machines are represented by the gas-meter, a device for measuring and recording the quantity or volume of passing gas. The wet meter (fr. 122, fig. 9), invented in 1807 by Samuel Clegg and improved in 1815 by Samuel Crossly, consists of an outer box partially filled with liquid (water, alcohol, or glycerin) to the level of about half the height of the box. Within this box is journalled an axis carrying a series of buckets, each capable of containing a definite quantity of gas, which is admitted through a pipe at the central part of the meter. The gas, which causes the buckets to rise successively and which maintains a continuous rotation, passes out through a pipe at the upper part of the meter, the quantity being measured by a series of multiplying gear wheels, which derive their motion from the axis ou which the buckets are secured, and which register on a series of dials hundreds, thousands, ten thousands, and hundred thousands of cubic feet. Valves are arranged to cut off the supply of gas when the water in the meter rises above or falls below certain limits. The dry meter, invented in 1820 by John Malam and improved in 1838 by Defries, operates on the principle of a bellows of known capacity alternately filled with gas and emptied, the quantity of passing gas being indicated by a register. The pressure of the gas is the motor, and the changes of motion are effected by induction and eduction valves, much as in a steam-engine. Gas-burners are tested by the meter shown in Figure io. To insure a uniform and steady light the test-meter is made with three diaphragms, and to show the internal working parts is glazed iu front and on top. The dial is so devised as to exhibit by one-minute observations the hourly rate of gas consumed.
ENT or TIM E: 1101t0LoGY.
the many inventions devised by the genius of man, none are of more general interest than those designed for dividing the day into fixed periods. The first divisions of time were doubtless made by observing the positions of the sun during his daily course, and by dis covering that surrounding objects throw shadows of different lengths and in different directions as the day advances. The result of these observa tions was the invention of the sun-dial, though the date when it was first employed cannot be fixed. The first recorded mention of its use is in Isaiah xxxviii. 8: " Behold, I will bring again the shadow of the degrees, which is gone clown in the sun-dial of Ahaz, ten degrees backward." As the intervention of clouds and the advent of night obstructed the light of the sun, the usefulness of the dial was limited, and something was demanded that should without interruption indicate the time during all hours of the day. This demand was met by the invention at an unknown date of the " water-clock " or clepsydra, which is supposed to have been of Egyptian origin. The oldest of which there is record is that mentioned by Plato (400 B. c.). The soft notes of flutes, played at regular intervals by this ingenious mechanism, indicated the time of clay. Fig ure i (fi/. 124) shows a form of clepsydra used in Egypt about zoo B. c. The water runs through a pipe (H) into a funnel (A) and drops into the cylinder E. The stopper B is for regulating the water-supply, while the superfluous water escapes through the pipe J. A floating piston moves a vertical rack which gears into a wheel bearing the hour-hand. The day of the Egyptians (from sunrise to sunset) was divided into twelve hours.
To regulate the flow of the water according to the length of the day (depending upon the season or latitude), the stopper B was adjusted by the stalk D, which was for this purpose provided with an index for any day in The most celebrated clepsydra of which there is a recorded description was that presented to Charlemagne by the caliph Haroun-al-Rasehid A. D. 807. Copper balls equal in number to the hours of time day dropped, one every hour, into a basin, and the termination of the hour was indicated by the sound produced by each ball. This clep sydra had twelve doors, which opened one after another every hour, and from each door as it opened there issued a rider with a lance in his hand. The several riders retained their positions outside the opened doors until the appearance of the twelfth rider, when all simultaneously withdrew, closing the doors with their lances. It is also stated that the clock was ornamented with movinp figures of many kinds.
Absolute precision in time-keepers, however, was not attained until there were invented clocks consisting of a series of wheels driven by weights and cords and controlled in their movements by regulating devices. With the construction of such instruments the history of the time-keepers of the present day begins, though it cannot be ascertained when the first wheel-and-weight clock was made. Pacificus, an archdea con of Verona in the ninth century, has been named as the inventor, though it is also asserted that this style of clocks was derived from the Arabians. Records of the twelfth century indicate with more certainty the existence of real clocks at that period. The first of which we learn particulars that indicate clocks of any similarity to those of modern con struction was presented in 1232 by the sultan of Egypt to the Emperor Frederick II. " It resembled a celestial globe, in which the sun, moon, and planets moved, being impelled by weights and wheels, so that they pointed out the hour, day, and night with certainty." This seems to indi cate that the fundamental principle of modern clocks is of Saracenic origin. We find that in the thirteenth century there were many tower clocks with striking works in the church steeples of Italy; in 1288 a striking clock was built for the Westminster tower with money derived from a fine imposed upon a chief justice. With the beginning of the fourteenth century such clocks became more general. The first detailed descrip tion of a clock dates from 137o, when a German, Heinrich von Wick, was employed by Charles V. of France to make turret clock for the king's palace (pl. '24, jig. 2). Although controlled by a " balance " (the pendulum not yet being invented), it had a striking resemblance to the turret clocks of recent years. It was the bell of this clock that gave the signal for the massacre on St. Bartholomew's Day. Portable time-keepers, in the form of watches impelled by the tension of a spring, were made early in the sixteenth century in Nuremberg, and it seems that Peter Hele invented them; they were known by the name of "Nuremberg eggs." (See Vol. II., p. 268.) A new epoch begins with the application of the pendulum and hair spring to control the action of the escapement, as these appliances pro vided the means for the attainment of the remarkable perfection of mod ern time-pieces. Christian Huygens (1629-1695) invented the pendulum clock, and Robert Hooke (1635-17°3) invented the hair-spring about 166o.