The work of the observer is difficult and requires much practice. He must give his directions to the thrower in no uncertain terms. When the thrower has missed his objective the observer will give positive directions for the next throw. Instead of saying "A yard too much to the left," he will say, "Throw a yard to the right." Positive directions, even if only half heard, are of some use; negative directions are certain to be both confused and confusing. The observer should be expert in the use of the periscope. HAND GRENADES AND PETARDS The hand grenade used by our allies on the western front is the bracelet grenade with automatic firing mechanism and consists of a ball of cast iron filled with an explosive and of a leather bracelet which is fastened to the wrist. To the bracelet is attached a piece of rope about 30 centimeters long, having an iron hook at its end. Just before the grenade is thrown, the hook is engaged in the ring of the roughened wire of the friction primer placed inside the fuse plug which closes the cast iron ball. When the grenade is thrown, the ring with the primer wire, held back by the hook of the bracelet, is wrenched off by a sudden movement of withdrawal from the wrist and the fuse is fired. The explosion takes place four or five seconds later. This grenade is supplied to the fighting zone ready for use. It is quite complicated. It can be thrown about 25 meters. The German grenade is composite; it can be thrown by hand or fired from a rifle. As a hand missile, it is used at short distances, 15 to 20 meters. It is composed of a copper rod to the extremity of which is fixed a cast iron cylinder, grooved to facilitate its breaking into small pieces at the moment of explosion. The explosive is placed inside this cylinder. A copper tube, also containing some explosive, is placed in the interior. It is surmounted by a complicated system for closing the grenade and for automatic ignition by percussion, which results in at least 50 per cent. of misfires. [Illustration: Plate 41b.--British hand grenade No. 1.]