764 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 Tues- 

 day, 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 miiltiple 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 lev- 

 ied 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 Emj)ire 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 adop- 

 tion 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 

 ns 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 meas- 

 ures). 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 con- 

 sulship of such or such a one. The later Jews used the era of the 

 Seleucidse, 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, 



