Page:Encyclopædia Britannica, Ninth Edition, v. 6.djvu/93

Rh SAFETY LAMPS.] COAL and when the air is dangerously charged with gas the light is extinguished. Various forms of safety-lamps have been introduced at different times, for the purpose of increasing the amount of light by substituting a glass cylinder for the lower portion of the wire gauze. The oldest of these is that of Dr Clanny, contemporary with those of Davy and Stepheuson. The air for supplying the flame, entering at the bottom of the gauze, and passing down the iiner side of the glass, protects the latter to some extent from becoming over heated, but a large amount of light is lost; by absorption in the glass, so that there is no great advantage over the ordinary Davy lamp to compensate for the extra weight and cost, especially as the safety property of the lamp depends upon the glass cylinder, which may be readily broken when subjected to the ordinary accidents of work ing. A more perfect form of lamp of the same character is that of Museler, which is extensively used in Belgium. It differs from Clanny s lamp by the addition of a conical chimney above the flame, which produces a rapid draught, and consequently a more perfect cooling of the glass cylinder by the down-draught of feed air for the flame. Boty s lamp, which was recommended by a commission of the Belgian Government as being safe in use, is essen tially that of Dr Clanny with Stephenson s perforated ring for admitting air at the level of the wick. Another Belgian variety is that of Eloin, in which the glass is shaped to the surface produced by the revolution of a parabolic arc, so as to disperse the light in parallel lines. The air is admitted by a Stephenson ring, combined with an Argand cap, the glass being surrounded by a brass chimney with a gauze top. In another form of the same lamp Museler s chimney is added. The locking of safety-lamps, so as to render them in capable of being opened by the miners when at work, is a point that has given play to a large amount of ingenuity. One of the most favourite devices is a combination of the wick-holder with the locking bolt, so that the latter cannot be withdrawn without lowering the wick and extinguishing the flame. Another method consists in the use of a lead rivet, uniting the two parts of the lamp, impressed with a seal, which cannot be removed without defacing the device. All this class of contrivances have the defect of only being efficacious when the miners are not provided with matches, or other means of obtaining a light. A more physically perfect method is that adopted by Bidder, where the locking bolt is magnetized and held in place by a force which can only be overcome by the application of a battery of heavy and powerful steel magnets. These are kept in the lamp cabin at the pit bottom, where the lamps are cleaned and served out lighted to the miners at the com mencement of the shift, and are collected before they return to the surface. When a Davy lamp is exposed to an atmosphere con taining less than 8 per cent, of marsh gas, the flame lengthens and becomes smoky ; when that amount is reached the flame returns to its usual size, but a column of blue flame rises to the top of the gauze. With 10 per cent, the flame of the wick is extinguished, the whole of the space within the gauze being filled with a blue flame of burning gas. If the lamp is allowed to remain too long in a fiery atmo sphere it becomes dangerous, as the gauze being heated to redness may fire the gas. The safety of the lamp is also endangered by an exposure to a current of gas moving at the rate of more than 6 or 8 feet per second, as the flame can then be readily driven through the gauze. It is there fore usual to protect the flame by a sliding shield of tin plate, horn, or mica from the direct action of any sudden outburst of gas in the workings. Lamps with glass cylin ders are generally very safe, except from the risk of acci dental breakage, which, however, is less frequent than might be imagined, and those taking air through a feed ring, such as Stephenson s, are readily extinguished in a foul atmosphere. The danger arising from gas in the workings may be considerably increased by the presence of coal dust in the air. This point has been the subject of investigation by Galloway, who found that an explosion may be produced by ignited particles of coal dust through the agency of a safety-lamp which under ordinary circumstances would be perfectly trustworthy. At Blanzy, in France, several fatal explosions have been traced to the firing of coal dust from the flame of a shot, even in cases where no fire-damp was present in the workings. An electric lamp, where the light is obtained from the Electric discharge in a Geissler vacuum tube, has been proposed amps, by Benoit-Dumas, instead of the ordinary safety lamps, or for use in exploring after explosions or in bad air ways. This consists of a box containing a galvanic battery, con sisting of two Bunsen cells, and a small induction coil, with connecting wires which convey the current to the lamp. The Bunsen cells may be conveniently replaced by a single bottle-ishaped bichromate battery. The cost and complication of this apparatus must necessarily limit its use. Apparatus, originating in France, known as aerophores, Aero- which enable the miner to carry sufficient fresh air for i llores - his own respiration, and to keep a lamp alight for a short time in a totally irrespirable atmosphere, have of late years come into use for the purposes of saving life after explosions, and repairing shafts and pit-work under water. There are two principal patterns, those of Galibert and Denayrouze. The former, which is the simplest, con sists of an air- tight bag of about 12 cubic feet capacity, con taining air at a little above atmospheric pressure, which is carried on the miner s back like a knapsack. The air, after being used, is returned with the products of respiration into the bag, and can be used over again until it becomes too impure for further use. It is obvious, therefore, that such an apparatus must be of very limited application, but its simplicity and cheapness are points in its favour for use in sudden emergencies. The Denayrouze apparatus consists of a series of sheet metal cylinders, containing air compressed to 300 or 350 S&amp;gt; to the square inch, which can be carried on the back, and served out at a pressure very slightly above that of the atmosphere by means of a reducing valve, whose construction is essentially the same in principle as that of the ordinary pressure regulator used in gas-works, i.e., a conical plug closed against its seat by the pressure of the air in the reservoir, which is constantly opposed by an external force tending to open it. This force is supplied by a disc of vulcanized india-rubber, which opens the valve at each inspiration, and allows a fresh supply of air to escape into the chamber of the regulator through the small aperture of the valve. Of course, all communication with the external air must be cut off, so that respiration can only take place through the mouth, the air-tube being attached by an india-rubber mask called a mouth-closer, and the nostrils closed by a spring clip. A similar regu lator valve, so constructed as to keep tlic india-rubber spring under a slight excess pressure in order to maintain a flow of air, is in connection with the lamp. This is of the ordinary Museler construction, with the addition of a chamber outside the gauze to receive the products of com bustion, which are discharged through a conical valve at the top, a reflux of the exterior gases being prevented by the pressure of a counter spring. The air is carried to the lamp by an india-rubber tube, which is sufficiently flexible to allow a certain freedom of motion. The dis tance that an explorer can penetrate with this apparatus is obviously limited by the capacity of the air-cylinders. VT. 10