THE HOURS. 



or sunrise, the disadvantages indicated above must attend such a 

 mode of reckoning time ; to which it may be added, that of all 

 diurnal phenomena, there is not one of which the observation is 

 attended with more uncertainty and risk of error than sunrise 

 and sunset. 



16. The English, French, Germans, and generally the moderns 

 in all the more civilised parts of the globe, commence the day at 

 midnight, and divide it into two equal series of twelve hours, so 

 that midday is twelve o'clock as well as midnight. According to 

 this system of reckoning, it is necessary, whenever an hour is 

 named, to indicate its relation to noon. The hours before noon 

 are indicated by the letters A.M., and those after noon by P.M., 

 being the initials of the Latin words ante meridiem (before mid- 

 day), and. post meridiem (after midday). 



Among ancient astronomers who adopted this mode of reckoning, 

 may be mentioned Hipparchus, who flourished about a hundred and 

 fifty years before our era, and among moderns Copernicus. 



The ancient Egyptians began the day at noon, in which they 

 were followed by Ptolemy, a celebrated astronomer, who nourished 

 at Alexandria in the second century of our era. This diurnal 

 epoch has been by general consent adopted by modern astronomers, 

 who divide the day into twenty-four successive hours, reckoned 

 from noon to noon. Thus, according to their manner of reckoning, 

 twenty minutes and an half after ten o'clock in the morning, 

 would be 22 h 20 m 30 s . 



17. Civil or common time, therefore, is half a day before 

 astronomical time, a circumstance which must always be carefully 

 allowed for in the comparison of dates expressed according to the 

 two modes of reckoning. 



Thus, for example, the first day of the year 1854, according to 

 civil reckoning, commenced at the moment of midnight, between 

 the 31st December, 1853, and 1st January, 1854. But according 

 to astronomical reckoning it commenced at midday on 1st January, 

 1854. It follows, therefore, that the twelve hours which pre- 

 ceded the noon of "1st January, 1854, were according to astrono- 

 mical reckoning the last twelve hours of the year 1853. 



In like manner, a certain hour of the forenoon, 5 A.M. of a day 

 (Tuesday, for example), according to civil time, is 17 h O m s of 

 the preceding day (Monday), according to astronomical time. 

 From noon, however, till midnight of any given day, the civil 

 and astronomical dates are exactly the same. 



III. THE DAY. 



18. A day then being adopted by common consent, and indeed 

 by the force of things, as the standard unit for the measure of 



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