another; at all some other part represented on Now when it tropics, which

drawn from the centre of the sun to that of the earth passes through those points where the equator and ecliptic cross one other times, it passes through of that oblique circle, which is the globe by the ecliptic line. passes through the equator or the are circles parallel to the equator, the sun and the clocks go together as far as regards this cause, but at other times they differ, because equal portions of the ecliptic pass over the meridian in unequal parts of time on account of its obliquity.

Charles. Can you explain this by a figure?

Tutor. It is easily shown by the globe which this figure Ns (Plate VI. Fig. 10.) may represent; will be the equator, the northern half the ecliptic, and the southern half. Make chalk or pencil marks a, b, c, d, e, f, g, h, all round the equator and ecliptic, at equal distances (suppose 20 degrees) from each other, beginning at Aries. Now by turning the globe on its axis, you will perceive that all the marks in the first quadrant of the ecliptic, that is, from Aries to Cancer, come sooner to the brazen meridian than their corresponding marks on the equator:-those from the beginning of Cancer to Libra come later :-those from Capricorn sooner-and those from Capricorn to Aries later.

Now time as measured by the sun-dial is represented by the marks on the ecliptic; that

measured by a good clock, by those on the equator.

Charles. Then while the sun is in the first and third quarters, or what is the same thing, while the earth is travelling through the second and fourth quarters, that is, from Cancer to Libra, and from Capricorn to Aries, the sun is faster than the clocks, and while it is travelling the other two quarters it is slower.

Tutor. Just so: because while the earth is travelling through the second and fourth quadrants, equal portions of the ecliptic come sooner to the meridian than their corresponding parts of the equator: and during its journey through the first and third quadrants, the equal parts of the ecliptic arrive later at the meridian than their corresponding parts of the equator.

James. If I understand what you have been saying, the dial and clocks ought to agree at the equinoxes, that is, on the 20th of March, and the 23d of September, but if I refer to the Ephemeris, I find that on the former day (1809) the clock is 8 minutes before the sun and on the latter day the clock in almost 8 minutes behind the sun.

Tutor. If this difference between time measured by the dial and clock depended only on the inclination of the earth's axis to the plane of its orbit, the clocks and dial ought to be together at the equinoxes, and also on the 21st of June and the 21st of December, that is, at the sum

mer and winter solstices; because, on those days, the apparent revolution of the sun is parallel to the equator. But I told you there was another cause why this difference subsisted.

Charles. You did: and that was the elliptic form of the earth's orbit.

Tutor. If the earth's motion in its orbit were uniform, which it would be if the orbit were circular, then the whole difference between equal time as shown by the clock, and apparent time as shown by the sun, would arise from the inclination of the earth's axis. But this is not the case; for the earth travels, when it is nearest the sun, that is, in the winter, more than a degree in 24 hours, and when it is farthest from the sun, that is, in summer, less than a degree in the same time: consequently from this cause the natural day would be of the greatest length when the earth was nearest the sun, for it must continue turning the longest time after an entire rotation, in order to bring the meridian of any place to the sun again: and the shortest day would be when the earth moves the slowest in her orbit. Now these inequalities, combined with those arising from the inclination of the earth's axis, make up that difference which is shown by the equation table, found in the Ephemeris, between good clocks and true sun-dials.

CONVERSATION XXXIV.

Of Leap Year.

James. Before we quit the subject of time, will you give us some account of what is called in our Almanacs Leap-Year?

Tutor. I will. The length of our year is, as you know, measured by the time which the earth takes in performing her journey round the sun, in the same manner as the length of the day is measured by its rotation on its axis. Now, to compute the exact time taken by the earth in its annual journey, was a work of considerable difficulty. Julius Cæsar was the first person who seems to have attained to any accuracy on this subject.

Charles. Do you mean the first Roman emperor, who landed also in Great-Britain?

Tutor. I do. He was not less celebrated as a man of science, than he was renowned as a general of him it was said,

Amidst the hurry of tumultuous war,

The stars, the gods, the heavens were still his care;
Nor did his skill to fix the rolling year,

Inferior to Eudoxus's art appear.

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

MW