THE SKYLARK.

I.

How sweet is the song of the Lark as she springs
To welcome the morning with joy on her wings;
The higher she rises the sweeter she sings,
And she sings when we hear her no more.

II.

When clouds and thick mists veil the sun from our sight,
She has mounted above them, and sings in his light;
There, far from the scenes that disturb or affright,
She loves her gay music to pour.

III.

'Tis thus with the Christian-he sees from afar
The day-spring appearing, the bright morning star ;
He quits this dark valley of sorrow and care,
For the land whence the radiance is given.

IV.

He sings on his way from this cloud-cover'd spot,
The higher he rises, the sweeter his note,
When we hear it no longer, the song ceases not,
But it blends with the chorus of Heaven.

fixed, the star will cross the middle of the field, and by means of a clock or a watch which shows seconds, the time which the star takes in crossing the telescope may be observed. By repeating the operation two or three times with the same star, considerable accuracy may be obtained.

If a person has no telescope, the experiment may be made with a small roll of card paper, having at one end an aperture somewhat larger than a pin hole, to fix the place of the eye.

The observer will by these means easily satisfy himself, that stars in the equator appear to move with the greatest rapidity; that two stars at equal distances north or south of the equator appear to move more slowly than an equatorial star, but at the same rate one with another; and that the slowness of this apparent motion increases very rapidly as stars are observed considerably distant from the equator. For example, it requires a change of declination amounting to 60°, that a star should move twice as slowly as at the equator; whereas an additional change of only 10°, from 60° to 70° of declination, causes a star to move more than four times as slowly as at the equator.

Such are the general phenomena presented by the southern part of the sky.

If the observer now turns his attention to the northern part of the heavens, he will perceive changes of a somewhat different kind. Stars near the pole appear to move very slowly. The motion of the pole star itself, which is distant only about 1° 30′ from the pole, (or nearly three times the width of the moon), is almost imperceptible to the unassisted eye ; and the motion of some very small stars, still nearer to the pole, is still slower.

If there were a star exactly at the pole it would appear absolutely

at rest.

It may now be enquired, what is the apparent motion of stars which are situated between the pole and the northern point of the horizon? They will be observed to move in the opposite direction to that of the motion of the stars above the pole; the rapidity of their apparent motion increasing as different stars are taken more and more remote from the pole.

It will now be perceived, that such stars as are not farther distant from the pole than the horizon is, that is, not farther distant, in degrees of a great circle, than the latitude of the place of observation, never set, but continue to revolve about the pole, preserving always the same distance from it. Such stars are called circumpolar.

The stars of the Great Bear are familiar instances; those in the constellations of Cassiopeia, Draco, Cygnus, and many of those in Bootes, are also well known. Among the principal single circumpolar stars, may be mentioned the Pole Star itself, Capella, in Auriga, and the brilliant star, Alpha Lyræ, which, at its lower passage, in the latitude of London, just skirts the northern horizon; and in more northern latitudes passes that horizon at a somewhat greater altitude. From this peculiarity in their motion, it follows, that some stars not very far distant from the North Pole, are, in northern latitudes, visible at all times of the year, when the sky is clear, and when the light of the sun does not overpower that of the star; whereas all other stars rise and set.

The observer may thus, by the evidence of his own eyes, satisfy himself of the following facts :

I. The whole heavens appear to revolve from east to west.

II. This motion is performed about an axis, the northern extremity of which, in northern latitudes, is above the horizon, and the southern extremity below the horizon.

III. In consequence of this general motion, every heavenly body appears to describe a circle, which is greater or less, as the body is more or less distant from the pole; the largest circle so described being the equinoctial, or celestial equator. If the diurnal circle, described by a star cuts the horizon, the star rises and sets; if it does not cut the horizon, the star revolves continually round the pole without setting.

We may now proceed a step farther; and point out some circumstances, respecting the apparent diurnal motion of the heavens, which are not so easily verified.

