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CLOSE POLLINATION

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CLOUDS

the pendulum might be kept going and the advantage of its uniform vibrations thus be obtained. This he accomplished by

the introduction of the escape-m e n t, a mechanism which is unlocked bythependulum at each beat and thus allows the train of wheels to a d vance. But in addition t o this the es-capement also gives the pendulum a little push,

which just makes up for the loss of energy which the pendulum sustains in swinging through the air and in unlocking the train.

The action of the escapement will be evident from the accompanying figure, in which the arrow indicates the direction in which the mechanism is driven by the spring or weight. In the upper part of the figure are represented the two pallets which receive alternate pushes to right and left as, one after another, the teeth of the wheel pass. A good clock of this type keeps better time than the sun; and accordingly we now use as our standard of time the period of revolution of a fictitious sun which revolves uniformly with the average speed of our actual sun. This is called mean solar time. The astronomer uses a clock in which the hour-hand rotates at the same rate at which the fixed stars appear to revolve about the earth. In other words, this instrument, which is called a sidereal clock, rotates at the same rate as the earth, which is the most uniform motion that we know anything about.

American clock-making from early days has had an interesting history, the men who have been connected with it being many and in their way characters, from Isaac Doolittle, in the old colonial times, who as a brass-founder built a bell-foundry and made brass wheel-clocks, to the era of the New Haven Clock Co., with its originators in Hiram Camp of Plymouth and Chauncey Jerome of Bristol, Conn. Interesting, too, is the story of Eli Terry, the father of wooden clock-making, as is that of the men who had to do with the New Haven concern—such as James E. English, H. M. Welch and Hiram Camp, the latter the inventor of a number of automatic

tools and machines for making parts of clock-works, and perhaps the greatest of American clock-makers.

The chronometer is merely a spring-clock in which an oscillating wheel called the balance is employed instead of a pendulum. This wheel is lightly tethered by a fine spring called a hair-spring, and its value lies in the fact that its vibrations occupy equal times. In 1714 the British government offered a reward of $100,000 to any one who would devise a means for getting longitude at sea within 30 miles. Stimulated, perhaps, by this offer, John Harrison (1693-1776), an English mechanician, invented the chronometer, which enabled navigators even then to determine their longitude within 18 miles.

A watch is merely a small chronometer that can be carried in the pocket. A striking clock is one fitted with a bell which is struck by a hammer at certain equal intervals, generally an hour. It is this form of instrument from which we derive our word clock, which originally meant a bell. Driving-clocks are really engines operated by a spring or weight. Telescopes in observatories are made to follow the stars by means of such driving-clocks.

Time-keeping before the invention of clocks was a very crude process. During the day the ancients were dependent upon the position of the sun, and during the night upon the positions of certain well-known fixed stars. Intervals of time were measured by allowing sand or water to run through funnel-shaped vessels, called hour-glasses and clepsydrae respectively. See Sir E. Beckett's Clocks, Watches and Bells; Benson's Time and Time-Tellers and Britten's Watch and Clock-Makers' Handbook.

Close Poriina'tion (in plants), the transfer of pollen from the stamen to the stigma of the same flower. See POLLINATION.

Clouds are masses of water-vapor condensed into very minute drops of water or frozen into very small particles of ice. Every one is familiar with fogs. Clouds are simply fogs formed at a considerable distance above the earth. They are generally white in appearance for the same reason that any transparent substance, such as glass or sugar, is white when broken up into small particles.

The classification of clouds generally employed is the one which divides them according to their external appearance into four different groups:

i. The cirrus or mare's tails clouds are composed of long white fibers or slender filaments. They are generally observed at great heights, and are probably composed of small crystals of ice. Glaisher in his balloon-ascents found cirrus clouds above him even at the height of 23,000 feet; while, on the other hand, cirrus clouds are

ANCHOR  ESCAPEMENT