Page:Whymper - Scrambles amongst the Alps.djvu/100

70 rock, the ordinary form of chisel is found best. Almost all the details of the machinery, the size of the gallery, the dimensions and number of the holes, and the manner of firing them, have been changed since the beginning; the general principles alone remain unaltered. The present system is as follows. A hole 4¾ inches in diameter is made to a depth of about a yard, towards the centre of the drift, but rather nearer to the floor than to the roof. Fifty to sixty holes, according to circumstances, of less diameter, but of about equal depth, are then driven into the remainder of the face. All the holes are then dried and cleaned by a jet of compressed air, the "affût" is withdrawn behind strong iron-bound doors, and six of the small holes nearest to the large are charged and fired. The force of the explosion goes in the direction of least resistance, that is towards the central hole, and a breach is made such as is indicated in the longitudinal section by the thick dotted line. The remaining holes are then charged and fired in sets of six or eight at a time, those nearest to the breach being exploded first. This system has been found more economical than firing a larger number