It follows, therefore, that the only burthen the memory need be charged with is the distribution of the regulars among the several months; because the other element, the solar epact (which also ranges from 1 to 7), may either be obtained from a short mental calculation, or, should the system come into general use, it would soon become a matter of public notoriety during the continuance of each current year.
Now, these solar epacts have several practical advantages over the Dominical letters. 1. They are numerical in themselves, and therefore they are found at once, and used directly, without the complication of converting figures into letters and letters into figures. 2. They increase progressively in every year; whereas the Dominical letters have a crab-like retrogressive progress, which impedes facility of practice. 3. The _rationale_ of the solar epacts is more easily explained and more readily understood: they are the accumulated odd days short of a complete week; consequently the accumulation must increase by 1 in every year, except in leap years, when it increases by 2; because in leap years there are 2 odd days over 52 complete weeks. But this irregularity in the epact of leap year does not come into operation until the additional day has actually been added to the year; that is, not until after the 29th of February. Or, as Bede describes it, "_in leap years one of the concurrent days is intermitted, but the number so intermitted must be used for January and February; after which, the epact obtained from cyclical tables_ (or from calculation) _must be used for the remaining months_." By which he means, that the epacts increase in arithmetical succession, except in leap years, when the series is interrupted by one number being passed over; the number so passed over being used for January and February only. Thus, 2 being the epact of 1851, 3 would be its natural successor for 1852; but, in consequence of this latter being leap year, 3 is intermitted (except for January and February), and 4 becomes the real epact, as obtained from calculation.
To calculate the solar epact for any year, Bede in another place gives the following rule:
"Si vis scire concurrentes septimanæ dies, sume annos Domini et eorum quartum partem adjice: his quoque quatuor adde, (quia) quinque concurrentes fuerunt anno Nativitatis Domini: hos partire per septem et remanent Epactæ Solis."