"John Phoenix," who, when a cadet, was called upon by
Professor Mahan to explain how he would defend a fort, mounting a
certain number of guns and garrisoned by a certain number of men, if
besieged by an army of another assumed strength in men and guns,
replied:

"I would immediately evacuate the fort and then besiege it and capture
it again in forty-one days."

Of course the fallacy of this reasoning was in the fact that the
besieging army is generally supposed to be four or five times as large
as the garrison of the fort; the primary object of forts being to
enable a small force to hold a position, at least for a time, against
a much larger force of the enemy.

Sieges have changed with the development of engines of war, from the
rude and muscular efforts of personal prowess like that described in
Ivanhoe, where the Black Knight cuts his way through the barriers with
his battle axe, to such sieges as those at Vicksburg, Petersburg, and
Plevna, where the individual counted for very little, and the results
depended upon the combined efforts of large numbers of men and
systematic siege operations. It should also be noticed that modern
sieges are not necessarily hampered by the rules laid down in text
books, but vary from them according to circumstances.

For example, many sieges have been carried to successful issues
without completely investing or surrounding the fortress. This was the
case at Petersburg, where General Lee was entirely free to move out,
or receive supplies and re-enforcements up to the very last stages of
the siege. In other cases, as at Fort Pulaski, Sumter, and Macon, the
breeching batteries were established at very much greater distances
than ever before attempted, and the preliminary siege operations were
very much abbreviated and some of them omitted altogether. This is not
an argument against having well defined rules and principles, but it
shows that the engineer must be prepared to cut loose from old rules
and customs whenever the changed state of circumstances requires
different treatment.

MILITARY BRIDGES.

In the movement of armies, especially on long marches in the enemy's
country, one of the greatest difficulties to be overcome is the
crossing of streams, and this is usually done by means of portable
bridges. These may be built of light trestles with adjustable legs to
suit the different depths, or of wooden or canvas boats supporting a
light roadway wide enough for a single line