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Bissextile or Leap years. But, since this excess is not quite 6 hours, this rule of adding one day to February every fourth year is interrupted, and the centennial years, which are not divisible by 400, are regarded as common years."

Hence, any year, (except a centennial year,) which is divisible by 4, is a Leap year, or consists of 366 days.

Centennial years which are divisible by 400 are regarded as Leap years; all others are considered as common years.

1796, 1804, 1808, 1812, 1816, 1820, 1824, 1828, 1832 1836, 1840, were all Leap years. 1800, not being divisible by 400, was a common year of 365 days; the same may be said of 1900; but the year 2000, being divisible by 400, will be a Leap year.

The number of days in the respective months may be recalled by recollecting the following versification:

Thirty days hath September,
April, June, and November-
All the rest have thirty-one,
Excepting February alone,
Which has but twenty-eight in fine,
Till Leap Year gives it twenty-nine.

Repeat the Table for Time. What is the length of the solar year to the nearest second? What is the more accurate value in decimals ? Into how many calendar months is the civil year divided ? Repeat their names and the number of days belonging to each. How often in general is an additional day added to February? What are such years styled ? Is the rule of counting every fourth year Leap year correct? Are centennial years, which are not divisible by 400, Leap years? Was 1800 a Leap year? Mention the next preceding and next following Leap year to 1800.

It is very desirable to be able readily to determine the number of days from any particular date to any other date. For this purpose, we will give the following

* There is still a further modification which takes place at the end of every 1000 years, which it is unnecessary to explain in this place.

TABLE,

SHOWING THE NUMBER OF DAYS FROM ANY DAY OF ONE MONTI TO THE BAX

DAY OF ANY OTHER MONTH IN THE SAME YEAR.

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As an example, suppose we wish the number of days from November 6th to the 15th of next April. We find No. vember in the left-hand vertical column, and April at the top line of the table, and at the intersection we find 151 days. So that from November 6th to April 6th is 151 days; consequently, adding 9, we find 160 for the number of days between November 6th and April 15th.

This table is constructed on the supposition of 28 days to February. When there are 29 days in February the proper allowance must be made.

EXAMPLES.

1. How many days from May 3d to the 4th of the next July?

Ans. 62 days.

2. How many days from July 4th to the 25th of the next December?

Ans. 174 days. 3. How many days from March 21st to the 23d of the next September?

Ans. 186 days. 4. How many days from September 23d to the 21st of the next March?

Ans. 179 days. 5. How many days from June 21st to the 22d of the next December?

Ans. 184 days. 6. How many days from December 22d o the 21st of the next June?

Ans. 181 days. 7. How many days from March 21st to the 21st of the next June?

Ans. 92 days. 8. How many days from Jan. 13th, 1848, to September 17th of the same year?

Ans. 248 days.

CIRCULAR MEASURE, OR MOTION.

77. By this is estimated Lauruse and Longitude, and the motion of the heaven.v oodle: a hich appear to move in circles. Every circle, whether great or small, is divided into 360 degrees.

TABLE.

60 seconds " . ... make 1 minute,'
60 minutes . . . . . " l degree, O
30 degrees ...... 6 1 sign, 8.
12 signs or 3600. .. circle, cr.

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• The sun appears to pass completely around the earth

in 24 hours, that is, it appears to move westward over 360° of longitude in 24 hours. Consequently, in one hour it will move over of 360°=15° of longitude. Hence, if the difference in the longitudes of two places is 150, it will be noon at the more easterly place, just one hour before it is noon at the other place. And in all cases, the difference in time of any two places will be at the rate of one hour for every 15° of longitude between the two places. As an example, suppose the city of Washington to be 770 west of Greenwich: it is required to find what time it is at Washington, when it is noon at Greenwich.

Dividing 770 by 15o we have 5 t for the number of hours difference in time, that is, 5h. 8m. And as the apparent motion of the sun is westward, it must be earlier at Washington than at Graprwich. Therefore, when it is noon at Greenwich, i ima 5h. em. before noon at Washington; that is, it is - Washi..gton 6h. 52m. A. M.

What use is made of Circular Motion ? Into how many degrees are all circles supposed to be divided ? Repeat the Table. Over how many degrees of longitude does the sun appear to move in 24 hours ? Over how many degrees in 1 hour : What is the difference of time corresponding to 770 ? When it is noon at Greenwich, what time is it at Washington, 770 west of Greenwich ?

78. Measures, &c., not included in the foregoing tables.

6 points make 1 line used in measuring length of 12 lines “ 1 inch clock pendulum rods. 4 inches 6 1 hand

s used in measuring the height

? of horses. 6 feet « 1 fathom used in measuring depths at 12 individual things make 1 dozen. 12 dozen ... ..6 1 gross. : 12 gross . . . . ." 1 great gross. 20 individual things 6 1 score. . 24 sheets of paper. 6 1 quire. 20 quires. ...6 1 ream. .12 pounds. .

sea.

1 quintal of fish. 2006 . ..." 1 barrel of pork or beef. 196 " ... " 1 barrel of flour. Repeat the above tables.

BOOKS.

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fol

79. A sheet folded into two leaves is called a folio.

“ folded into four leaves is called a quarto,

or 4to. folded into eight leaves is called an oc

tavo, or 8vo. 66 folded into twelve leaves is called a duo

decimo, or 12mo. 66 folded into eighteen leaves is called an

• 18mo.

When a sheet is folded into two leaves what is it called? How called when folded into four leaves ? How, when folded into eight leaves ? How, when folded into twelve leaves? How, when folded into eighteen leaves ?

REDUCTION.

80. REDUCTION is the changing of numbers from one name or denomination to another, without altering their value.

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