Part 25 (2/2)

Chapter XX

VARIOUS MECHANISMS

CLOCKS AND WATCHES:--A short history of ti power--The escape balance--The cylinder escapement--The lever escape 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 threshi+ng--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 fortime is the sun-dial That of Ahaz s 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 amarked on the inside

Subsequently a very si 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 the153) In 807 AD Charle 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 tiht 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 elve o'clock twelve horsemen in miniature issued forth at the salasses were introduced about 330 AD Except for special purposes, such as tis, they have not been of any practical value

The clepsydra naturally suggested to the istering ti on soht-driven clock_ is attributed, like a good s, to Archimedes, the famous Sicilian mathematician of the third century BC; but no record exists of any actual clock coht prior to 1120 AD So wethe era of the clock as we know it

About 1500 Peter Hele of Nureht, and the _watch_ appeared soon afterwards (1525 AD) The pendulu the uished Dutch mechanician, in 1659

To Tho,” is ascribed the honour of first fitting a _hairspring_ to the escapement of a watch, in or about the year 1660 He also introduced the _cylinder escapeh many improvements have been made since his time, Tompion manufactured clocks and watches which were excellent timekeepers, and as a reward for the benefits conferred on his fellows during his lifetiranted the exceptional honour of a resting-place in Westminster Abbey

THE CONSTRUCTION OF TIMEPIECES

A clock or watch contains three ht or a spring; (2) the train of wheels operated by the driving force; (3) the agent for controlling the e clocks is usually a pendulu balance To these may be added, in the case of clocks, the apparatus for striking the hour

THE DRIVING POWER

_Weights_ are used only in large clocks, such as one finds in halls, towers, and observatories The great advantage of e power is exerted _Springs_ occupy hts, and are indispensable for portable timepieces The employment of them caused trouble to early experimenters on account of the decrease in pohich necessarily accoue overcame the difficulty in 1525 by the invention of the _fusee_, a kind of conical pulley interposed between the barrel, or circular dru, and the train of wheels which the spring has to drive The principle of the ”dru 201 Thesteel ribbon fixed at one end to an arbor (the watchhtly wound The arbor and spring are inserted in the barrel The arbor is prevented fro in its effort to uncoil causes the barrel to rotate

[Illustration: FIG 201]

A string of catgut (or a very fine chain) is connected at one end to the circu fixed to the larger end of the fusee, which is attached to the driving-wheel of the watch or clock by the intervention of a ratchet and click (not shown) To wind the spring the fusee is turned backward by means of a key applied to the square end A of the fusee arbor, and this draws the string fro causes the fusee to rotate by pulling the string off it, coil by coil, and so drives the train of wheels But while thethe string froe of the larger radius as its energy becomes lessened

The fusee is still used forand pendulum, and occasionally for watches In the latter it has been rendered unnecessary by the introduction of the _going-barrel_ by Satch barrel to drive the train of wheels This kind of dru the operation of winding, which is perfor the dru is attached A ratchet prevents the arbor frooing-barrel has been made satisfactory by the improvements in the various escapement actions

THE ESCAPEMENT

[Illustration: FIG 202]

The spring or weight transs to the _escape the rate at which the wheels are to revolve In clocks a _penduluent Galileo, when a student at Pisa, noticed that certain hanging la on their cords at an equal rate; and on investigation he discovered the principle that the shorter a pendulu to and fro As has already been observed, Huygens first applied the principle to the governing of clocks In Fig 202 we have a simple representation of the ”dead-beat”