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time. 45

C H A P T E R I W. Time. “The last white grain Fell through, and with the tremulous hand of age The old astrologer reversed the glass; And, as the voiceless monitor went on, Wasting and wasting with the precious hour, He looked upon it with a moving lip, And, starting, turned his gaze upon the heavens, Cursing the clouds impatiently.” — Willis. WE have now determined the relative situation of our earth with regard to the heavenly bodies, and its size compared with them, and we are prepared to investigate the causes of some of the changes which we witness upon its surface. Previous to this, we will devote a few chapters to Time and the Calendar, for the familiar expression of a day, or an hour, or a year, seldom conveys to the mind the exact meaning which belongs to those terms. We may consider time to be a definite portion, that is, a portion which can be measured, of indefinite duration, or, as Young poetically expresses it: “From old Eternity’s mysterious orb, Was Time cut off, and cast beneath the skies.” Time was personified by the Ancients, under the figure of an old man with scythe and hour-glass, and a single tuft of hair on the forehead. The scythe was emblematic of that all-powerful influence which cuts down every thing as it sweeps past. Man, and his works, perish, and crumble before it, as the grain falls before the mower's scythe. Nor is the emblem unappropriate. The keen edge, while it sweeps through the field of ripe grain, suddenly laying low the proud stalk, cuts down many a flower, and tender stem. The hour-glass, held in the outstretched hand, portrayed the passing moment, and the sand, in its ceaseless flow, marked the ebbing of the current of life. We cannot

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refrain from quoting a beautiful little poem, from “Hone's Every Day Book,” entitled

INSCRIPTION, -
Fort MY DAUGHTERS’ Hour-GLAss.

Mark the golden grains that pass,
Brightly thro' this channell’d glass,
Measuring by their ceaseless fall,
Heaven’s most precious gift to all !
Busy, till its sands be done,
See the shining current run;
But, th’ allotted numbers shed,
Another hour of life hath fled !
Its task perform’d, its travail past,
Like mortal man, it rests at last !—
Yet let some hand invert its frame,
And all its powers return the same,
Whilst any golden grains remain,
'Twill work its little hour again,-
But who shall turn the glass for man,
When all his golden grains have ran 7
Who shall collect his scattered sand,
Dispersed by Time's unsparing hand 7
Never can one grain be found,
Howe'er we anxious search around!

Then, daughters since this truth is plain, That Time once gone, ne’er comes again,Improv’d bid every moment pass— See how the sand rolls down your glass 1'.' The forelock was also emblematical, indicating that if we would improve the time, we must take it by the forelock, and that time once passed left no hold by which it could be reclaimed. Such was the beautiful emblem of time devised by the ancients, and which we still retain. The diurnal revolution of the earth, or rather, as it was once believed, the revolution of the heavens around the earth, was observed at a very early day to be performed with the utmost regularity. The return of night, and approach of day, the duration of the night and day, are the first great natural phenomena which engage attention, and we may suppose, therefore, that the apparent revolution of the stars around the earth was at a very early period, employed to determine equal intervals of time. Sun-dials were undoubtedly the earliest means employed

