Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/826

808 BLASTING is the process by which portions of rock, or other hard substances, are disintegrated by means of an explosive agent, such as gunpowder. It is largely resorted to in quarrying, tunnelling, and muring operations. Of late years there has been rapid advance in the art, through the discovery of new explosives, through improve ments in appliances for firing, &c. ; so that the older method of blasting has, in many instances, given place to a more complex system, with which much better results are obtained. The simpler process may be described thus. When a blast is to be made, a hole to receive powder is first bored in the rock ; such holes vary in diameter from ^ inch to 2^- inches, in depth from a few inches to many feet, and in direction from the vertical to the horizontal. The borer, or jumper, with which the hole is made is a steel pointed drill ; it is struck by a hammer, and is turned partly round after each blow, to make the hole cylindrical. One man may do all this alone, but generally, in the case of larger holes, a man, in sitting posture, directs the jumper, supplies the hole with water, and clears out the powdered stone at intervals with a scraper, while another man, or two, or three, are engaged in striking. A small rope of straw or hemp is twisted round the jumper at the orifice of the hole to prevent squirting up of the water. In the case of soft rock a loaded drill is sometimes used, which acts merely by its own weight. On the other hand, in substances like pyrites, or compact magnetic iron ore, which cannot be penetrated with steel drills, holes for blasting may be made by the gradual action of an acid (commonly muriatic) admitted through a vertical glass tube. When a sufficient depth is reached, the hole is cleaned and dried, and a charge of powder put in. A small taper rod of copper, the needle or nail, is inserted so as to reach to the bottom of the charge ; then the rest of the hole is filled up with some such material as dry sand or tough clay, which forms the &quot; tamping&quot; or wadding, and which is firmly rammed down in small quantities successively by means of the tamping bar, a copper-faced punch of such thickness that it nearly fills the hole, and having a groove in it to receive the nail. This operation requires great care, because of the danger of eliciting sparks through collision. The hole being now fully charged, the nail is withdrawn, leaving a small vent hole, into which is then introduced an oaten straw filled with powder (or a series of ! such). To this is attached a slow match of paper steeped j in saltpetre. The match is touched with fire ; the alarm is given to retire to a safe distance ; and presently the explosion takes place, the rock opening with a sharp report, and fragments of stone being often shot into the air in all directions. An improvement on this method of firing consists in the employment of Bickford s patent fuse, which may be described as a perforated rope or hose containing an inflammable composition. A suitable length of fuse is placed in contact with the charge before tamping, and carried up to the mouth of the hole. On being lighted it burns at the rate of 2 to 3 feet per minute, giving the miners an opportunity to escape before explosion. A water-tight form of the fuse is often used in submarine blasting, the shot or charge being then made up in cartridge form. Blasting, however, is often done on a much larger scale than that just indicated. As an example of the large ! blasts, or &quot; mines,&quot; where great blocks of rock have to be i removed at once, we might take some of the operations ! carried on at the Holyhead quarries a number of years ago ! for the harbour works. An entrance gallery, 5 feet 6 I inches by 3 feet 6 inches was first driven from the face of the rock (hard quartzose schist), an extent of 34 feet, where a shaft, 3 feet 6 inches by 3 feet 6 inches, was sunk to a depth of 14 feet. From this, level galleries were driven some distance right and left, with four short headings, at intervals, returning towards the face of the rock and terminating in chambers for the charges. Tiie four charges, amounting in all to 12,000 K&amp;gt; of powder, were enclosed in canvas bags coated with tar. They were calculated at the rate of 1 Ib of powder to 3 tons of rock. For tamping a stiff red clay was used ; it was well rammed up close to the bags of powder (leaving a small air space round these), and continued to the mouth of the gallery. The charges were fired simultaneously by means of platinum wire heated by a Grove s battery. The total quantity of rock removed was about 40,000 tons ; it was separated into various sized blocks. Similarly, the Rounddown Cliff at Dover was overthrown in 1843 for railway purposes by 18,500 Ib of powder, in three separate charges, fired simul taneously from a Voltaic battery ; a saving of 7000 was thus effected by the South-Eastern Railway Company. In reviewing recent developments of the art of blasting, the application of machinery in the boring of rocks naturally claims some attention. A good rock-boring machine, at least where used in connection with simultaneous firing by electricity, ensures considerable economy in time and labour over the old method of hand-boring. Of such machines, in which the jumper is repeatedly driven agoinst the rock by compressed air or steam, being also made to rotate slightly at each blow, there are several varieties ; the Buiieigh rock drill is one of the best. It was used in the Hoosac tunnel in Massachusetts from 18G9 ; and the last 5220 yards were completed with only eight of these machines. The rock was gneiss alternating with quartz. With hand-boring, the progress per minute was about 16 yards; with the Burleigh drill it was 48 yards, and the work was about one-third cheaper. According to Engineering, the cost of the Mont Cenis tunnel was 195 per linear yard ; that of the Hoosac tunnel, notwithstanding much harder rock, only 180. In the recent large blastings at Hellgate, New York, the Buiieigh machines also established their superiority, and came to be used exclusively. Among other boring-machines may be men tioned the &quot; Diamond &quot; drill, and the systems of Law, Ingersoll, M Kean, Bergstroem, Sachs, Doeriug. The general properties of ordinary blasting-powder are well known ; it requires to be kept dry, and when dry, a spark of fire will cause it to explode. Various efforts have of late years been made towards the employment of more powerful explosive agents for blasting purposes. The violent oxidizing power of chlorate of potash marked it out as available for explosive mixtures ; and sundry preparations containing this substance have been made (some of them highly dangerous). Ilorsley s Blasting- Powder, consisting of powdered nut galls and chlorate, may be taken as a type of these mixtures, and as the safest of them. It is both more violent and more rapid in explosion than ordinary blasting-powder, and does not give off any smoke or unpleasant smell when it explodes. It must be kept dry, and it is liable to explode through friction ; the expensiveness of its ingredients is also a drawback. Gun- cotton was discovered by Schonbein in 1846, but owing to disastrous accidents occurring in the three years which followed, it was abandoned in this country and in France for sixteen years. Through the researches, meanwhile, of an Austrian, Baron von Lenk, it again came into notice in 1864, and a Government committee investigated the merits of the gun-cotton twist or rope made according to the Austrian system. For blasting hard rock its general superiority in effect to powder was recognized ; and the absence of smoke, where the resistance opposed to the gun-cotton was sufficient to develop its full explosive force, was specially remarked upon. The want of rigidity of the material was objectionable ; and several accidents 