This motion of the heavens is absolutely uniform; that is, it goes on continually, at precisely the same rate. The whole period of this revolution, called a sidereal day, is supposed to be divided into twenty-four sidereal hours. The uniformity of the motion of the heavens, is a fact by no means easy to be proved; because we have no other exactly equable motion, with which we can compare it. But in proportion as a clock or a watch can be regulated so as to mark time uniformly, in the same proportion it is found to agree with the apparent motion of the heavens. Thus we are furnished with an abso

lutely exact measure of time.

Suppose a telescope set so, that a star passes across the middle point of the telescope, (a wire being placed in the telescope for the purpose of facilitating the observation), and that the time, marked by the

clock, or a watch, is observed. After twenty-four sidereal hours, the star will have come to the same position again; and, if the telescope has remained fixed, will be exactly upon the wire, at the end of that interval of time. If the clock or watch should be found to mark always an equal interval of time, between two successive passages of a star over the middle point of the field of a fixed telescope, the clock or watch would be going uniformly, however it might gain or lose. If such a clock or watch could be further adjusted, so as to mark precisely twenty-four hours in that interval, it would mark sidereal time. This is the time by which observatory clocks are regulated.

The reason of the difference between sidereal time, and mean solar time, must be reserved, till we can speak of the annual motion of some of the heavenly bodies. For the present it will be sufficient to observe, that, if a clock were regulated accurately, to mean solar time, which is the time commonly employed, a star would return to the same position, after an interval of 23 h. 56 m. 4 s. as shewn by the clock; or the clock would appear to lose 3 m. 56 s., when referred to the stars.

Thus the rate of a clock or watch, or its daily gain or loss, can always be ascertained by referring to the stars; and even without a telescope, such an observation can be made, with considerable accuracy, by placing a mark, to fix the position of the eye, such as a nail driven into the side of a window; or, what is better, a small hole in a card, and by noticing on successive nights, the time at which a star disappears behind any fixed object. If the disappearance takes place, 3 m. 56 s. earlier every night, as shewn by the clock, the clock keeps true mean time. If otherwise, the gain or loss of the clock in twenty-four sidereal hours is known.

It may, here, be mentioned, that in comparing a clock with the heavens, the proper method is first to observe the second pointed by the clock, and then to count the beats by the ear, thus leaving the eye at liberty to observe the star.

Before concluding these remarks upon the apparent daily motion of the heavens, we may mention one circumstance, which is likely to mislead persons who have not paid attention to the subject, and which has caused considerable difficulty even to those who have studied the question most deeply. If the stars in a constellation are observed, and their relative distances fixed in the mind, when the stars are at a considerable altitude; and if the same stars are again observed when they are near the horizon, they will appear much farther apart than before, and thus seem to have changed their relative position. The

three stars in Orion's belt, and the stars of the Great Bear, are very favourably placed for observing this appearance. The sun and moon, in like manner, appear sensibly larger when near the horizon. Now this is entirely an optical delusion. If the distance of the stars from one another be measured with an instrument fitted for the purpose, that distance is found to be absolutely unaltered, except by a very small quantity arising from unequal refraction. One cause which occasions this apparent increase of relative distance, is probably, the faintness of the light near the horizon, in consequence of the greater thickness of the air through which the light passes; such faintness suggesting the idea of greater distance from the spectator, and consequently, that of greater distance between the objects themselves, or between different parts of the same object. Another cause, of the same nature, seems to be, that the concave of the sky does not appear to us to be a hemisphere, but a considerably less portion of a sphere, terminated on all sides by the horizon: and we feel, in fact, as if we could draw a much longer line along the horizon before we reached the sky, than if we drew a line vertically upwards. There is an illusion of the very same kind, when we stand upon the sea side, where the coast is extended to the right and left. We know well, that we can see exactly as far, when we look out to sea, as when we look along the shore. Yet a point in the horizon near the line of coast, appears very much more distant than another point taken in that part of the horizon which lies directly out at sea.

Having thus pointed out, in the apparent diurnal motion of the heavens, some circumstances which may be verified by reference to the heavens themselves, we propose next, to notice some of the changes depending upon the period of the year.