By the assistance of a seconds watch the following interesting
calculations may be made:
If a traveller, when on a precipice or on the top of a building,
wish to ascertain the height, he should drop a stone, or any other
substance sufficiently heavy not to be impeded by the resistance of
the atmosphere; and the number of seconds which elapse before it
reaches the bottom, carefully noted on a seconds watch, will give
the height. For the stone will fall through the space of 16-1/8
feet during the first second, and will increase in rapidity as the
square of the time employed in the fall: if, therefore, 16-1/8 be
multiplied by the number of seconds the stone has taken to fall,
this product also multiplied by the same number of seconds will
give the height. Suppose the stone takes five seconds to reach the
bottom:
16-1/8 × 5 = 80-5/8 × 5 = 403-1/8, height of the precipice.
The Count Xavier de Maistre, in his _Expédition nocturne autour
de ma Chambre_, anxious to ascertain the exact height of his room
from the ground on which Turin is built, tells us he proceeded
as follows: “My heart beat quickly, and I just counted three
pulsations from the instant I dropped my slipper until I heard
the sound as it fell in the street, which, according to the
calculations made of the time taken by bodies in their accelerated
fall, and of that employed by the sonorous undulations of the
air to arrive from the street to my ear, gave the height of my
apartment as 94 feet 3 inches 1 tenth (French measure), supposing
that my heart, agitated as it was, beat 120 times in a minute.”
A person travelling may ascertain his rate of walking by the aid
of a slight string with a piece of lead at one end, and the use of
a seconds watch; the string being knotted at distances of 44 feet,
the 120th part of an English mile, and bearing the same proportion
to a mile that half a minute bears to an hour. If the traveller,
when going at his usual rate, drops the lead, and suffers the
string to slip through his hand, the number of knots which pass in
half a minute indicate the number of miles he walks in an hour.
This contrivance is similar to a _log-line_ for ascertaining a
ship’s rate at sea: the lead is enclosed in wood (whence the name
_log_), that it may float, and the divisions, which are called
_knots_, are measured for nautical miles. Thus, if ten knots are
passed in half a minute, they show that the vessel is sailing at
the rate of ten knots, or miles, an hour: a seconds watch would
here be of great service, but the half-minute sand-glass is in
general use.
The rapidity of a river may be ascertained by throwing in a light
floating substance, which, if not agitated by the wind, will move
with the same celerity as the water: the distance it floats in a
certain number of seconds will give the rapidity of the stream; and
this indicates the height of its source, the nature of its bottom,
&c.--See _Sir Howard Douglas on Bridges_. _Thomson’s Time and
Time-keepers._