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a year commence with D, as the dominical letter; C, at the 28th of February, must in that case stand for Saturday; C also must be against the 29th, and, of course, being for the Lord's day, must be dominical; or, if D be repeated, C, at the 7th of March, becomes dominical, and thus continues through the year. The

next year would commence two days later in the week. On account of this leaping in the retrograde order of the letters, the seven occupy five years in a revolution, when leap year is twice included; six, when it is once included. Hence the days of the week return to the same days of the month in five or six years, according as bissextile is twice or but once included. In 28 years, the seven letters will always have five revolutions, except at the end of the centuries, when leap year is omitted.

The table following shows the dominical letter for 6000 years of the Christian era, according to the new style, or Gregorian calendar:+

A Table of Dominical Letters for 6000 Years of the Christian

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The dominical letter for any year

of the first century is found in the column of letters under 100, opposite to the year. For a year in any century after the first, find the century preceding the year at the top under this, and opposite to the year of the century, in the column for years less than 100, is the dominical letter sought.

EXAMPLE.

Under 1800, opposite to 31 in the left hand column, is B, the dominical letter for 1831.

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

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The following couplet is designed to assist the memory.

All days decline; great blessings end; 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.

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What are periods? How long is the Chaldean period? What takes place at the termination of this period? How is the Julian period formed? How long is it? What is its use? Of how many years does the cycle of the Sun consist? What is effected by this cycle? How long is the cycle of the Moon, or golden number? What takes place at the expiration of this cycle? What is the Roman indiction? How are the cycle of the Sun, golden number and indiction found? What is the epact? How is the Julian epact found? How is the Gregorian? Are the golden number, epact, and indiction of much use? What is the difference between old and new style in the present century Which is the proper day for the celebration of Washington's "birth? What constitutes a year? Of what does the civil solar year consist? How long is a lunar year? What was added to make this year coincide with the solar year? How did the Jews compute their time? What defect was there in their computation? Of what did the year of the Greeks consist? Was their year easily connected with the solar year? When were the Olympic games celebrated? At what season was the first month of the Grecian year? How did the ancient Romans compute time? They reckoned by lunar years, established by whom? Who established the Julian calendar? In this calendar how many years were common? When was the day added in leap year? Why was it called bissextile? On what day of March did the vernal equinox happen at the time of the Council of Nice, in the year 325? In 1582, what did Pope Gregory XIII. observe? To correct the style, what did he do? When was the Gregorian, or new style, adopted in England? Where does the Julian, or old style, still prevail? What principle did Pope Gregory endeavor to establish? By his principle, how many bissextiles are to be omitted in four hundred years? How can it be ascertained what bissextiles are to be omitted? Has any bissextile been of late omitted? Does the civil year, as now established, exactly coincide with the tropical? What omission would bring the two computations to great exactness? What is a sidereal lunar month? What is the principal lunar month? Of how many kinds are civil months? Of what does the weekly month consist? What is the true legal month? What are the other months? Of what origin are they? How are the names? What was the sixth month formerly called? Why was it called August? How was the compliment heightened? What seems to have been the foundation of weeks? What is an artificial day? What is a natural day? When did the ancient Egyptians begin their day? What nations now begin their day at midnight? When did the Jews begin their day? How did they divide the night and the day? What modern nations begin their day at sunset? What ancient nations commenced their day at sunrising? What moderns? When does the nautical, or sea-day, commence ? How are the hours marked? When does the astronomical day begin? How is it reckoned? What is an hour? From whom did the Greeks learn their method of dividing the day? Who divided the day and the night each into four watches? How long was a watch? After hours, in what

order do all the remaining divisions of time proceed? How were the first seven letters of the alphabet formerly placed in almanacs? Who introduced them? Why was one of these called dominical? If a common year begins on Monday, on what day will the next year begin? What prevents the dominical letters from proceeding in a retrograde order, and returning again at the end of seven years? In what order were the letters placed in almanacs? What causes two dominical letters in leap year? In how long time do the days of the week return to the same days of the month? What is a convenient couplet? Why is it convenient? If you know the letter at the beginning of a month, and the dominical letter, how will you find on what day of the week the month begins?

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 ob

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