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flow uniformly in an unchangeable course, which alone serves to measure with accuracy the change of all other things. For, unless we correct the vulgar measures of time, which are gross and inaccurate, by proper equations, the conclusions are always found to be incorrect and erroneous. Time may be conceived to be divided into successive parts, that may be less and less without end, though, with respect to any one particular being, there may be a least sensible time, as well as a minimum sensibile in other magnitudes. But however various the flux of time may appear to different intellectual beings, it cannot be thought to depend upon the idea of any created being whatever.'
Our ideas relative to the measure of time, therefore, include those of motion; and when a body describes equal spaces with a uniform velocity, we readily conceive that the times of their description are equal to each other, or that an equal interval of duration elapses between the commencement and termination of each motion. This may be familiarly illustrated by conceiving the minute hand of a watch to pass with a uniform motion over the whole circumference of the dial plate; then, all the divisions on that plate being supposed to be equal to each other, the time employed by the hand in passing over each of them must be the same, and each complete revolution of the hand will also be performed in the same time. Now, as the stars are fixed bodies at an immense distance from the Earth, and the diurnal rotation of the Earth is a uniform motion, this illustration may be readily transferred from the watch to the heavens; for, if we suppose an imaginary line to extend from any point on the Earth's surface to a star situated in the vertical plane passing through that point and the pole of the world, the termination of this line
would describe a circle in the heavens, with a uniform motion occasioned by the diurnal revolution of the Earth; and consequently not only every equal part of this circle would be described in an equal time, but each consecutive revolution would also be completed in precisely the same period of duration. This uniform revolution of the Earth on its axis, which causes any star always to return to the plane of the same meridian, after equal and successive intervals, therefore, becomes a proper measure of time, and one which the laws of nature appear to have rendered immutable. Each of these revolutions is denominated a sidereal day; and the measure of duration as referred to this standard is called sidereal time.
The Sun, however, is the most conspicuous object in the heavens, and therefore the best adapted for the division of time into periods for the common and practical purposes of life; and hence his successive returns to the same meridian, though after unequal intervals of duration, have generally been adopted. Duration, as thus measured by the apparent motion of the Sun, is denominated apparent or solar time, and each interval between two consecutive returns of the Sun to the same meridian constitutes a solar day, which is divided into 24 hours.
But as the Earth has an annual motion in its orbit as well as a diurnal rotation about its axis, it advances through a certain part of its orbit during each of its diurnal rotations; it must therefore have actually performed more than one complete revolution on its axis, before the Sun appcars again on the same meridian; for the diurnal reyolution of the Earth is evidently completed, when the plane of the terrestrial meridian becomes again parallel to its former position. If the Earth had not any motion in its orbit, the
time that elapsed between the Sun leaving any meridian, and arriving at that meridian again, would always be equal to the time in which it performed one complete revolution on its axis. But as the Earth moves nearly a degree in its orbit while it performs one diurnal revolution, the length of one of these revolutions must always be less than 24 hours, or the time which elapses between the arrival of the Sun at the meridian in two successive days. Instead of the Earth's motion in its orbit, if the Sun be supposed to move in an opposite direction, the effect will be the same. For when the Sun is in the meri. dian of any place on one day, and that place is carried round to the same position the next day, the Sun will have moved from his former situation; so that the place on the Earth must move through an additional arc before the Sun will again be in the meridian, and consequently it will have performed more than a complete revolution. The time which is occupied in describing this additional arc will evidently be the same part of 24 hours, as the arc is of a whole revolution of the Sun round the Earth, or of 360°. Now the Sun in one day moves through an arc equal to 59'8":3; therefore we have 360°: 59'8":3 ::24 h. : 3 m. 56 s.6. Hence 24 h. -3 m. 56 s.6 - 23 h. 56 m. 384, the real time in which the Earth performs a revolution about its axis. As the stars do not change their places like the Sun, they appear to describe circles in the heavens, or to revolve about the Earth in 23 h. 56 m. 3s:4; so that this is the length of the sidereal day, and 24 h. that of the solar day. Consequently, in 365 days, the Earth will perform just 366 complete revolutions about its axis.
(To be continued.]
The Naturalist's Diary
For MARCH 1819.
Thus all looked gay and full of cheer
To welcome the new-liveried year'. Such is Sir Henry Wotton's description of Spring, which will generally apply to the month of March, in forward seasons. And I do easily believe,' says old Izaac Walton, that peace and patience, and a calm çontent, did ever dwell in the cheerful heart of Sir Henry Wotton; because I know, that when he was beyond 70 years of age, he made this description of a part of the present pleasure that possessed him, as he sat quietly in à summer's evening on a bank fishing. The superabundant moisture of the earth is now dried up, and the process of vegetation is gradually brought on: those trees which, in the last month, were budding, now begin to put forth their leaves; and the various appearances of nature announce the approach of SPRING. The latest springs, however, are always the most favourable, because, as the young buds do not appear so soon, they are not liable to be cut off by chilling blasts.
The melody of birds now gradually swells upon the ear. The throstle (turdus musicus), second only to the nightingale in song, charms us with the sweetness and variety of its lays. The linnet and the goldfinch join the general concert in this month, and the golden-crowned wren (motacilla regulus) begins its song. Rooks build and repair their nests. Rooks, crows, and pigeons, it has been
* See the remainder of the poem in Walton's Angler, p. 122.
There is a beautiful Elegy on the Approach of Spring,' in T. T. for 1817, p. 85.
proved, are by no means so detrimental to the farmer as is generally imagined, though many of them still commit great havoc among these birds, and use every means in their power to frighten them away. (See T. T. for 1816, pp. 86, 87.)
Among the numerous singing birds which delight us with their notes in the spring, the lark must not be forgotten. The melody of this little creature continues during the whole of summer. It is chiefly, however, in the morning and evening that its strains are heard; and as it chaunts its mellow notes on the wing, it is the peculiar favourite of every person who has taste to relish the beauties of nature at the most tranquil seasons of the day, particularly at dawn. See some interesting particulars of this bird in T.T. for 1817, p. 76.
The CAPTIVE LARK.
Where feathery snares were set
Laid struggling in the net.
And blessings on your store !
To see my face no more..
Of birds will vouch for me :
Let innocence go free.
"I took, 'twas all I did;
Justice will sure forbid!'
Their craving brood to rear,
Which held her husband dear.
Did she for mercy call;