Robespierre fell in 1794;
Napoleon in 1815, and Charles X. in 1830.
Now, the remarkable fact in connection with these dates is, that the
sum of the digits composing them, added to the dates, gives the date
of the fall of the successor. Robespierre fell in 1794; 1 + 7 + 9 + 4
= 21, 1794 + 21 = 1815, the date of the fall of Napoleon; 1 + 8 + 1 +
5 = 15, and 1815 + 15 = 1830, the date of the fall of Charles X.
There is a singular rule which has been supposed to determine the
length of the reigning Pope's life, in the earlier half of a century.
Add his number to that of his predecessor, to that add ten, and the
result gives the year of his death.
Pius VII. succeeded Pius VI.; 6 + 7 = 13; add 10, and the sum is 23.
Pius VII. died in 1823.
Leo XII. succeeded Pius VII.; 12 + 7 + 10 = 29; and Leo XII. died in
1829.
Pius VIII. succeeded Leo XII.; 8 + 12 + 10 = 30; and Pius VIII. died
in 1830.
However, this calculation does not always apply.
Gregory XVI. ought to have died in 1834, but he did not actually
vacate his see till 1846.
It is also well known that an ancient tradition forbids the hope of
any of St. Peter's successors, _pervenire ad annos Petri_; i. e., to
reign 25 years.