PIONEER AND PONTOON SERVICE. 173 



thrown towards X, the other is pressed into the side walls, h. The line ao 

 is called the line of least resistance, gckcg is the sphere of action, and hh 

 the crater of the mine, of which the upper surface is called the surface of 

 explosion, its circumference the circle of rupture, the lines o h radii of the 

 crater, and the lines ha radii of explosion. The sphere of action extends 

 to m and n, where the earth is loosened. Towards g the effect is less. 



In a properly loaded mine (Jig. 20), the radius, oh, of the surface of 

 explosion is equal to the line of least resistance, ao. If a gallery, mn 

 (Jig. 25), lies If times the line of least resistance from a properly loaded 

 mine, it is sufficient, to avoid any impression, that it be removed to double 

 that line by propping it with wood. If the diameter of the circle of explo- 

 sion is greater than twice the line of least resistance, the mine is said to be 

 overcharged ; if less, it is undercharged. If the craters of two mines, c dfg 

 and hgik (Jig. 22), overlap, they must, if they do not lie so near that 

 their spheres of activity, trs, pass into each other, receive a stronger 

 charge to produce the ordinary crater. Fig. 23 shows four mines whose 

 circles of explosion overlap. Fougasses are made by digging pits from 

 eight to ten feet deep, and placing in them well pitched wooden boxeSj 

 loaded with powder, bombs, and grenades, and provided with saucissons. 

 These mines, A (Jig. 28), are used against the saps along the capitals ; the 

 fougasses, B, are to destroy part of the crowning of the covered-way ; the 

 mines, C, are directed against the lodgment in the re-entering place-of-arms ; 

 the mines, D, defend the foot of the breach ; the mines, E, are to destroy 

 the hostile lodgments in the interior of bastion or ravelin. When time is 

 lacking, fougasses can be established as at F, and are then called rosaries. 

 Where saucissons cross each other, as at m, they are laid one under the 

 other, being sunk at m and brought up again by other shafts at n and p. 

 Fig. 31 shows the effect of a mine in solid mason- work ; ^^. 32, upon a 

 vault ; Jig. 34, as globe of compression against a gallery. 



2. Subterranean Warfare. 



As the approaches of the assailants are made usually only along the 

 capitals, the gallery mp (Jig. 13), running forward from the foot of the 

 counterscarp, is the only one established at first ; it is regarded as a listening 

 gallery, but fourneaux are placed in it also. Subsequently small galleries, 

 ab and cd, are run across through this ; finally, for the purpose of blowing 

 up the breach and counter-batteries, the crosses eff and the galleries mlnm 

 are established, from which the new crosses, egg and e h h, can be thrown out. 

 The gallery mlnm is called the magistral gallery. Frequently, also, several 

 rows of mines are established, one above the other (pi. 51, Jig. 26), D E F, 

 so as to explode them in succession. In order to extend the circle of 

 efficiency of the counter-mines further than, from want of air, it can be 

 carried by the listening galleries, the gallery CCCC (Jig. 29) is con 

 structed, parallel with the magistral gallery GGGG, and called the envelop- 

 ing gallery. From this the enemy's fourth parallel is blown into the air. 



649 



