The measures of defence resorted to by the garrison will, of course, be subordinate, in some degree, to those of attack.  As soon as any danger of an investment is apprehended, the commanding general should collect into the place all the necessary provisions, forage, military munitions, &c., to be found in the surrounding country; all useless persons should be expelled from the garrison; a supply of timber for the works of the engineers and artillery, fascines, gabions, palisades, &c., prepared; all ground within cannon range around the work levelled; hedges and trees cut down; holes filled up; temporary buildings demolished or burnt; and all obstacles capable of covering an enemy and interrupting the fire of the work, removed.

During this period the engineer troops and working parties detached from the other arms will be most actively employed.  As soon as the investing corps makes its appearance, bodies of light troops are thrown out to cut off reconnoitring parties, and, if possible, to draw the enemy into ambush.  To facilitate these exterior operations, and to prevent a surprise, several guns of long range are placed on the salients of the bastions and demi-lunes, and others, loaded with grape, in the embrasures of the flanks, so as to sweep the ditches.  About one-third of the garrison may be employed in exterior operations, and the other two-thirds in arranging the means of defence in the interior.

Second period.—­As soon as the engineers have completed their reconnaissances and determined on the front of attack, and all the other preparations are made, the general will direct the opening of the trenches.  The ground being previously marked out, battalions of light troops, termed guards of the trenches, as soon as it is dark, are placed about thirty yards in front of the first parallel, (A.  Fig. 54,) with smaller sections, and sentinels about the same distance further in advance.  These guards lie down, or otherwise conceal themselves from the fire of the work.  The engineer troops and detachments of workmen being first marched to the depots and supplied with all the necessary tools for carrying on the work, now commence their labors under the protection of these guards.  By daybreak the construction of the first parallel, and the trenches connecting it with the depots, will be sufficiently advanced to cover the men from the fire of the place; the guards will therefore be withdrawn, and the workmen continue their labors during the day to give the trenches the proper size and form.

The parallels are the long lines of trench which envelop the besieged work, and serve both as covered ways for the circulation of the besiegers, and as means of defence against sorties from the garrison; they are therefore arranged with banquettes for musketry fire.  The boyaux are trenches run in a zigzag direction along the capitals of the front of attack, and are intended exclusively for the circulation of the troops; they have no banquettes.  The first parallel is about six hundred yards from the place, and consequently beyond the reach of grape.  It is constructed by the simple sap.  After the first night, the guards, instead of advancing in front of the work, are placed in the trenches.