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. _Traite d'art militaire et de la fortification._ Gay de Vernon. _Art de la guerre._ Rogniat. _Essai general de fortification, &c._ Bousmard. _Aide-memoire portatif a l'usage des officiers du genie._ Laisne. A very valuable and useful book. _Aide-memoire de l'ingenieur militaire._ Grivet. _Cours d'art militaire._ Laurillard Fallot. _Cours de fortification, &c._ Lavart. _Le livre de la guerre._ Perrot. _Journaux des sieges dans la peninsule._ Belmas. _Journal of Sieges in Spain._ John Jones. Both of the above are works of great value. _Cours d'art militaire et de fortification militaire._ Francois. _Architettura militare._ Marchi. _Essai sur la fortification._ Baltard. _La fortification._ Bar-le-Duc. _Elemens de fortification._ Bellaire. _La science des ingenieurs._ Belidor. _L'art universel des fortifications._ Bitainvieu. _Nouvelle maniere de fortifier les places._ Blondel. _Les sept sieges de Lille._ Brun Lavaine. _Defense des places fortes._ Carnot. _Memoire sur la fortification._ Carnot. _Defense de Saragosse._ Cavallero. _Memoires sur la fortification._ Choumara. _Nouvelle fortification._ Coehorn. _Theorie de la fortification._ Cugnot. _Des fortifications,_ &c. &c. Darcon. _Relation de la defense de Dantzik._ D'Artois.