The Polytechnic Society of Cornwall, in comparing the rate of mortality among men working at different depths, (accidents deducted,) estimates that in works of 400 to 500 metres in depth, where ladders are used, the lives of the men are shortened by twenty years. However this may be, it is certain that the prolonged use of ladders gives rise to serious derangements of the organs of respiration, and renders a certain number of men unfit for work before they are thirty years old.

The time required for lowering and raising a shift of men by cables is not as easy to estimate as that required where ladders alone are used. It depends, in fact, on two variable elements-the rate of speed, and the number of men that can be lifted at each time.

The rate of speed varies according to the importance of the workings: in shafts without guides it is often from one to two metres per second; in shafts provided with guides and cages, it is from three to twelve metres per second; but when the men are taken up and down in the cages, the speed is often slackened, keeping it about three to six metres per second. The number of men carried at once is from two to three in the small workings, and sixteen to twenty, or more, in mines of greater extent.

A comparison of the time required for the descent by ladders and by lowering in cages may be made as follows: Assuming that there are 200 men in a shift and that the depth is 400 metres, the rate of speed. averaging, say five metres, and that eight men are carried at once-at five metres per second, to ascend or descend 400 metres requires 409—80 seconds. To this must be added about two minutes (120 seconds) for the stepping in and out of the men, and the starting and stopping of the engine, which makes altogether 120+80=200 seconds. Lowering 8 men at once, we have 200-25 journeys in all for the shift; the time will therefore be 25 × 200 seconds 5,000 seconds 1 hour 24 minutes. Doubling this for the entire time in going into and out of the mine, will be 2 hours 48 minutes, which is half the time taken for the ascent and descent of the same number of men by ladders. But these figures are not absolute; they may vary widely, either more or less, according to the extent of the workings.

The advantages and disadvantages of the rope are inversely to those of the ladders; the health of the men does not suffer, but there is less security, and accidents are much more sericus.

Accidents by ropes and by ladders are as 3 to 2; but this ratio is still increased by the fact that of 100 accidents to men, 94 are killed and 6 injured.

These deplorable consequences from this method of transportation of miners caused the Prussian government to prohibit the lowering or rais ing of men by the cages in the mines of Prussia.

In order to estimate the time required for the ascent and descent of miners by the man-engine, let us take our standard example, 400 metres of depth, and 200 men to send down or lift up for each shift.

36

Allowing the stages to be 6 metres distant from each other, and the man-engine to make 6 double strokes per minute, in one minute a man will then have passed upon and from 6 stages; he will then have been lifted 6.00 × 6=36.00, and consequently will rise the 400 metres: in 400 - 12 minutes, in round numbers. Each double stroke thereafter will deliver another man at the surface, or, which is the same thing, the machine will lift 6 men per minute; the 200 men will therefore arrive at the surface in 200: 34 minutes in round numbers, which, added to the 12 minutes required for the whole ascent of the first man on the stages gives in all 46 minutes; doubling this for the lowering and lifting of one shift of men, and we have 92 minutes (1 hour and 32 minutes) for the

whole, and that without either danger or fatigue. So that for 200 men and 400 metres of depth, the ascent by ladders requires 6 hours; by hoisting, varying from 1 to 4 hours; by the man-engine, only 1 hour. The fitting up of a man-engine is doubtless a considerable expense, but it is soon repaid by the time saved, and the prevention of muscular fatigue of the miner.

For further details respecting the construction and working of manengines reference may be made to the following-named works, from which a part of the information here presented has been compiled: Burat's Materiel des Houillères; Portefeuille de Cockerill; Zeitschrift des œsterreichischen Ingenieur-Vereins, 10ter Jahrgang; Annales des Travaux Publics de Belgique, vols. 4 and 6; Annales des Mines de France, 5me, vol. xv; Revue Universelle, vols. iv, v, vi, xiv, xvi.

CHAPTER LXXVI.

VENTILATION.

Very few of the mines of the West are so deep and extensive as to require any elaborate and extensive contrivances for their proper ventilation. In most cases, their position and construction are such that a current of air circulates spontaneously through them by reason of one of the openings being at a greater altitude than some other, as, for example, one or more shafts with tunnels leading to them from the hillside. If the air in the mine is warmer than that outside, it rises in the shaft, and is replaced by the influx of the colder air through the tunnel. But the conditions essential to ventilation in this way are not always found, and it becomes necessary to resort to artificial means to supply the miners at the extreme points of the mine with fresh air. In driving long tunnels, especially where powder is used, the air is rapidly vitiated, and soon becomes unfit to breathe.

There are two ways in which ventilation may be effected, either by drawing the impure air out, or by forcing pure air in. A good example of the first-named method was presented at the Latrobe tunnel, Virginia City, which was driven for the greater part of the distance without a ventilating shaft, one only having been sunk not far from the entrance. Tin pipes were first used to convey the air and were placed along the top of the tunnel extending from a few yards back of the face to the bottom of the air-shaft. But it was found difficult to maintain a good draught, and the metal pipe was replaced with one of wood, made of boards about eight inches wide, and rabbeted so as to form tight joints. With this arrangement no difficulty was experienced; the heated air at the end of the tunnel escaped constantly through this tube and rose in the shaft, while the pure air from the outside flowing in at the mouth of the tunnel took its place. In this instance it was evident that the non-conducting quality of the wood prevented the air from becoming cooled in its passage before it reached the shaft.

The method of producing a draught by means of a fire or by connecting the ventilating pipes with the ash pit of the furnace fires is well known and is often resorted to on a small scale in California.

