Dectection and Measurement of Inflammable Gas in Mines. 129 



When moderately large quantities of gas are present, such 

 as two or three per cent., the difference in colour of the 

 light emitted by the two wires becomes so striking as to cause 

 some difficulty in judging of the exact position of equal 

 illumination of the screen, a difficulty of course felt more or 

 less with all photometers when examining lights of different 

 tint. This difficulty is avoided (so far as the object of this 

 instrument is concerned) by covering one half (Y, fig. 2) of 

 that side of the screen that is illuminated by the exposed 

 spiral with yellowish red paper, the other half, X, remaining 

 white ; and for quantities below 2 per cent, the white surface 

 X is compared with the white surface Z ; but above that 

 amount the yellow surface Y is employed, its tint neutralizing 

 the very white light of the exposed wire and rendering the 

 comparison easy. 



I must now say a few words relative to the practical use of 

 the instrument. 



In the modern systems of colliery-ventilation the main 

 current of air that descends the downcast shaft is soon sub- 

 divided into several separate currents (or u splits," as they are 

 called). These, passing along the principal travelling- ways, are 

 conveyed to the different districts where the men are working, 

 and, after ventilating a certain number of working-places and 

 becoming more or less contaminated with the combustible gas 

 that is there evolved from the freshly exposed surface of the 

 coal, are carried by air- ways known as " returns " to the up- 

 cast shaft, often skirting in this part of their course large areas 

 of broken ground (or goaf) where the coal has been entirely 

 removed. 



Now it is for the examination of the air passing in these 

 different returns that I consider this instrument specially 

 adapted, as it at once enables the manager of a colliery to 

 determine the quantity of gas evolved in the different working- 

 districts, and thus regulate the proportion of his various 

 currents of air so as to make the best use of the given 

 ventilating-power that he has at his disposal, and not have 

 some of his air-ways considerably contaminated with gas 

 whilst others are practically free (a condition I have frequently 

 found to be present). Again, if a regular account of the ob- 

 served percentages of gas be kept, the quantity of air passing 

 and the barometric pressure being also noted, it will soon 

 become evident within what limits it usually varies; and 

 should at any time an abnormal increase be observable, it will 

 be desirable to follow up the return until the source is ascer- 

 tained and precautions, if necessary, taken. 



The instrument thus employed, and the results carefully 



