The dominical letter being known, it is easy to find the day of the week on which any month begins, by the following table.

The following couplet is de-A table showing at what days of the

signed to assist the memory.

months the letters formerly stood; and where the dominical letters now stand.

All days decline; great blessings end [D. Letters. A B C D E F G Good Christians find a during friend.

The first letters of these twelve words are the same as those at the beginning of each month.

If the letter set at the first! day of a month be before or after the dominical letter in any year, the day on which the month begins, is before or after the Lord's day; and is as far distant as the commencing letter is distant from the dominical letter. Thus, if A be the dominical letter, January begins on the Sabbath; February and March on Wednesday. If B be the dominical letter, January begins on Saturday, February and March on Tuesday.

CHAPTER X.

Obliquity.

THE obliquity of the equator to the plane of the ecliptic being the cause of the variety of seasons, the different length of days and nights, and the pleasing vicissitudes resulting from the varying year, is well deserving a place, even in a compendium of astronomy. The principal inquiry is, whether the obliquity remains the same, or is subject to a constant diminution.

The obliquity of the ecliptic to the equator,' says Dr. Brewster, was long considered a constant quantity. Even so late as the end of the 17th century, the difference between the obliquity, as determined by ancient and modern astronomers, was generally attributed to inaccuracy of observation,) and a want of knowledge of the parallaxes and refraction of the heavenly bodies. It appears, however, from the most accurate modern observations, at great intervals, that the obliquity of the ecliptic is diminishing By comparing about 160 observations of the ecliptic, made by ancient and modern observers, with the obliquity of 23° 28′ 16′′, as observed by Tobias Mayer, in 1756, we have found, that the diminution of the obliquity of the ecliptic during a century is (51"; a result, which accords wonderfully with the best observations.' This would bring the obliquity at the present time, 1831, to 23° 27′ 38′′.

The above statement, though contrary to the opinion of some philosophers, is in accordance with the true principles of Newtonian philosophy, and is corroborated by the best modern astronomers. Professor Vince, having stated the observations of many authors, ancient and modern, concludes; It is manifest, from these observations, that the obliquity of the ecliptic

continually decreases; and the irregularity, which here appears in the diminution, we may ascribe to the inaccuracy of the observations; as we know that they are subject to greater errors than the irregularity of this variation.'

The following table will give an idea of the diminution of the obliquity for many centuries. It was extracted from Rees' Cyclopædia.

A small difference will be seen between the statement of Dr. Rees and that of Professor Vince, respecting the obliquity, as observed by some of these authors. But as the general principle is not affected, it may be useless to attempt a reconciliation.

The part of the table on the left was taken from observation. It will be found very nearly to coincide with that on the right, formed by calculation from the observations of the most accurate modern astronomers.

The attraction of the moon on the spheroidical figure of the earth is without doubt the cause of the diminution above stated. [See a demonstration of this in the author's larger work on astronomy.] The action of the sun upon the same figure must have an effect similar to that of the moon; but far less in quantity on account of his immense distance. The principle is

nearly similar to that of the tides, as may be seen in the demonstration.

If such be the cause of diminution, the obliquity must continually decrease, and in time become extinct. But at the present ratio of diminution, such an event cannot occur under about 160,000 years; period stretching beyond the most distant wish of the present inhabitants respecting the concerns of this world.

The variety of seasons it is true must cease at such an event. But long ere that time the earth may be 'dissolved; or it may be renovated, so as to produce 'seed time and harvest, and summer and winter.'

Why does the obliquity of the equator to the plane of the ecliptic deserve a place in a compendium of astronomy? How was this obliquity long considered? To what was the difference between the obliquity, as determined by ancient and modern astronomers, long attributed? Does it appear now, that the obliquity is increasing or diminishing? What is the diminution in a century? What is the cause of the diminution? If the moon's attraction on the spheroidical figure of the carth be the cause of diminution in the obliquity, what must in fine be the effect? According to the present diminution of the obliquity, how many years would be required to bring the equator to the ecliptic? Ought the present inhabitants to be anxious respecting such an event?

CHAPTER XI.

Parallax.

THE true place of a heavenly body is the situation, in which it would appear, if seen from the centre of the earth; the apparent, where it is seen from the surface. The angular difference between these, the true and apparent place, is what is understood by parallax. It is equal to the angle, under which the semi-diameter of the earth would appear at the sun or a planet. Par

allax is greatest at the horizon, called horizontal parallax. Decreasing from this it becomes nothing at the zenith. Plate v. Fig. 7, let A B D be the earth, C its centre; M N O P the place of the moon at different altitudes. When the moon is at M, she would appear from the earth's centre among the stars at E; but as seen from the surface at A, she appears at F. When at N. she would be seen from the centre at G; but from A she seems at H. At O, her parallax being lessened, she would be seen from the different stations at 1 and at K. Having Having no parallax at P, she appears at the same place from C and from A, being seen in the zenith as at Z.

This is called diurnal parallax. Annual parallax is the difference between the apparent place of a heavenly body, as seen from opposite points of the earth's orbit. The diameter of this orbit is about 190 millions of miles. A spectator at one season of the year, as the 20th of June, must be the whole of this diameter distant from his place at the opposite season, the 20th of December. Hence an object, unless inconceivably distant, as seen from one part, must appear in a very different place, from the same object, as seen from the opposite part.

Diurnal parallax, usually denominated parallax without epithet, increases with the nearness of the body to the earth. The moon being the nearest heavenly body has the largest parallax; while the fixed stars being immensely distant, have no perceptible parallax, the semi-diameter of the earth appearing at that distance no more than a point.

Parallax always depresses a body, making it appear below its true place.

The horizontal parallax of the moon has long been known. It is of great importance in the calculation and projection of eclipses.

The parallax of the sun has been an object of attention from the greatest antiquity. Aristarchus, an astron