The force of the explosion is markedly affected by the strength of the caps, the effect being greater for low-grade powders. For 40 per cent dynamite the explosion caused by a 5X cap is 15 per cent stronger than that caused by a 3X cap. For 60 per cent dynamite the difference is only 6 per cent. The deterioration of the caps will reduce the strength of an explosion noticeably. With straight dynamite, 3X caps are generally used, but with gelatine dynamite 6X or heavier caps must be used. Caps may be tested by exploding them in a confined space and noting the report and the effect on the shell. A full strength cap will tear the shell into minute pieces, while a deteriorated cap will merely tear it into three or four large pieces. An ordinary blasting cap is shown in Fig. 120 together with other equipment for blasting. Firing by electricity is generally safer and more satisfactory than by the use of ordinary caps and powder fuses. The explosion is more certain and its exact time is under the control of the operator. Fig. 121 shows a section through an electric blasting cap or detonator, commonly called an electric fuse. Delayed action electric detonators are made by inserting a slow-burning substance between the platinum bridge and the detonating substance. The time of delay is controlled by the depth of the slow-burning substance. Delayed action detonators are useful in tunnel work where it is desired to explode the charge in three or four stages in order that the debris from one charge may be out of the way of the following, and that the forces of the explosions may not serve to nullify each other. [Illustration: FIG. 120.—Blasting Supplies. Courtesy, Aetna Powder Co. ] =176. Care in Handling.=—Some of the don’ts in the handling of explosives recommended by the U. S. Army Engineer Field Manual are: in the use of nitro-glycerine explosives of all kinds— (_a_) Don’t store detonators with explosives. Detonators should be kept by themselves. (_b_) Don’t open packages of explosives in a store house. (_c_) Don’t open packages of explosives with a nail puller, pick or chisel. Packages should be opened with a hard wood wedge and mallet, outside of the magazine and at some distance from it. (_d_) Don’t store explosives in a hot or damp place. All explosives spoil rapidly if so stored. (_e_) Don’t store explosives containing nitro-glycerine so that the cartridges stand on end. The nitro-glycerine is more likely to leak from the cartridges when they stand on end than it is when they lie on their sides. (_f_) Don’t use explosives that are frozen or partly frozen. The charge may not explode completely and serious accidents may result. If the explosion is not complete the full strength of the charge is not exerted and larger quantities of harmful gases are given off. [Illustration: FIG. 121.—Electric Fuse. Full size. ]