laced the
incandescent electric lamps that were to doom it.

[38] If, of course, there is no safety-valve in proper working order
included in the installation.

Chapter XX.

VARIOUS MECHANISMS.

CLOCKS AND WATCHES:--A short history of timepieces--The
construction of timepieces--The driving power--The
escapement--Compensating pendulums--The spring balance--The
cylinder escapement--The lever escapement--Compensated
balance-wheels--Keyless winding mechanism for watches--The hour
hand train. LOCKS:--The Chubb lock--The Yale lock. THE CYCLE:--The
gearing of a cycle--The free wheel--The change-speed gear.
AGRICULTURAL MACHINES:--The threshing-machine--Mowing-machines.
SOME NATURAL PHENOMENA:--Why sun-heat varies in intensity--The
tides--Why high tide varies daily.

CLOCKS AND WATCHES.

A SHORT HISTORY OF TIMEPIECES.

The oldest device for measuring time is the sun-dial. That of Ahaz
mentioned in the Second Book of Kings is the earliest dial of which we
have record. The obelisks of the Egyptians and the curious stone pillars
of the Druidic age also probably served as shadow-casters.

The clepsydra, or water-clock, also of great antiquity, was the first
contrivance for gauging the passage of the hours independently of the
motion of the earth. In its simplest form it was a measure into which
water fell drop by drop, hour levels being marked on the inside.
Subsequently a very simple mechanism was added to drive a pointer--a
float carrying a vertical rack, engaging with a cog on the pointer
spindle; or a string from the float passed over a pulley attached to the
pointer and rotated it as the float rose, after the manner of the wheel
barometer (Fig. 153). In 807 A.D. Charlemagne received from the King of
Persia a water-clock which struck the hours. It is thus described in
Gifford's "History of France":--"The dial was composed of twelve small
doors, which represented the division of the hours. Each door opened at
the hour it was intended to represent, and out of it came a small number
of little balls, which fell one by one, at equal distances of time, on a
brass drum. It might be told by the eye what hour it was by the number
of doors that were open, and by the ear by the number of balls that
fell. When it was twelve o'clock twelve horsemen in miniature issued
forth at the same time and shut all the doors."

Sand-glasses were introduced about 330 A.D. Except for s