We might illustrate this by numerous examples.

A single one, however, must suffice.

About the period of the last war, eight new forts were constructed for the defence of New York harbor, at an expense of some two millions of dollars.

Six of these were _circular_, and the other two were _star forts_--systems which had been discarded in Europe for nearly two thousand years! Three of these works are now entirely abandoned, two others are useless, and large sums of money have recently been expended on the other three in an attempt to remedy their faults, and render them susceptible of a good defence.
Moreover, a number of the works which were constructed by our engineers before that corps was made to feel the influence of the scientific education introduced through the medium of the Military Academy--we say, a considerable number of our fortifications, constructed by engineers who owed their appointment to political influence, are not only wrong in their plans, but have been made of such wretched materials and workmanship that they are already crumbling into ruins.
A fortification, in its most simple form, consists of a mound of earth, termed, the _rampart_, which encloses the space fortified; a _parapet_, surmounting the rampart and covering the men and guns from the enemy's projectiles; a _scarp wall,_ which sustains the pressure of the earth of the rampart and parapet, and presents an insurmountable obstacle to an assault by storm; a wide and deep _ditch_, which prevents the enemy from approaching near the body of the place; a _counterscarp wall_, which sustains the earth on the exterior of the ditch; a _covered way_, which occupies the space between the counterscarp and a mound of earth called a _glacis_, thrown up a few yards in front of the ditch for the purpose of covering the scarp of the main work.
The work by which the space fortified is immediately enveloped, is called the _enceinte_, or _body of the place_.

Other works are usually added to the enceinte to strengthen the weak points of the fortification, or to lengthen the siege by forcing the enemy to gain possession of them before he can breach the body of the place: these are termed _outworks_, when enveloped by the covered way, and _advanced works_, when placed exterior to the covered way, but in some way connected with the main work; but if entirely beyond the glacis, and not within supporting distance of the fortress, they are called _detached works_.
In a bastioned front the principal outwork is the _demi-lune_, which is placed in front of the curtain; it serves to cover the main entrance to the work, and to place the adjacent bastions in strong re-enterings.
The _tenaille_ is a small low work placed in the ditch, to cover the scarp wall of the curtain and flanks from the fire of the besieger's batteries erected along the crest of the glacis.
The _places of arms_, are points where troops are assembled in order to act on the exterior of the work.