201/2
" _d y_ (general slope, exclusive of inequalities) 353/4
" _a x_ (ditto, ditto, to point of cliff above _x_) 231/2 ]

For instance, no mountain in the Alps produces a more vigorous
impression of peakedness than the Matterhorn. In Professor Forbes's
work on the Alps, it is spoken of as an "obelisk" of rock, and
represented with little exaggeration in his seventh plate under the
outline Fig. 31. Naturally, in glancing, whether at the plate or the
mountain, we assume the mass to be a peak, and suppose the line _a b_ to
be the steep slope of its side. But that line is a perspective line. It
is in reality _perfectly horizontal_, corresponding to _e f_ in the
penthouse roof, Fig. 30.

[Illustration: FIG. 33.

Angles with the horizon _x y_.

_a f_ 56 deg.
_a e_ 123/4
_e b_ (from point to point) 441/2
_b c_ ( ditto, ditto ) 671/4
_c d_ (overhanging) 79 deg.
_a x_ (irrespective of irregularities) 56
_a y_ 383/4 ]

Sec. 12. I say "perfectly horizontal," meaning, of course, in general
tendency. It is more or less irregular and broken, but so nearly
horizontal that, after some prolonged examination of the data I have
collected about the Matterhorn, I am at this moment in doubt _which is
its top_. For as, in order to examine the beds on its flanks, I walked
up the Zmutt glacier, I saw that the line _a b_ in Fig. 31 gradually
lost its steepness; and about half-way up the glacier, the conjectural
summit _a_ then bearing nearly S. E. (forty degrees east of south), I
found the contour was as in Fig. 32. In Fig. 33, I have given the
contour as seen from Zermatt; and in all three, the same letters
indicate the same points. In the Figures 32 and 33 I measured the angles
with the greatest care,[61] from the base lines _x y_, which are
accurately horizontal; and their general truth, irrespective of mere
ruggedness, may be depended upon. Now in this flank view, Fig. 32, what
_was_ the summit at Zermatt, _a_, becomes quite subordinate, and the
point _b_, far down the flank in Forbes's view taken from the
Riffelhorn, is here the apparent summit. I was for some time in
considerable doubt which of the appearances was most trustworthy; and
believe now that they are _both_ deceptive;