even and a useful number of feet or inches, which he will always be able
to recollect, and refer to as a unit of measurement. The distance between
the eyes is instantly determined, and, I believe, never varies, while
measurements of stature, and certainly those of girth of limb, become
very different when a man is exhausted by long travel and bad diet. It is
therefore particularly useful for measuring small objects. To find it,
hold a stick at arm's-length, at right angles to the line of sight; then,
looking past its end to a distant object, shut first one eye and then the
other, until you have satisfied yourself of the exact point on the stick
that covers the distant object as seen by the one eye, when the end of
the stick exactly covers the same object, as seen by the other eye. A
stone's throw is a good standard of reference for greater distances.
Cricketers estimate distance by the length between wickets. 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