The high explosives detonate with full force only when exploded with a detonating fuze. Under favorable conditions one fuze will detonate a very large charge, but cases arise in which a portion of the charge explodes and the remainder does not. To insure the best results, therefore, it is desirable to distribute fuzes throughout a large charge, at the rate of perhaps one fuze to each 50 lbs. These fuzes should contain from 20 to 30 grains of fulminate of mercury, which is itself very sensitive to shock, and has in a high degree the power of detonating the other explosives. One fuze only (or, for safety against defects, two or three) need be connected with the firing apparatus, the others serving to reinforce and carry on the wave of explosion after it is started--differing in this respect from their use with charges of gunpowder.[19] =65. Character and Construction of Fuzes.=--Formerly, for firing mines, trains of powders put up in linen tubes, quick-match, and other similar devices were used. Electric-blasting apparatus is now in such common use that it will always be available for any extended mining operations. For single mines with small charges it may, however, be necessary sometimes to resort to the older method of firing, the apparatus for which can be readily improvised. But even in these cases “Bickford” or “Safety” fuze will usually be available, and may be used alone for firing gunpowder, or with a common fulminate-of-mercury “blasting-cap” for high explosives. It burns at the rate of about 4 feet per minute. Very quick-burning fuzes are also made which may be used at times (e. g., _Bickford Instantaneous_, which burns at the rate of 120 feet a second; _Gomez Lightning_, which burns so rapidly that it may almost be said to detonate; etc.) Great care must be taken not to mistake them for the common Bickford. =66. Electric Fuzes= are made of three general classes: First, those which are fired by a spark from a high-tension machine; second, those which are ignited by a current from a battery or “dynamo;” third, those which can be fired by either. (Abels, etc.) =67.= Those of the second class are manufactured in large quantities, and, in connection with a portable dynamo or “blasting-battery,” are almost universally used for blasting operations throughout the United States.