Balance, from the 21st June to the 23rd September, the patches on the ecliptic will arrive later, the clock then appears too fast. From the first of the Balance to the first of the Goat (the 23rd of September to the 21st of December), they will arrive sooner; the clock again appears too slow. And from the first of the Goat to the first of the Ram, (the 21st of December to the 20th of March), the clock is again too fast.

This experiment is interesting, is easily performed, am is very satisfactory, as far as it goes. The clock and the sun would be accurately together at the equinoxes and solstices, if their agreement depended only upon the obliquity of the ecliptic to the eqtor; if the Earth did not revolve in an elliptical orbit, and if the consequences of that revolution did not prevent such an agreement.

The elliptical form of the Earth's orbit, is a principal cause of the difference between time as shewn by the sun and the clock. The Earth is at different distances from the Sun in different parts of its orbit, and not being at all times attracted in an equal degree by the Sun, the Earth does not always move with the same velocity: so that the Earth is not always precisely at its places of aphelion and perihelion at the time of the solstices, nor exactly at its mean distance from the Sun at the time of the equinoxes.

Moreover, the Earth cannot, from the elliptical form of its orbit, turn round on its axis to the Sun, accurately once in 24 hours. This also in the cause of slight variations in the length of the Solar days, as will become apparent on considering the following diagram; in which, it must be acknowledged, that this cause of difference amongst them is considerably exaggerated. But we are now speaking of causes generally, without attempting to deûne consequences precisely.

Let the circle represent an orbit, of which Z, the centre, would in that case be the station of the Sun; and let the oval within it represent another orbit, of which S is the station of the Sun. Then let us suppose the Earth to be at M, on which the meridian (the line crossing it) will be opposite to the Sun whether it be at S or Z.

Now it must be observed that as the Earth's orbit nearly approaches a circle, so, like a circle, it may be imagined to consist of 360 degrees; and as the Earth passes through 360 degrees in 365 days, it is nearly 1 degree that the Earth proceeds eastward every day. Owing to its proceeding at this rate, it must, day after day, turn a little more round towards the Sun before the same meridian can come opposite to the Sun, that is now opposite to it at M, than it would if the Earth did not move forward in its orbit. Let the Earth have moved through 1-8th part of its orbit, (one quarter of the upper half from M) and it will be apparent that the Earth moving in the ellipsis or oval, must in its daily revolutions, turn somewhat more, before its meridian will be opposite to the Sun S-its day would be longer, than the day of the Earth moving in the circle,-—which would have to turn less round to the Sun Z. The dif

ference between the two will be still more apparent by the time they have proceeded through a quarter of their orbits. But in the second quarter, the Earth moving in the oval would have to turn less,—its natural day would not then be so long as that of the Earth moving in the circle-the meridian of which would have to turn more round to its Sun. By the time the Earth has got half way round, the meridian will be opposite to the Suns at Z and S; so that though the Earth moving in the oval had to turn most in the first quarter, this was compensated by its having to turn less in the second; and thus, on an average, in proceeding through half its orbit, it kept pace with the Earth moving in a circle, whose motion, as the Sun is precisely in the centre, is equable. The same will happen in passing through the other half. The Earth moving in the oval has to turn less in the third quarter; so that what it gains on the clock there, it loses by turning more in the fourth: hence, at the end of that quarter, the clock and sun agree, as they did when they set out.

It is then the united operation of these causes that produces the variations between time, as shewn by a sun dial, and time, as measured by a clock; and it is from these causes, that the only days in the year on which the Sun and Clock now agree, are the 15th of April, the 15th of June, the 1st of September, and the 24th of December.

The motion of a clock is supposed to be uniform. But it is impossible that it should be uniform from the very nature of the materials of which it is composed; for these are variously affected not only by heat and cold; but also by the imperfection of workmanship, and their wearing unequally. But all these imperfections are in

great measure counterbalanced; for the astronomer and mathematician, being well acquainted with all the variations arising from the different causes just pointed out, are able to equalize them all-to construct tables for the Equation of Time; and by these tables our public clocks are regulated with great exactness.

A concise Equation Table, adapted to the second Year after Leap Year, and within a Minute of the truth for every Year; shewing, to the nearest full Minute, how much a Clock should be faster or slower than the Sun. By Mr. Smeaton.

This table is near enough for regulating common clocks and watches. It may be easily copied by the pen, and being doubled, may be put into a pocket book.

LECTURE VIII.

Of the Fixed Stars.

Ir the object of the evenings we have devoted to Astronomy had not been to store our minds with the leading truths of the science, as resulting from the observations of the astronomer, and the calculations of the mathematician; if this had not been our object, rather than to form ourselves into observers of the motions of the celestial bodies, I should certainly have preferred another arrangement, and should have begun with the Fixed Stars, instead of ending with them: for the acquirement of an intimate acquaintance with them, —with the relative magnitudes and situations of all such as are visible to us,-is the first step to be taken by the young astronomer.

Hitherto, I have adverted to the fixed stars, only in so far as they were connected with what I have termed the Astronomy of the Earth; and although the consideration of them forms the peculiar object of the present evening, it seems advantageous first to advert to the subject of Light; because, as it pervades the universe, its claim to some attention is indisputable; but it would be truly advantageous to us, if it were possible to convey any satisfactory and conclusive information respecting its nature, because it would, in that case, tend to enlarge our views of the universe.