The measures of defence during this period must embrace every thing calculated to retard the works of the besiegers.  This may be most effectually accomplished by maintaining a constant fire of grape and musketry on the heads of the sap, and throwing grenades, shells, &c., into the trenches, to harass and destroy the workmen.  As the musketry fire of the besiegers now becomes very destructive to the artillerists at the guns, strong musket-proof blinds are arranged to mask the mouths of the embrasures when the guns are not in battery, and also sloping blindages to cover the men when serving at the pieces.  The possession of the outworks should be disputed inch by inch, and when the besiegers have reached the ditch of the body of the place, sorties, and every species of projectile, should be employed to drive off the sappers, and to retard the construction of their works.  In fine, all the resources of the engineer’s art should be put in requisition for the defence of the breach, and the final assault should be vigorously resisted by the bayonet, and by a well-sustained fire from all the collateral works.

With respect to the relative strength of the opposing forces it may be well to remark, that if the fortress is properly constructed the garrison will be able to resist a besieging army six times as numerous as itself.  Such is the estimate of the best engineers.[48]

[Footnote 48:  A good knowledge of the several subjects discussed in this chapter may be derived from the writings of Vauban, Cormontaigne, and Noizet de St. Paul, on the attack and defence of places and field fortification; the several manuels used in the French service on sapping, mining, and pontoniering; Col.  Pasley’s experiments on the operations of a siege, sapping, mining, &c.; Douglas’s work on military bridges; Macauley’s work on field fortification; and Professor Mahan’s Treatise on Field Fortification. This last is undoubtedly the very best work that has ever been written on field fortification, and every officer going into the field should supply himself with a copy.

The following are recommended as books of reference on subjects discussed in the three preceding chapters.

Memorial pour la fortification permanente et passagere. Cormontaigne.

Defense des places. Cormontaigne.

Attaque des places. Cormontaigne.

Attaque des places. Vauban.

Traite des mines. Vauban.

Memorial pour la castrametation et la fortification passagere. Lafitte-Clave.

Exercice sur les fortifications. Davigneau.

Memorial de l’officier du genie. A periodical of rare merit, containing most valuable military and scientific matter.  It is conducted by officers of the French corps of engineers.  It has already reached its fourteenth number, each number forming a volume.

Traite complet de fortification. Noizet de St. Paul.