A horn-work consists of a front of fortification, and two wings resting on the faces of bastions of a front of the fortress.  It sometimes has also a demi-lune or bonnet, as in the case of demi-tenaillons. (Fig. 43.)

A crown-work consists of two fronts of fortification, and two wings.  (Fig. 44.) It is sometimes made double, and even triple.

These works are also employed as advanced works, and placed entirely in front of the glacis.  They have generally been added to a fortress for the purpose of occupying some important piece of ground not included within the limits of the main work.  They may be constructed with covered ways, and sometimes it may be found advantageous to secure them by retrenchments.

A detached work may be made in any form deemed best suited to the site.  Being but remotely connected with the fortress, the latter will exercise but slight influence on the character of its plan or construction.  They are usually of limited extent and slight relief, partaking much of the nature of field-works.[45]

[Footnote 45:  The general principles of permanent fortification may be best learned from the writings of Cormontaigne, St. Paul de Noizet, and Laurillard-Fallot.  A list of valuable books of reference on the several branches of military engineering will be given at the close of the next chapter.]

CHAPTER XIV.

FIELD-ENGINEERING.

Field-Engineering includes the making of military reconnaissances, temporary fortifications, and military roads; the planning and construction of military bridges; the attack and defence of military works;—­in fine, all the various duties of engineer troops, either in the operations of a campaign, or in the dispositions on the battle-field.

Military reconnaissance.—­By this term is meant an examination of a portion of the theatre of war, to ascertain its military character and resources.  If the examination be made of a large district of country, and for an entire campaign, the reconnaissance is general; if made for collecting detailed information respecting a proposed line of march, the passage of a river, the position of an enemy, &c., it is termed special.

In making a general reconnaissance, great care should be taken to collect accurate information respecting the general topography of the country; the character of the mountains, forests, and water-courses; the nature of the roads, canals, and railways; the quality of the soil, and the amount of provisions and forage it produces; the population and character of the cities, towns, and villages, the commercial and manufacturing resources of every part of the country, and the means of transportation to be found in each district.  The plan of military operations will be based on the information thus obtained, and any serious error in the reconnaissance may involve the results of the campaign, and even the fate of the war.