Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/480

Rh 462 MINING [MEANS OF ACCESS. A is better than B, because it not only affords a greater inclination for the ladders, but also renders it less likely that a man will drop through the opening (manhole) in the platform (sollar) if he loses his hold and falls. These may seem trifling matters ; but, leaving aside the question of safety, the economy derived from fixing the ladders at the best inclination is by no means small. To make this apparent we must recollect the depths to and from which men have to climb, viz., 300, 400, and even 500 yards. It is important, therefore, to save every unnecessary expenditure of energy, which, though trifling for one ladder, becomes considerable when repeated a great number of times. When a mine has reached a depth of 200 yards, and a fortiori when it exceeds it, mechanical appliances should be introduced for raising and lowering the men, because time and strength are wasted by climbing. Medical men also are agreed that excessive ladder-climbing is injurious to the health of the miner. Therefore, both upon hygienic and financial grounds, one of the first thoughts in working a mine should be the conveyance of the men up and down the shafts by machinery with the least possible fatigue. Cages. In collieries and other mines worked by perpendicular shafts, it has long been customary to raise and lower the men by the ordinary winding machinery already described. In the United Kingdom it is necessary that guides should be used if the shaft exceeds 50 yards in depth ; safety- catches and disengaging hooks (COAL, vol. vi. p. 75) are frequently applied for the purpose of preventing accidents. The simplicity of this method of ingress and egress naturally renders it popular, and statistics prove that, where proper precautions are used, it is exceedingly safe. Man- The first man-engine was put up in the Harz in 1833, and engines, nine years later a similar machine was fixed in Tresavean mine in Cornwall. Since that time this very useful means of conveying workmen up and down shafts has been resorted to in other mining districts, and especially in Belgium and Westphalia. Two kinds of man-engine are in use, the double-rod machine and the single-rod machine. The double-rod or original man-engine consists of two reciprocating rods like the main rods of pumps, carry ing small platforms upon which the men stand. The stroke is from 4 to 16 feet, and the little platforms are so arranged that they are always opposite each other at the beginning and end of each stroke. Figs. 95 and 96 represent the rods in the two final positions. A man who wishes to descend steps upon platform b (fig. 95) ; the rod B goes down, and A goes up, so that b (fig. 96) is brought opposite c. The man steps across from b to c, and then the rod A makes a down-stroke, B an up-stroke. Platform c is now oppo site d (fig. 95), and the man again steps across ; and thus, by constantly stepping from the rod as it com pletes its down-stroke, the man is A gradually conveyed to the bottom of the shaft. By reversing the process, or, in other words, by stepping off on to the opposite platform as soon as the rod has completed its up stroke, the man is raised to the sur face, without any fatigue beyond that of the very slight effort of stepping sideways. If each rod makes four up and down strokes of 10 feet each per minute, the rate of ascent or descent will be 80 feet per minute. The single-rod man-engine has one rod carrying steps, whilst fixed plat forms are arranged in the shaft so as to correspond exactly with them (fig. 97). If a man wants to go down, he steps on to A when the up- stroke is completed; the rod goes pj 95 Fi&amp;lt;*. 96. Fig. 97. down and A is brought down oppo site to the fixed platform b, on to which lie steps off. He then waits on b until the rod has finished its up-strokc. B is brought opposite b ; he steps on to B, the rod goes down and he is brought opposite c, where he steps off again and waits. By reversing the operation he is gradually lifted to the top of the shaft. The single- rod engine may be used by men going up while others are going T

Ladders. Man-engines. Cages. Average annual number of men travelling. 73,942 75 7,191 41 (54,071 74 Average annual death-rate per 1000 0-101 0-570 0-115 down, provided that there is sufficient room upon the fixed plat forms (sollars). The best plan is to have sollars right and left, as shown in the figure, and then the ascending men step off to the left, for instance, while the descending men take the right-hand sollars! The ascending man steps on to the man-engine as soon as the descending man steps off, and so the rod may be always carrying men up or down. The usual stroke in Cornwall is 12 feet, and there are from three to five or six strokes a minute. &quot;With five strokes the men descend 10 fathoms a minute, or in other words a descent or ascent of 300 fathoms occupies half an hour. The reciprocating motion is best obtained from a crank, because in this case the speed is diminished gradually at the dead points, and the danger of an accident in stepping off and on is thereby dimin ished; man-engines, however, are sometimes driven by direct-acting engines. Man-engine rods are constructed of wood or iron ; and at An- dreasberg in the Harz each rod is replaced by two wire ropes. Like a pump rod the man-engine rod requires proper balance bobs and catches, and for the safety of the men a handle is provided at a convenient height above each step. The man-engine has one great advantage over the cage, which consists in the fact that it can be safely applied in inclined and even crooked shafts ; and it is for this reason that man-engines have been adopted in many metal mines unprovided with vertical shafts. Careful comparisons as regards safety of travelling have been made in Prussia between ladders, man-engines, and cages. The average accidental death-rate is shown by the accompanying table, which gives averages for a period of ten years, 1871 to 1880: The table shows that the cage is nearly as safe as ladders. In reality, if the actual distance travelled were taken into account, the cage would appear to be safer, because we may fairly assume that the mines in which men are hoisted by cages are on the whole very much deeper than those in which men ascend and descend by ladders. The man-engine appears to be decidedly more dangerous than either the cage or ladders. Here again a distinction requires to be made between the single-rod and the double-rod machines, and the Prussian statistics include many of the latter. It will be readily understood that a fall in a naked shaft with few fixed platforms is much more likely to be fatal than a fall in the shaft of a single-rod man-engine which is closed with the exception of the manhole at intervals of 12 feet. The Belgian warocqueres are rendered safer than the Harz or Saxon man-engines by having a railing round the back of each platform on the rod. Some of the double-rod machines are made with large platforms so that two persons can stand on them, one going up and the other going down, or both travelling in the same direction. The use of double-rod man-engines has been entirely abandoned in the United Kingdom. The death-rate from accidents on man-engines in Cornwall and Devon during the nine years 1873 to 1881 was 17 per 1000 persons using them, whilst the annual death-rate per 1000 persons using ladders was slightly higher, viz., 19. If the actual distance travelled were taken into account, the scale would turn more decidedly in favour of the man-engine. The cost of raising and lowering men by the man-engine is not great. At Dolcoath, a tin mine in Cornwall approaching 400 fathoms in depth (see figs. 62, 63), it is reckoned thatl^d. per man per day covers all expenses, including interest upon the capital expended and depreciation of plant. 13. Dressing or Mechanical Preparation of Ores. In a Dressing large number of cases the mineral, as it is raised from the of ores - mine, is not ready for sale. It usually requires to be sub jected to mechanical processes whereby the good ore is entirely or partly freed from valueless veinstone. These processes, which in a few special instances are aided by calcination in furnaces, are known as the dressing or mechanical preparation of the ores. As a rule the valuable ore is specifically heavier than the veinstone, and most of the separating processes are based upon the fact that the heavy particles of ore will fall in water more quickly than the light particles of veinstone. The processes of mechanical preparation may be classified as follows : (1) washing and hand-sorting; (2) disintegra tion, or reduction in size ; (3) classification by size or by equivalence ; (4) concentration. (1) Sometimes the ore coming from the mine requires Washing, simply to be freed from adhering particles of clay in order