*THE POPULAR SCIENCE MONTHLY.*

almanac) what day of the week corresponds to any day of any month of any year, and it is constructed in this manner.

As every common year consists of fifty-two weeks and one day, supposing the 1st of January of any year to fall upon Sunday, A will be the Sunday letter for that year. The last day of that year will also be Sunday, and Monday will be the 1st of January of next year; and, as A is always affixed to the first day of the year, G will become the Sunday letter for that year. The next year will begin with Tuesday, which will make its Sunday letter F, etc.; hence, if there were no leap-year, the Sunday letter of each succeeding year would be removed one letter further backward, and in seven years the cycle would be complete, and the Sunday letter of the eighth year would again be A. But, as every leap-year has fifty-two weeks and two days, the letter C, which always belongs to the 28th of February, is also affixed to the 29th, which puts the Sunday letter for the remainder of the year one letter further back. Leap-year has therefore two Sunday letters instead of one, as in common years. This change takes place every four years; the other, as we have seen, would take place in seven years. Hence a complete cycle of the Sunday letter consists of the multiple of seven and four twenty-eight years; i. e., in any given century, the Sunday letters will again follow each other in exactly the same order every twenty-eight years.

Indiction Cycle.—This is a cycle of fifteen years established by the Emperor Constantine, at the termination of which a tax was levied to pay the soldiers whose term of enlistment was fifteen years. It was afterward ordered by the Council of Nice that this cycle, beginning a. d. 312, should be substituted as the epoch from which all dates should be reckoned instead of that of the Olympiads, which, until that time, seems still to have been used in the Eastern Empire of the Romans.

The epoch from which we now compute years i. e., the birth of Christ—was not used until about the year 500. The universal adoption of this by all Christendom has obviated the necessity of many of the cycles and epochs used prior to that time, and it is impossible for us now to estimate the difficulties the earlier chronologists had to encounter in their attempts to locate events and to regulate them by some fixed standard (illustrated by different modern weights and measures). The Greeks reckoned by Olympiads—cycles of four years beginning 776 b. c. The Romans' great epoch was the founding of their city, 752 b. c. They also used the lustrum, a cycle of four years; and events are very frequently recorded to have occurred in the consulship of such or such a one. The later Jews used the era of the Seleucidæ, 312 b. c., which era the Nestorians, it is said, still use. Prior to the adoption of our own era, the Christians used the era of Diocletian, 284 a. d.

To harmonize the conflicting and troublesome eras, one Scaliger,