Pacing yards
should be practised. It is well to dot or burn with the lens of your
opera-glass a scale of inches on the gun-stock and pocket-knife.
Velocity of Sound.--Sound flies at 380 yards or about 1000 feet in a
second, speaking in round numbers: it is easy to measure rough distances
by the flash of a gun and its report; for even a storm of wind only makes
4 per cent. difference, one way or the other, in the velocity of sound.
Measurement of Angles.--Rude Measurements.--I find that a capital
substitute for a very rude sextant is afforded by the outstretched hand
and arm. The span between the middle finger and the thumb subtends an
angle of about 15 degrees, and that between the forefinger and the thumb
an angle of 11 1/4 degrees, or one point of the compass. Just as a person
may learn to walk yards accurately, so may he learn to span out these
angular distances accurately; and the horizon, however broken it may be,
is always before his eyes to check him. Thus, if he begins from a tree,
or even from a book on his shelves and spans all round until he comes to
the tree or book again, he should make twenty-four of the larger spans
and thirty-two of the lesser ones. These two angles of 15 degrees and 11
1/4 degrees are particularly important. The sun travels through 15
degrees in each hour; and therefore, by "spanning" along its course, as
estimated, from the place where it would stand at noon (aided in this by
the compass), the hour before or after noon, and, similarly after sunrise
or before sunset, can be instantly reckoned.
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