70 



SAFETY-FUZE 



SAFETY-LAMP 



these three plates were riveted together by 

 hydraulic pressure in such a way that the rivete 

 well out into tin- \viilt>r holes of the centre plate 

 ami therefore cannot be punched. As the nveta 

 are made with a strand of hard steel, neither can 

 they he drilled. See Protection from Fire and 

 Tkieeet, by G. H. Chubb ( 1875). Various kinds of 

 fireproof chambers are built with vaulted roofs 

 And sides of strong masonry. 



Safety-fuze. See BLASTING, and FUZE. 



Safety-lamp. It has been long known that 

 when methane, marsh-gas, or light carburetted 

 hydrogen, which is frequently disengaged in large 

 quantities from coal-seams, is mixed with ten times 

 its volume of atmospheric air, it liecomes highly ex- 

 plosive. Moreover, this gas the./? re -i/n/nii of miners 

 in exploding renders ten times it bulk of atmo- 

 spheric air unfit for respiration, iiiul the choke-damp 

 thus produced is often as fatal to miners as the prim- 

 ary explosion. With the view of discovering some 

 means of preventing these dangerous results, Davy 

 instituted those important observations on flame 

 which led him to the invention of the safety-lamp. 

 lie found that when two vessels tilled with a gas- 

 eous explosive mixture are connected by a narrow 

 tube, and the contents of one fired, the flame is 

 not communicated to the other, provided the 

 diameter of the tube, its length, and the conduct- 

 ing power for heat of its material bear certain 

 proportions to each other; the flame being extin- 

 guished by cooling, and its transmission rendered 

 impossible. In this experiment high conducting 

 power and diminished diameter compensate for 

 diminution in length; and to such an extent may 

 this shortening of length he carried that metallic 

 gauze, which may be looked uiion as a series of 

 very short square tubes arranged side by side, com- 

 pletely arrests the passage of flame in explosive 

 mixtures. The following are Davv's directions 

 regarding the structure of his lamp : ' The a|>erture8 

 in the gauze should not be more than 5 'jd of an 

 inch square. As the (ire-damp is not influenced by 

 ignited wire, the thickness of the wire is not of 

 importance ; but wire from ,Vt|' to Vth of an inch 

 in diameter is the most convenient. Iron-wire and 

 brass-wire gauze of the required degree of fineness 

 are made for sieves by all wire workers, but iron- 

 wire gauze is to be preferred : when 

 of the proper degree of thickness, it j 

 can neither melt nor burn ; and the 

 coat of black rust which soon forms 

 upon it superficially defends the in- 

 terior from the action of the air. The 

 cage or cylinder should be made of 

 double joinings, the gauze IM-JUX 

 folded over so as to leave no aper- 

 tures. When it is cylindrical, it 

 should not be more than two inches 

 in diameter ; for in larger cylinders 



tli miliiistion of the fire -dump 



renders the top inconveniently hot, 

 and a double top is always a proper 



Iirecaution, fixed at the distance of 

 talf or three-quarters of an inch above 

 the first top. The gauze cylinder 

 I -lionld be fastened to the lamp by a 

 C<ritijfcJ screw of four or five turns, and fitted 

 "^ to the screw by a tight ring. All 

 Fig. 1. joinings should be made with hard 

 Davy Lamp, wilder ; and the security depends upon 

 the circumstance that no a|>erture 

 exist* in the apparatus larger than in the wire 

 gauze.' The oil is supplied to the interior by the 

 pipe projecting from the right side of the figure, 

 and the wick i- trimmed by a wire bent at the 

 upper end, and pawed through the liottom of the 

 wrap, so that the gauze need not be removed for 



this process. (The wire is here shown in the 

 figure.) When a lighted lamp of this kind is intro- 

 duced into an explosive mixture of air and fire- 

 damp the flame is seen gradually to enlarge as the 

 proportion of firv-diiuip increjiscs, until at length 

 it fills the entire gaii/e cylinder. Whenever tliis 

 pale enlarged flame is seen the miner* should 

 depart to a place of safety, for although no explo- 

 sion can occur while the gauze is sound, yet at 

 that high temperature the metal becomes rapidly 

 oxidised, and might easily break ; and a single 

 aperture of sufficient size would then occasion a 

 destructive explosion. In a strong current of air 

 the heated gas may lie blown through t lie apertures 

 of the gauze before its temperature is sufficiently 

 reduced to prevent an explosion ; but such a con- 

 tingency may lie guarded against by placing a 

 screen between the draught and the lamp. 



The first lamp which would safely burn in an 

 explosive mixture of gas and air was contrived in 

 1813 by Dr W. Reid Clauny of Sunderland. Into 

 i hit- lamp fresh air was blown through water, and 

 heated air escaped through water by means of a 

 recurved tube. Such a lamji was unfit for ordinary 

 use. George Stephenson invented a safety-lamp 

 which was tried at the Killingworth pits in 1815, 

 and the reader will find its merit* discussed in 

 Smiles 's Life of George Slephenson. Both Clannv 

 and Stephenson applied wire gauze cylinders to 

 their lamps after Davy's came into use, or at least 

 after a communication about it had been made to 

 the Royal Society in 1815. Stephenson 's lamp, or 

 as it is called the 'Geordie,' has a glass cylinder 

 inside the wire gauze, the former having a cap of 

 perforated copper. Small orifices below the glass 

 admit the necessary air, and when the air becomes 

 highly explosive the light goes out, so that the 

 lamp does not get overheated. To enable it to 

 burn well this lamp requires to be either held or 

 suspended. What has long lieen known as Clanny's 

 lamp (not his original lamp) consists of a cylinder 

 of thick glass round the light, and on the top of it 

 resting on a metal rinjj is a narrower cylinder of 

 wire gauze through which the feed air enters. In 

 the first or earlier form of this lamp there is im 

 perfect combustion and it is not very safe, but 

 when the gauze is protected by a metal jacket 

 or bonnet it appears to be secure in currents having 

 a velocity of 25 feet per second. The Mueseler 

 lamp resembles the Clauny in having a glass 

 cylinder below and wire gauze above, but within 

 i In' gauze ton there is a central chimney opening 

 just above tlie flame. The peculiar construction 

 of this presents an obst ruction of two gauzes to the 

 inlet air, while the heated outgoing air only passes 

 through one. Consequently the strong upward 

 draught tends to draw the fresh air briskly through 

 the gauze to the wick, thus keeping the two 

 currents separate and ensuring a good combustion. 



For many years after the Davy, tin- Stephenson, 

 and the Clannv lamp were introduced, the air 

 currents in coalmines seldom reached a speed of 

 6 feet per second. Nowadays, owing to improved 

 ventilation, this speed sometimes exceeds 20 feet 

 per SIT. ,nd iii the main airways, while in some 

 mines it is not greatly less at the faces where the 

 men are hewing the coal. The old forms of these 

 lamps, though safe under former conditions, are 

 consequently no longer secure. But the Davy 

 lamp can lie rendered safe by enclosing it in a 

 lantern, and when so protected against strong 

 current* it is called the 'tin-can Davy.' Never- 

 theleiw, this lamp is falling out of use on account of 

 the miserable light which it gives. In the Final 

 Report of the ('omini-sionent on Accidents in 

 Mines, published in 1886, the subject of safety- 

 lamps is very carefully gone into. After describing 

 a considerable number of these which had been 