The only example of mechanical ventilation worthy of special mention which came under the writer's observation in California was at the Princeton gold mine, Mariposa Estate. Foul air was generated to such an extent in the southern part of the mine that it could not be entered. A simple centrifugal fan-wheel, about ten feet in diameter and two feet

wide, was erected over the nearest shaft. This fan-wheel was not inclosed, but revolved between two vertical temporary walls of boards, thus leaving the arms and fans fully exposed to view. The mouth of the shaft was tightly closed, excepting two openings connected by box tubes with large openings in the walls of boards, around the axis of the fan-wheel. When this wheel was put in motion by a band from the engine, it produced a strong current of air up the shaft, and cleared the workings of foul air in a short time. It was evident that a much smaller blower would have answered the purpose.

For forcing pure air into tunnels and drifts of slight extent an ordinary wind-sail or a fan-wheel driven by hand or attached to the horsewhim is usually employed; but these, of course, from their want of forcing power, are not very effective. The object in using them in most cases is not so much to supply pure air as to give a cooling current or blast of air near the workmen.

In several of the mines upon the Comstock lode, the "Indiana Blower," Root's rotary compression blower, is used with great success in ventilating. Mr. James G. Fair, superintendent of the Hale and Norcross Silver Mining Company in March, 1869, had the blower in constant use, day and night, since its adoption by the company in August, 1868. Although of only medium size, it supplied sufficient air to enable them by the use of branch pipes and dampers to prospect simultaneously at different depths, and at considerable distances apart upon the same level. It furnished air to two gangs of miners in separate drifts on the 1,030-foot level, and partially ventilated the level 100 feet above that, and with but a portion of the power the blower could have utilized.

The Potosi Company have had a No. 2 blower for ventilating the levels below the 900-foot station; and they have been successfully used upon the Yellow Jacket mine since the great fire. Mr. Winters, the superintendent, in August last, in a letter to the agents, said, "I have great pleasure in stating that they work admirably. If we had been without them, it would have been quite impossible to work our mine. since the great fire in this and the adjoining mines."

This blower gives a "positive" or force-blast, taking in and forcing forward a definite quantity of air at each revolution. Its construction is shown by the figures annexed.

The first figure shows the external form of the case, and pulleys at each end for the reception of driving-belts. The cross-section of the interior shows the inlet and outlet and the form of the arms or wings. The case is usually made of cast

iron, with the cylindrical parts bored out, and the head-plates faced off truly upon a boringmill, arranged for the purpose. The friction is confined to the journals and cog-wheels. The arms or wings do not touch in running, but move as closely to gether as possible without being actually in contact. They are about two feet long and make from 100 to 300 revolutions a minute. A machine exhibited at the Paris Exposition was said to produce a pressure equal to

one-third of an atmosphere, or five pounds to the square inch, when driven at the rate of 250 turns per minute.

In the coal mines of Pennsylvania more and more attention is now given to ventilation, as not only the ordinary difficulties of working, but also the liability to accumulations of dangerous gases increase with the depth of the mines. The different systems of natural and artificial ventilation, including the use of the furnace, steam-jet, and blower, have been vigorously discussed during the past year or two by Messrs. Rothwell and Harden, mining engineers of Wilkesbarre, in the columns of the New York Engineering and Mining Journal. The latter gentleman seems to esteem the furnace more highly than the former, who in most eases prefers fans. The arguments on both sides were interesting and valuable; and those directed against the furnace were emphasized shortly afterward by the terrible catastrophe at Avondale, where apparatus of this kind set fire to a column of gas and burned the brattice of the shaft and the breaker over it, closing the only entrance to the mine and sacrificing a large number of lives. A fan has been substituted for the furnace at Avondale; yet under some circumstances a fan is inferior. For instance, when a fan is disabled or interrupted its effect ceases at once. Thus, at some critical moment in a mine, or in some very fiery mine, where every moment is a critical one, the fan ventilation might instantly and totally cease, while a furnace, though neglected or interrupted, would continue to act, though with diminishing effect, for hours. The precaution of maintaining a duplicate fan always in reserve is calculated to remove the objection.

MECHANICAL VENTILATION OF MINES ABROAD.

For the extensive collieries of Great Britain and the continent of Europe powerful means of ventilation are required, and the subject receives great attention among mining engineers and constructors. The miners not only have to contend with the air vitiated by their own respiration, by the animals employed, and by the lamps, but the coal beds themselves give off large volumes of deleterious gases, and the dreaded ire-damp, the collier's great destroyer. Coal mines therefore require more elaborate and costly preparations for ventilation than any other. The mean depth of the English coal mines is 180 metres; of those in Belgium, at Charleroi, 360 metres; at Centre, 350 metres; and at Mens, 416 metres.

A few years ago nearly all the important collieries of England were ventilated by means of furnaces placed near or at the bottom of ventilating shafts, by which the air was rarefied and made to ascend. In such furnaces, from ten to twenty tons of coal were burned daily. This required large furnaces and costly excavations, and galleries of large size for the air-courses. In Belgium and France mechanical ventilation has been carried to a great degree of perfection. This system is said to be well established by experience as much cheaper and less dangerous than the use of furnaces, and is gradually replacing the furnace ventilation. Mechanical ventilators may be grouped in three classes: 1, centrifu gal ventilators, or fans; 2, rotary pumps, or force-blowers; 3, piston machines, with reciprocating motion.

Ventilators of the first class are of great dimensions, capable of delivering immense volumes of air. They have been in use for about thirty years, and have undergone many changes and improvements. Like most other ventilators, they act by aspiration, producing to a certain extent throughout the mine a lower barometric pressure than is found in the external air. This depression varies in general from five to seven centimetres of water.

Guibal's ventilator.-Guibal's ventilator, after having undergone many