Page:The American Cyclopædia (1879) Volume II.djvu/718

 698 BLASTING is closed, but its sides are perforated with nu- merous small holes. As the blast circulates through these the charcoal burns vividly, pro- ducing intense heat and melting away the sides of the cavity. The tubes must be frequently withdrawn to hook out the fragments of cinder which accumulate; and as the size of the chamber increases more charcoal is continually dropped into the hole by the side of the tubes, the hole being left open for the escape of the gases. In the course of a few hours the cavity will be sufficiently large to hold 20 or 80 Ibs. of powder. In granitic rocks the effect of this operation is very remarkable ; the ingredi- ents melt down into a liquid slag, and if a bucket of cold water is dashed in upon the highly heated surface, this is scaled off in large flakes by the sudden chill, and by the mechanical ac- tion of the high steam which is instantly gen- erated. In hard silicious rocks, as the firm sandstones of the Shawangunk range, the rock crumbles down to sand, and this is blown out of the hole as the process goes on, covering the surface around. In calcareous rocks the stone is burned to quicklime, and a large cavity is rapidly produced. The heat generated in this operation is so great, that wrought-iron pipes have been melted down by coming into too close a contact with the charcoal. The en- larged size of the hole at the bottom is par- ticularly favorable for the explosive force of the powder to be exerted to the best advantage. Huge masses of rock are lifted up, and cracks of great extent are opened to a depth not reached by the ordinary method of blasting. These cracks afford convenient opportunities for the use of the sand blast, and thus very large quantities of rock are broken up with comparatively small expense for drilling. Fir- ing a number of charges simultaneously by the galvanic battery is sometimes adopted with great advantage, where large bodies are to be moved. The effect produced by the same quantity of powder is much greater than if the charges were separately exploded. The same mode of firing is also conveniently applied to blasting under water. This method has been said to have been first practised in England in 1839, by Gen. Pasley in removing the wreck of the Royal George, and by Mr. Alan Steven- son in submarine rock blasting. But in vol. xxi. of the " American Journal of Science," for 1831, is a letter of Dr. Hare, describing the operations of Mr. Moses Shaw, who had already applied the electrical machine to this purpose, and then by advice of Dr. Hare was making use of the galvanic battery ; and in vol. xxvi. of the same journal (1834) the apparatus is fully described, with drawings which show that the arrangement was essentially the same with that now in use. In 1843 three charges of 18,000 Ibs. of powder were fired simultaneously by this means at Dover, by Mr. William Cubitt. A chalk cliff 400 feet high was thrown down with little report, and the beach was covered with 400,000 cubic yards of chalk rock. It is estimated that the saving to the Southeastern railway company in this operation over the ordinary process was not less than 7,000. Very successful blasting was performed at the Holyhead quarries in England in January, 1867, for supplying stone for the breakwater at that place. The accompanying diagram (fig. 1) exhibits the ground plan of the galleries and return chambers. These latter were placed 3 ft. below the level of the ground line of the face of the quarry, because it had been found by experience that if they were placed above the level, a wall of rock would be left standing, expensive to remove. The method of estima- ting the total quantities of powder for loading the four chambers was as follows: The cubical content of the mass to be dislodged was divided FIG. 1. Original Face of Eock, 210 ft. long, 115 ft. high. by 12, the minimum number of cubic feet per ton, and the quotient by 5, it being estimated in this case that one pound of powder was re- quired to dislodge five tons of rock. The length of the face of the rock being 210 ft., its height 115 ft., and the horizontal depth to be removed 40 ft., the proper quantity of powder was therefore, in round numbers, 16,000 Ibs. The quantities applicable to charges No. 1, 2, 3, and 4, the lines of least resistance being re- spectively 26, 25, 20, and 27 ft, were 4,200, 4,500, 2,300, and 5,000 Ibs. That these esti- mates were very nearly correct appears from the fact that the force of the powder was main- ly expended in displacing and breaking up the rock, but little concussion of air being produced. The report of Col. Servante of the royal engi- neers, who was sent to witness the explo- sion, says: " The mass was quietly overthrown down to the level of the quarry ground line, with very littl noise, and scarcely a stone was thrown into the air." The quantity of rock detached was found to be 120,000 tons, in blocks of from 3 to 40 tons, averaging 7i tons of stone to one pound of powder. The opera- tions were conducted by Mr. 0. G. Reitheimer, the engineer employed by the Messrs. Rigby, the proprietors of the quarry. The galleries and shaft were tamped with clay, and the tamping was extended through the entrance gallery to the surface of the rock. The de- scription of the operations performed in the demolition of the Russian docks at Sebastopol by the English and French engineers, which is contained in vol. vi. of the " Professional Pa- pers of the Corps of Royal Engineers" of Great Britain, presents interesting examples of blasting. The choice of the explosive com-