Wherever defence only is contemplated, the protection of wire is essential, but it will be well to bear in mind that too many entanglements may prove inconvenient at the time of an attack. We have seen that in such cases, the difficulty is solved by digging communication trenches under the wires. =3. Mines and counter-mines.= Since both parties dug themselves in, much use has been made of mines and counter-mines, especially in 1915 and a part of 1916. The aim of the mine is to throw the enemy into sudden consternation and disorder, while destroying an advanced trench, or work. The French considered the mine as a weapon with which it would be possible to remedy at certain points the defects of their line. They caused heavy losses to both parties, but are not so much used now for the very good reason that, where the fronts have undergone no change since 1914, the soil has been so greatly disturbed that it would be absolutely impossible to make the necessary excavations. We shall simply recall that in June, 1917, the British, prior to their attack on Messines, set off twenty mines, each containing twenty-three thousand kilograms of explosive, and as the Germans were not aware of their construction, the effect of the explosions was terrible, producing huge craters, seventy metres deep and several hundred metres in circumference. Mines are dug by hand or electric drills. The latter have the disadvantage of making too much noise. Greatly improved systems of “listening posts” permit the enemy’s operations to be detected and opposed, and in all cases, excepting at Messines, where the British bored their galleries at more than fifty metres underground, mine-digging was carried on under great difficulties. The best means to neutralize the danger of a mine whose construction is discovered, is to reach it as quickly as possible by excavating a “counter-mine,” and blow it up before the enemy has a chance to set it off. The success of such a counter-mine is termed a _camouflet_ inflicted on the enemy. In our opinion, mines and counter-mines will play a less and less important part in the present war, but it will nevertheless be necessary for our armies to be provided with the means of operating them whenever the Command may deem them advisable. =4. Special railway troops.--Transportation by roads.= Since the beginning of the war, France has been lacking in special technical corps, especially for the railways. Her ante-bellum system was insufficient to cope with the rapid development of the military operations, and the reorganization rendered necessary by trench warfare along an extensive front. A large proportion of the railroad employees was at first mobilized to assist and reinforce the railway troops. Then, after a few months, a large number of these men had to be sent back to their former civilian duties, in order to assure the economical life of the country.