is
ever turned by the weather-cock to meet the full force of the wind,
which, blowing upon it, drives it back against a spring. To this spring
is affixed a chain passing over, pullies toward another pencil, fixed
above a sheet of paper, and moving faithfully, more or less, as the
wind blows harder or softer. And thus the "gentle zephyr" and the fresh
breeze, and the heavy gale, and, when it comes, the furious hurricane,
are made to note down their character and force. The sheet of paper on
which the uncertain element, the wind, is bearing witness against
itself, is fixed upon a frame moved by clock-work. Steady as the
progress of time, this ingenious mechanism draws the paper under the
suspended pencils. Thus each minute and each hour has its written
record, without human help or inspection. Once a day only, an assistant
comes to put a new blank sheet in the place of that which has been
covered by the moving pencils, and the latter is taken away to be bound
up in a volume. The book might with truth be lettered, "The History of
the Wind; written by Itself"--an AEolian autobiography.

Close by is another contrivance for registering in decimals of an inch
the quantity of rain that falls. The drops are caught, and passing down
a tube, a permanent mark is made by which the quantity is determined.

The eastern turret is devoted to the Time Ball and its mechanism. Far
out at sea--away from all sources of information but those to be asked
of the planets, his compass, his quadrant, his chronometer, and his
almanack, the mariner feels the value of _time_ in a way which the
landsman can scarcely conceive. If his chronometer is right, he may feel
safe; let him have reason to doubt its accuracy, and he knows how the
perils surrounding him are increased. An error of a few seconds in his
time may place him in danger--an error of a few minutes may lead him to
steer blindly to his certain wreck. Hence his desire when he is leaving
port to have his time-pieces right to a second; and hence the
expenditure of thought, and labor, and money, at the Greenwich
Observatory, to afford the shipping of the great port of London, and the
English navy, the exact time--true to the tenth of a second, or six
hundredth of a minute--and to afford them also a book, the Nautical
Almanack, containing a mass of astronomical facts, on which they may
base their calculations, with full reliance as to their accuracy. Every
day for the last seventeen years, at fi