Julius Cæsar, who was well acquainted with the learning of the Egyptians, fixed the length of the year to be 365 days and six hours, which made it six hours longer than the Egyptian year. Now, in order to allow for the odd six hours in each year, he introduced an additional day every fourth year, which accordingly consists of 366 days, and is called Leap-Year, while the other three have only 365 days each. From him it was denominated the Julian year.

James. It is also called Bissextile in the Almanacs: what does that mean?

Tutor. The Romans inserted the intercalary day between the 23d and 24th of February: and because the 23d of February, in their calendar, was called sexto calendas Martii, the sixth of the calends of March; the intercalated day was called bis sexto calendas Martii, the second sixth of the calends of March, and hence the year of intercalation had the appellation of Bissextile. This day was chosen at Rome, on account of the expulsion of Tarquin from the throne, which happened on the 23d of February. We introduce, in Leap-Year, a new day in the same month, namely, the 29th.

Charles. Is there any rule for knowing what year is Leap-Year?

Tutor. It is known by dividing the date of the year by 4, if there be no remainder, it is LeapYear; thus 1799 divided by 4, leaves a remain

der of 3, showing that it is .the 3d year after Leap-Year. These two lines contain the rule:

Divide by 4; what's left shall be

For Leap-Year 0; for past 1, 2, 3.

James. The year, however, does not consist of 365 days and 6 hours, but of 365 days, 5 hours, 48 minutes, and 49 seconds.* Will not this occasion some error?

Tutor. It will; and by subtracting the latter number from the former, you will find that the error amounts to 11 minutes and 11 seconds every year, or to a whole day in about 130 years: notwithstanding this, the Julian year continued to be in general use till the year 1582, when Pope Gregory XIII. undertook to rectify the error, which, at that time, amounted to ten days. He accordingly commanded the ten days between the 4th and 15th of October in that year to be suppressed, so that the 5th day of that month was called the 15th. This alteration took place through the greater part of Europe, and the year was afterwards called the Gregorian year, or New Style. In this country, the method of reckoning, according to the New Style, was not admitted into our calendars until the year 1752, when the error amounted to nearly 11 days, which were taken from the month of September, by calling the 3d of that month the 14th.

* See Conversation XXX,

Charles. By what means will this accuracy be maintained?

Tutor. The error amounting to one whole day in about 130 years, it is settled by an act of parliament, that the year 1800 and the year 1900, which are, according to the rule just given, Leap-Years, shall be computed as common years, having only 365 days in each: and that every four hundredth year afterwards should be common years also. If this method be adhered to, the present mode of reckoning will not vary a single day from true time, in less than 5000 years.

By the same act of parliament, the legal beginning of the year was changed from the 25th of March to the 1st of January. So that the succeeding months of January, February, and March, up to the 24th day, which would, by the Old Style, have been reckoned part of the year 1752, were accounted as the first three months of the year 1753. Hence we sometimes see such a date as this, Feb. 10, 1774-5, that is, according to the Old Style it was 1774, but according to the New it is 1775, because now the year begins in January instead of March.

VOL. I. R

CONVERSATION XXXV.

Of the Moon.

Tutor. You are now, gentlemen, acquainted with the reasons for the division of time into days and years.

Charles. These divisions have their foundation in nature, the former depending upon the rotation of the earth on its axis; the latter upon its revolution in an elliptic orbit about the sun as a centre of motion.

James. Is there any natural reason for the division of years into weeks, or of days into hours, minutes, and seconds?

Tutor. These divisions were invented entirely for the convenienee of mankind, and are accordingly different in different countries. There is, however, another division of time marked out by nature.

Charles. What is that, sir?

Tutor. The length of the month: not indeed that month which consists of four weeks, nor that by which the year is divided into twelve parts. These are both arbitrary. But by a month is meant the time which the moon takes in performing her journey round the earth:

Then mark'd astronomers with keener eyes
The moon's refulgent journey through the skies.

DARWIN.

James. How many days does the moon take for this purpose?

Tutor. If you refer to the time in which the moon revolves from one point of the heavens to the same point again, it consists of 27 days, 7 hours, and 48 minutes, this is called the periodical month but if you refer to the time passed from new moon to new moon again, the month consists of 29 days, 12 hours, and 44 minutes, this is called the synodical month.

Charles. Pray explain the reason of this difference.

Tutor. It is occasioned by the earth's annual motion in its orbit. Let us refer to our watch as an example. The two hands are together at 12 o'clock; now when the minute-hand has made a complete revolution, are they together again?

James. No; for the hour-hand is advanced the twelfth part of its revolution, which in order that the other may overtake, it must travel five minutes more than the hour.

Tutor. And something more, for the hourhand does not wait at the figure I, till the other comes up and therefore they will not be together till between 5 and 6 minutes after one.

Now apply this to the earth and moon, suppose (Plate vII. Fig. 11.) s to be the sun; r the earth in a part of its orbit Q L; and E to be the position of the moon; if the earth had no mo