a term in chronology, expressing the solar equation, necessary to prevent the new moon from happening a day too late; by which it is opposed to proemptosis, which signifies the lunar equation necessary to prevent the new moon from happening a day too soon. The new moon's running a little backward, that is, coming a day too soon, at the end of three hundred twelve years and a half; by the proemptosis a day is added every three hundred years, and another every two thousand four hundred years. On the other hand, by the metemptosis, a bifextile is suppressed every one hundred and thirty-four years; that is, three times in four hundred years. These alterations are never made but at the end of each century; that period being very remarkable, and rendering the practice of the calendar easy.

There are three rules for making this addition or suppression of the bifextile day, and by consequence for changing the index of the epacts. 1. When there is a metemptosis, the next following, or lower index, must be taken. 2. When there is a proemptosis without a metemptosis, the next preceding or superior index is to be taken. 3. When there are both a metemptosis and proemptosis, or when there is neither the one nor the other, the same index is preserved.