THE EOYAL AETILLE11Y INSTITUTION. 
509 
AC, AD, &c.; if then the points of the same level be joined—such as 1,1, 1, 
2, 2, 2 —we shall have obtained the contours of the slopes. The sketcher 
must use his own judgment in sketching in the ground between the cross- 
sections. Of course, the greater the number of cross-sections, the more 
accurate the result should be. 
Fig. 7. 
The protractor and plummet can also be used as a clinometer, for 
measuring angles of elevation and depression. (Yide Fig. 7.) If a sketcher, 
standing on the top of a hill, directs the upper edge of his protractor to 
a point in the watercourse the same height as his eye, that angle of 
depression will be contained between the 90° on the protractor and the 
plummet line—which must be clamped carefully with the thumb of 
the left hand before moving the protractor, unless a friend is at hand 
to read the observed angle. By taking reciprocal angles of elevation and 
depression, a careful sketcher will find that his observations will be within 
half a degree of the truth. Having the angle of inclination, and the distance 
in plan between the top and bottom of the hill, from the scale of shade, the 
number and distance apart of the contours can be marked off. Supposing 
the slope to be 10° and the scale of drawing 6 inches to the mile, the 
shaded space marked 10° would give the horizontal equivalent for a 50 feet 
contour; if the scale of the drawing was 1 2 inches to the mile, the contours 
shewn would have a vertical interval of 25 feet. This, to my mind, is the 
best property of the scale of shade. The method of running cross-sections, 
while giving the most correct result, takes a long time in its operation; but 
when time is an object, and an approximation to truth only required, a few 
angles of inclination will be sufficient to guide the sketcher in filling in the 
contours of the different hills. This method will be most exact when the 
inclines are tolerably the same throughout; where the change of inclination 
is very marked, two angles of inclination may be observed instead of one, 
and the contours marked off accordingly. We shall then have sufficient data 
for depicting the ground by hachure shading. 
The object of shading appears to be, to group the contours so that a 
general idea of the features of the ground may be conveyed to the mind at 
a glance; or, in other words, that the map may be deciphered as a whole, 
instead of as a number of isolated parts. The plan of grouping the contours 
by hachure shading, has certainly the advantage of enabling the sketcher to 
depict the minor ramifications of ground, as each stroke coincides with a 
contour at the spot it is drawn; but the time it takes in drawing (if anything 
like accuracy is attempted), is certainly an objection to this style of shading. 
