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ALFRED SWEETING, PRINTER, BARTLETT'S BUILDINGS, LONDON,
Explanation of the column headed "Thermomctrical Register."
THE Thermometrical Register commences with Nov. 1848, and closes with Oct. 1849 inclusive. These registers have been carefully copied from those made at the Royal Observatory, Greenwich, and which are also deposited with the Royal Society. They show the highest and lowest ranges within each twenty-four hours, from twelve different readings. Each month is placed with its corresponding month, as affording the most ready and advantageous means of comparison, although by this arrangement the register of the last two months of 1848 follows the ten months of 1849, which are all we can give up to the day of publication.
Explanation of the columns headed "Length of day,' ," "Day's increase or decrease," Day breaks," and " Twilight ends."
THE column headed "Length of day" contains the number of hours and minutes between sunrise and sunset. The column headed "Day's increase' expresses the number of hours and minutes which the day has increased since the shortest day; and, where the column is headed Day's decrease," it expresses the number of hours and minutes which the day has decreased since the longest day; for example, the length of the longest day in 1850 is put down in the column "Length of day" at 16h. 34m.; and on the 6th of July following we find that the length of the day, or the number of hours and minutes between sunrise and sunset, is set down at 16h. 24m.; hence the day has decreased 10m. since the longest day, and, accordingly, in the column " Day's decrease," we find opposite July the 6th Oh. 10m.
Equation of Time.
IN this Almanac the calculations are all made for mean time (given by the clock), instead of apparent time (given by the sun-dial), which latter had been used up to the year 1833. It must be obvious that, for all practical purposes, mean time is the most useful; and to obtain it from apparent time, the columns in the Almanac headed "Equation of Time" should be used. The column "Equation of Time" ought, for example, to be consulted when persons are desirous of setting their clock by a sun-dial. When clock after sun is written above the number of minutes and seconds opposite to the day, then the clock ought to be set so much slower than the sun-dial, and the contrary.
THE Moon's age is set down in days and the nearest tenths of days from the time of change. Thus it is New Moon on the 12th of February at 6h. 29m. morning, and therefore at noon on the 13th she is 1 day 5h. 31m. old, which is set down as 1 day and twotenths. The fraction of the day of course continues the same throughout the lunation.
LIST OF THE CORRESPONDENCE OF ERAS WITH THE YEAR 1850. [In those Eras which begin with the Christian year, the year alone is stated; in those which begin at a different season, the month in which the 1st of January, 1850, occurs is also given.]
Year of the World (Constantinopolitan account)
Era of Nabonassar
Persian Era of Yezdegird III. (Parsee account).
Armenian common year...
Ditto ecclesiastical year