHY 253 



sketching topography ; but if a range-finder is available, its use 

 will facilitate the work enormously. 



Difference of level can be at once obtained from a theodolite 

 angle of elevation or depression by the formula : 



angle in sees, x dist. in miles 



Di£f . of level in feet = 



34 



Hills are best shown by contours. We do not, of course, Contom 

 pretend that our contours are a fixed distance apart, but we ^^^' 

 must endeavour to draw them approximately so, calling each 

 contour line, 25, 50, or 100 feet apart, as the scale may require, 

 and estimating the height of each spur with the assistance of a 

 pocket barometer, if we have one, which will give us roughly 

 the height of each station above the sea, if we read it when we 

 land, and whenever we have occasion to do so. Each contour 

 must be continued on from one hill to the other, or until it 

 meets itself again round the hill ; and as their number and 

 closeness together will roughly indicate the height of the liill, 

 we must be careful not to get more on one side of a hill than 

 another, or the value of this method will be lost, and the con- 

 tours will simply show the shape of each spur, without reference 

 to its relation in height, steepness, etc., to the next one, which 

 is what we want to show as well. These contours will perhaps 

 not appear in the finished chart, in which the mountains may 

 be delineated in a different manner, but they will form an 

 excellent guide for the amount of shade to be put on to the 

 different hills and slopes, and it is the readiest and quickest 

 method of showing this at the time. 



The simplest form of clinometer is a bullet suspended from Use of 

 the zero-mark on a protractor, which is held with its edge ^jg^er. 

 uppermost in the direction of the slope, the angle of which it 

 is desired to measure. The thread holding the bullet indicates 

 the number of degrees from the horizontal at which the edge 

 of the protractor is inclined. For different angles of slope a 

 scale is constructed showing the distance apart at wliich the 

 contours should be drawn corresponding to the scale of the 

 chart. This is principally of service when working on large 

 scales and dealing with long even slopes. 



A useful and rapid method of sketching topography on a large 

 scale is to select some commanding knoll for a station, and to 



