TIME.

264. Time is naturally divided into days and years; the former are caused by the revolution of the Earth on its axis, the latter by its revolution round the sun.

The following are the names of the 12 calendar months into which the civi or legal year is divided, with the number of days in each.

January,

written (Jan.) the first month, has 31 days.

February,

(Feb.)

second

28

The number of days in each month may be easily remembered from the foll lowing lines:

OBS. 1. A Solar year is the exact time in which the earth revolves round the sun, and contains 365 days, 5 hours, 48 minutes, and 48 seconds.

2. Since the civil year contains 365 days and 6 hours, (nearly,) it is plain that in four years a whole day will be gained, and therefore every fourth year must have 366 days. This day was originally added to the year, by repeating the sixth of the Calends of March in the Roman calendar, which corresponds with

ضرت

QUEST.-264. How is time naturally divided? Recite the Table. Obs What is a solar year? How is leap year occasioned? To which month is the odd day added?

the 24th of February in ours. It was called the intercalary day, from the Latin intercalo, to insert.

The year in whch this day is added, is called Bissextile, from the Latin bis, twice, and sextilis, the sixth. It is also called "Leap Year," because it leaps over a day more than a common year.

3. The civil or legal year is often called the Julian year, from Julius Cæsar emperor of Rome, who adapted the calendar or register of the civil year to the supposed length of the solar year, oy adding 1 day to every fourth year.

265. In process of time, as mathematical and astronomical science advanced, it was found that the length of a solar year was only 365 d. 5 hrs. 48 min. 48 sec., or 11 min. 12 sec. less than 3654 days, which in 400 years amounted to about 3 days; consequently, the Julian calendar was behind the solar time. This error at the time of Pope Gregory XIII., amounted to 10 days, which he corrected in 1582 by suppressing 10 days in the month of October, the day after the 4th being called the 15th. Hence this calendar is sometimes called the Gregorian calendar.

OBS. 1. This correction was not adopted in England till 1752, when the error amounted to 11 days. By Act of Parliament, 11 days, after the 2d of September, were therefore omitted; and the civil year by the same Act, was made to commence on the 1st of January, instead of the 25th of March, as it had done previously.

2. Dates reckoned by the old method or Julian calendar, are called Old Style; and those reckoned by the new method, are called New Style.

To change any date from Old to New Style, we must add 11 days to it; and if the given date in Old Style, is between the 1st of January and the 25th of March, we must add 1 to the year in New Style.

Russia still reckons dates according to Old Style. The difference now amounts to 12 days.

266. To ascertain whether a year is LEAP YEAR.

The remainder, if any,

Divide the given year by 4, and if there is no remainder, it is Leap year. shows how many years have clapsed since a Leap year occurred. Thus, dividing the year 1847 by 4, the remainder is 3; hence it is 3 years since the last leap year, and the ensuing year will be leap year.

OBS. 1. To this rule there is an exception. For, we have seen, that a solzi year is 11 min. and 12 sec. less than a Julian year, which is 365 days. This error, in 400 years, amounts to about 3 days; consequently, if 1 day is added

QUEST.-266. How do you ascertain whether a year is leap year?

every fourth year; that is, if we have 100 leap years in 400 years, according to the Julian calendar, the reckoning would fall 3 days behind the solar time. Thus, reckoning from the commencement of the Christian era, when it was January 1st, 401 by the Julian time, it was January 4th by the solar time.

2. To remedy this error only 1 centennial year in four is regarded a leap year; or, which is the same in effect, whenever the centennial year, or the number expressing the century, is not divisible by 4, that year is not a leap year, while the other centennial years are. Thus, 17, 18, 19, denoting 1700, 1800, and 1900, are not divisible by 4, consequently they are not leap years, though according to the rule above they would be; on the other hand 16 and 20, denoting 1600 and 2000, are divisible by 4, and are therefore leap years. There is still a slight error, but it is so small that in 5000 years it scarcely amounts to a day.

CIRCULAR MEASURE, OR MOTION.

267. Circular Measure is applied to the divisions of the circle, and is used in reckoning latitude and longitude, and the motion of the heavenly bodies.

This measure is often called Angular Measure, and is chiefly used by astronomers, navigators, and surveyors.

90°

270°

180°

Note.-The division of the circumference of the circle into 360 equal parts, took its on gin from the length of the year, which, (in round numbers) was supposed to contain 360 days, or 12 months of 30 days each. The 12 signs QUEST-267. In what is Circular Measure used? Repeat the Table. Obs. How is the circumference of every circle divided? On what does the length of a degree depend?

correspon 1 to the 12 months. The term minutes, is from the Latin minuum, which signifies a small part. The term seconds, is an abbreviated expression for second minutes, or minutes of the second order.

268. Since the earth turns on its axis from west to east once in 24 hours, it evidently revolves 15° per hour; or 1° in 4 minutes, and 1' in 4 seconds of time. Hence,

When the difference of longitude between two places is 1', the difference in the time, or the hour of the day at these two places, is 4 seconds; if the difference of longitude is 1°, the difference of time is A minutes; if 2°, the difference of time is 8 minutes, &c.

Thus, when it is noon at London, in Philadelphia, which is about 75° west from London, it is only 7 o'clock, A. M. For, if the earth revolves 1° in 4 minutes, to revolve 75°, it will require 75 times as long, and 4×75=300 min., or 5 hours.

OBS. 1. Since the earth revolves from west to east, it is manifest, that the time is carlier as we go eastward, and later as we go westward.

9. This principle affords navigators and others a convenient and useful method of ascertaining the difference of time between two places, when the difference of their longitude is known; also, for ascertaining the difference of logitude between two places, when the difference in their time is known.

MISCELLANEOUS TABLE.

269. The following denominations not included in the precding Tables, are frequently used.

Note.-Formerly it was customary to allow 112 lbs. for a quintal.

QUEST.-268. When the difference of longitude between two places is 1', what is the difference of time? When 1°, what is the difference of time?

PAPER AND BOOKS.

270. The terms folio, quarto, octavo, &c., applied to books, denote the number of leaves into which a sheet of paper is folded.

Note.-American paper is usually rather larger than English paper of the

same name.

QUEST.-270. What do the terms, folio, quarto, &c., denote, when applied to books What is a folio? A quarto? An octavo? A duodecimo? An 18mo.? A 36mo. ?