DIALS AND CLEPSYDRAE. . 47

to mark the passage of time, and are in common use even at the present day. Every country tavern is furnished with its meridian or noon-line, which oftentimes is nothing more than a scratch or mark in the floor, and the gnomon, or shadow-stick, is the side of a window or door. In our younger days, we have watched with far more interest, the shadow approach the humble line drawn on the floor of a tinker's shop, than'in more mature years the steady passage of a star over the wires of a transit telescope. And we have not forgotten those days of sun-dial memory, when we were, unconsciously, children playing with time. We find allusions to the dial in the Old Testament. The dial of Ahaz, which was, undoubtedly, a large public edifice. Such was the dial constructed by Dionysius, and such the dial used by the Chinese, and in India. Sun-dials were liable to many objections; they could only be used when the sun was shining, and consequently at night, or in cloudy weather they were worthless. The Clepsydra, or water-clock, was therefore invented at an early date. It is said that they were found among the ancient Britons, at the time of the invasion by Julius Caesar. The first water-clocks were made of long cylindrical vessels, with a small perforation at the bottom. These being filled with water, marked the passage of time by the descent of the fluid column. Various ornamental contrivances were subsequently introduced, but they were all dependent upon the same principle. We will imagine one of the early philosophers, with his waterclock, starting the stream when some well known star was occulted, or hidden by a distant object, the tube being long enough to continue the stream until the next night. As the heavens move on, we find him watching the descent of the liquid, and at the approach of the succeeding evening, when the same star is again occulted by the same object, he marks the level of the liquid in his tube, and selecting another star, for the first has gone out of sight, he fills the tube, and at the given signal, when the star passes behind the hill, or other occulting object, he permits the water to flow. On the succeeding evening, as this star is again hidden, he observes the fluid, and finds it at precisely the same level as before, and thus arrives at the conclusion that the stars all revolve around the earth in the same time, or, more philosophically speaking, he learns that the earth turns uniformly on its axis—performing each revolution in exactly the same interval of time. The space thus obtained on the clepsydra, for a revolution of the heavens, we may imagine him dividing into portions that will mark the subdivisions of the day. These divisions would not all be equal, but decrease in length as the height of the fluid column decreased. His instrument thus adjusted to measure the flight of time, we may suppose him to observe the exact instant of sunset, and after an interval of a day, again making the same observation. He would find upon careful observation that this interval was longer than the interval required for a star to revolve around the earth, by about 4 minutes, if his instrument would detect so small a quantity. In other words, he would find that the sun was apparently moving backward in the heavens. And now, he is, perhaps, for a moment puzzled which measure of time to adopt, that of the stars, or of the sum. Convenience points out the latter, and consequently astronomers regulate their time measurers to divide the solar day into 24 hours; the other is called the siderial day, and is about four minutes shorter. For a long time, even after Copernicus and Galileo had established the fact of a rotation of the earth on its axis, there were no means of measuring intervals of time more correctly than by the water-clock. It is true, that instruments made of wheels, and moved by weights, were, in Galileo's time, in use, but as they were without any regulators, the time was too inaccurately measured to be of any service. The discoveries which were being made by Tycho Brahe, and Kepler, demanded some more accurate method of registering the time. It is related that Galileo, observing the swinging of a suspended lamp, in a Church at Pisa, and noticing that the vibrations, whether long or short, were performed in equal times, conceived the idea of adapting such a contrivance, now called a pendulum, to measure intervals of time. His apparatus was rude enough, and it was necessary to employ a boy to occasionally give the pendulum a slight push when it was near resting. It does not appear, at first

Siderial, dAY. 49

thought, that long and short vibrations will be performed in the same time—yet this is true, at least when the pendulum is quite long, and the arcs over which it swings are of moderate lengths. Huygens conceived the idea of applying the pendulum to the clock, as a regulator, and succeeded in accomplishing this, and thus gave to the world an accurate measurer of time. The clock thus perfected, became so accurate, that it was necessary to contrive some more accurate means to regulate it. Hitherto, the successive occultations of some star, observed without the aid of a telescope, had been sufficient, and the time of noon, or 12 o'clock, was obtained by sun-dials, and other means, with sufficient accuracy, for the instruments hitherto employed. Any occurrence, which takes place at regular intervals, may be adopted as a regulator of time, but the revolution of the earth on its axis is by far the most accurate. For certain reasons, which will be given presently, the sun is apparently subject to such irregularities, that the solar days, or exact interval, from the time the sun is on the meridian, until his return to it again at the successive revolution, are of unequal lengths. In other words, the solar day is variable. Now the real revolution of the earth on its axis, is the time in which any given meridian, or situation on the earth, moves from a particular star, back to that star again. Thus:

Let A, B, C, D, be the earth, its north pole N, being towards us, and suppose it revolving in the order of the letters. Let N D be the meridian, or north and south line passing through some particular spot, Greenwich, for example, shown at E, and let the star S, be upon the meridian, that is, if this line was extended to the heavens, or, more properly, a plane passing through this

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