lamps that were to doom it.
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