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We give above a representation of the earth, as it would probably appear to a spectator removed to the distance of the moon. The same hemisphere of the moon is always turned towards the earth, this is caused by a revolution on its axis in the same time that it revolves around the earth. Consequently, a spectator on the moon, would always behold the earth as a stationary body in the heavens, as we should behold the sun, if the earth turned on its axis but once in 365 days. The apparent size of the earth, seen from the moon, would be a globe of about four times the diameter of the moon. In the imaginary view we have given, the great Indian Ocean is directly in front, the Pacific at the right, and the Atlantic at the left. The large inland seas are shown; also, Europe, Africa, Asia, and New Holland; and around its north pole are fields of ice, and cloudy patches are over the whole sur. face. Such a vast globe, suspended apparently in the heavens, and revolving on its axis with a motion easily perceptible, must be a magnificent spectacle, and if the moon is really inhabited, well worth a journey round half its surface to behold.

TIME.

45

CHAPTER IV.

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:

66

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

*

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

INSCRIPTION,

FOR MY DAUGHTERS' HOUR-GLASS.

Mark the golden grains that pass,
Brightly thro' this channel'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?
Who shall collect his scattered sand,
Dispersed by Time's unsparing hand?
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!"

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 CLEPSYDRÆ.

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 Cæsar.

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 sun. 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

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