Each of the twelve great highways which ran from the gates of Rome was
bordered on either side, at a short distance from the city wall, by the
hidden Christian cemeteries. The only one of the catacombs of which even
a partial survey has been made is that of St. Agnes, of a portion of
which the Padre Marchi published a map in 1845. "It is calculated to
contain about an eighth part of that cemetery. The greatest length of
the portion thus measured is not more than seven hundred feet, and its
greatest width about five hundred and fifty; nevertheless, if we measure
all the streets that it contains, their united length scarcely falls
short of two English miles. This would give fifteen or sixteen miles for
all the streets in the cemetery of St. Agnes."[B] Taking this as a fair
average of the size of the catacombs, for some are larger and some
smaller, we must assign to the streets of graves already known a total
length of about three hundred miles, with a probability that the unknown
ones are at least of equal length. This conclusion appears startling,
when one thinks of the close arrangement of the lines of graves along
the walls of these passages. The height of the passages varies greatly,
and with it the number of graves, one above another; but the Padre
Marchi, who is competent authority, estimates the average number at ten,
that is, five on each side, for every seven feet,--which would give a
population of the dead, for the three hundred miles, of not less than
two millions and a quarter. No one who has visited the catacombs can
believe, surprising as this number may seem, that the Padre Marchi's
calculation is an extravagant one as to the number of graves in a given
space. We have ourselves counted eleven graves, one over another, on
each side of the passage, and there is no space lost between the head
of one grave and the foot of another. Everywhere there is economy of
space,--the economy of men working on a hard material, difficult to be
removed, and laboring in a confined space, with the need of haste.