Having already described the general influence of permanent fortifications as a means of national defence, we shall here speak merely of the principles of their construction. It is not proposed to enter into any technical discussion of matters that especially belong to the instruction of the engineer, but merely to give the nomenclature and use of the more important parts of a military work; in a word, such general information as should belong to officers of every grade and corps of an army. The first species of fortification among the ancients was of course very simple, consisting merely of an earthen mound, or palisades. A wall was afterwards used, and a ditch was then added to the wall. It was found that a straight wall could be easily breached by the enemy's battering-rams; to remedy this evil, towers were built at short intervals from each other, forming a broken line of salient and re-entering parts. These towers or salient points gradually assumed a shape approximating to the modern bastion. After the invention of gunpowder and the application of cannon to the attack and defence of places, it became necessary to arrange earthen ramparts behind the thin walls of the ancient works, for the reception of the new artillery. Moreover these walls were soon found inadequate to resist the missiles of the besiegers, and it became necessary to replace them by parapets of earth. In order to cover the retaining walls of these parapets from the besieging batteries, it was also found to be necessary to lower these walls as much as possible, and to raise the counterscarps. The traces or plans of the works, however, received no material change till about the close of the fifteenth century. It is not known who first changed the ancient towers into bastions. Some attribute it to an Italian, and with considerable show of reason, for a bastion was built at Turin as early as 1461. Achmet Pacha, it is said, fortified Otranto in this way, in 1480, but whether the system was previously known among the Turks cannot be determined. Others attribute the invention to Ziska, the celebrated leader of the Hussites. It is most probable that the transition from the tower to the bastion was a very gradual one, and that the change was perfected in several countries at about the same time.