The wheels are made of smaller members than those that we are accustomed to see in our own materiel. It must be remembered, however, that the French designed their wheels for much better roads than ours; and, in comparison with our materiel throughout, this fact must be borne in mind. The wheels must be carefully watched, especially through the dry weather, for they tend to check and crack. Remedies are tire shrinking and soaking in water over night, followed by a thorough and careful application of linseed oil. Fast travel, as when the piece is coupled to a truck, must be avoided if the life of the carriage is to be assured. According to the French drill of the gun squads, in going into action, the piece is first unlimbered and the trail is then laid on the ground, and the cannoneers change posts to the extent of the gunner going to his position by his sight; while the remaining cannoneers lift the trail again and set the spade. This is slow and cumbersome work. Much better results may be obtained by setting the spade at the time that the piece is unlimbered, without moving the cannoneers from their posts. CHAPTER XII EXPLOSIVES, AMMUNITION AND FUSES. EXPLOSIVES. As a matter of practical interest, explosives may be divided into three classes, namely: (1) Progressive or =propelling explosives= called =low explosives=. (2) Detonating or =disruptive explosives=, termed =high explosives=. (3) =Detonators= or exploders, known as =fulminates=. The first includes all classes of gun powders used in firearms of all kinds; the second, explosives used in shell, torpedoes, and for demolitions; the third, those explosives used to originate explosive reactions in the two first classes. Corresponding names are given to the phenomena characteristic of each class of explosives, (1) explosions proper, of low order, progressive, or combustions, (2) detonations, of high order, (3) fulminations, this last possessing exceptional brusqueness. The explosion of low order is marked by more or less progression; the time element is involved as a controlling factor, the time required to complete the explosive reaction being large compared with that of the other forms of explosion. The second class of explosion is of a different nature. The explosive reaction is not limited or confined to the surfaces exposed but appears to progress in all directions throughout the mass radially from the point of initial explosion. It has been determined experimentally that the velocity of propagation of the explosive wave throughout a mass of guncotton is from 17,000 to 21,000 feet per second. Fulmination is a class of explosion still more brusque than the last. The abruptness of their explosion and the consequent sharpness of the blow and the concentration of heat on the point of ignition constituting their efficiency as originators of explosions of the first two classes.