
_ Parr J.] EXAGGERATED ESTIMATES OF SLOPES. 39 
average angle of declivity may mount as high as 65°, 
But such steep slopes are of limited extent. Declivities 
exceeding 40°, and bearing a large proportion to the 
total dimensions of hill or mountain, are always found 
to consist of naked rock. In estimating angles of inclina- 
tion from a distance, the student will learn by practice 
how apt is the eye to be deceived by perspective and to 
exaggerate the true declivity, sometimes to mistake a 
horizontal for a highly inclined or vertical line. The 
mountain outline shown in Fig. 2 presents a slope of 25° 
between a and b, of 45° between b and ¢, of 17° between ¢ 
and d, of 40° between d and e, and of 70° between e and f. 


















































































































































































Wig. 2—ANGLES OF SLOPE WHERE THE HYE MAY BE DECEIVED BY 
PERSPECTIVE. (AFTER Rusxin.) a, Mountain OUTLINE; B, THE 
SAME OUTLINE AS SHOWN BY A CoTTAGE Roor. 
At a great distance, or with bad conditions of atmosphere, 
these might be believed to be the real declivities. Yet 
if the same angles be observed in another way (as on a 
cottage roof at B),we may learn that an apparently in- 
clined surface may really be horizontal* (as from a to 6 and 
from ¢ to d), and that by the effect of perspective, slopes may 
be made to appear much steeper than they really are. 
Much evil has resulted in geological research from 
the use of exaggerated angles of slope in sections and 
diagrams. It is therefore desirable that the student should 
from the beginning accustom himself to the drawing of 
outlines as nearly as possible on a true scale. The ac- 
companying section of the Alps by De la Beche (Fig. 3) is 
of interest in this respect as one of the earliest illustrations 
1 Mr. Ruskin has well illustrated this point. See Modern Painters, vol. iv 
whence the illustrations in the text are taken. 
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