IV. FRENCH SYSTEM. 481. The basis of this system of weights is the weight in vacuo of a cubic decimetre of distilled water. This weight is called a kilogramme, and is the unit of the French system. It is equal to 2.204737 pounds avoirdupois. The other denominations are as follows: 10 milligrammes = 1 centigramme; 10 centigrammes = 1 decigramme; 10 decigrammes = 1 gramme; 10 grammes=1 decagramme; 10 decagrammes = 1 hectogramme; 10 hectogrammes 1 kilogramme; 10 kilogrammes = 1 quintal; = quintals = 1 ton of sea-water. 10 482. TIME is a part of duration. The time in which the earth revolves on its axis is called a day. The time in which it goes round the sun is called a solar year. Time is divided into parts according to the following The number of days in each month may be remembered by the following: Thirty days hath September, April, June, and November; All the rest have thirty-one, Excepting February, twenty-eight alone. NOTES.-1. Days are numbered in each month from the first day of the month. 2. Months are numbered from January to December. 3. The centuries are numbered from the beginning of the Christian Era. The year 30, for example, at its commencement, was called the 30th year of the first century, though neither the century nor the year had elapsed. Thus, June 2d, 1856, was the 6th month of the 56th year of the 19th century. 4. The civil day begins and ends at 12 o'clock at night. In the civil day, the hours are reckoned from that time. Dates. 1. The length of the solar year is 365 da. 5 hr. 48 m. 48 sec., very nearly. It is desirable to have the periods and dates of the civil year correspond to those of the solar year; else, the summer months of the one would in time become the winter months of the other, thereby producing great confusion in dates and history. 2. The common civil year is reckoned at 365 da., and the solar year at 365 da. 6 hr. The 6 hours accumulate for 4 years before they are counted, when they amount to 1 day, and are added to February; and the year is called a bissextile, or leap-year. 3. The odd 6 hours have been so added, that the leap years occur in those numbers which are divisible by 4. Thus, 1856, 1860, 1864 &c., are leap-years; and when any number is not divisible by 4, the remainder denotes how many years have passed since a leap-year. 4. This method of disposing of the fractional part of the year would be without error, if the solar year were exactly 365 da. 6 hr. in length; but it is not; it is only 365 da. 5 hr. 48 m. 48 sec. long : hence, the leap-year is reckoned at too much, and to correct this error, every centennial year is reckoned as a common year. But this makes an error again, on the other side, and every fourth centennial year the day is retained. Thus, 1800 was not, and 1900 will not be, reckoned a leap-year: the error will then be on the other side, and 2000 will be a leap-year. This disposition of the fractional part of the year causes the civil and solar years to correspond very nearly, and indicates the following rule for finding the leap-years: Rule.-Every year which is divisible by 4 is a leap year, unless it is a centennial year, and then it is not a leap-year unless the number of the century is also divisible by 4. 5. The registration of the days, by reckoning the civil year at 365 da., was established by the Roman emperor, Julius Cæsar, and hence this period is sometimes called the Julian year. The error, arising from the fractional part, continued to increase until 1582, when it amounted to 10 days; that is, as the year had been reckoned too long, the number of days had been too few, and the count, in the civil year, was behind the count in the solar year. In this year (1582), Pope Gregory decreed the 4th day of October to be called the 14th, and this brought the civil and the solar years together. The new calendar is sometimes called the Gregorian Calendar. 6 The method of dating by the old count, is called Old Style, and by the new, New Style. The difference is now 12 days. In Russia, they still use the old style; hence, their dates are 12 days behind ours. Their 4th of January is our 16th. VIII. UNITS OF CIRCULAR MEASURE, 483. Angular, or circular measure, is used in estimating latitude and longitude, in measuring the motions of the heavenly bodies, and also in measuring angles. The circumference of every circle is supposed to be divided into 360 equal parts, called degrees. Each degree is divided into 60 minutes, and each minute into 60 seconds. |