j February 9, 191 1] 



NATURE 



489 



collecting samples of the afterdamp), are described and 

 illustrated. One of two methods of ascertaining velocity, 

 which appears to be fairly satisfactory, consists in fixing 

 strips of tin-foil, 4 inches long by \ inch wide, in a 

 horizontal position inside the gallery at intervals of 50 feet 

 from each other, which, on being melted successively by 

 the passage of the flame, break electric contacts in the 

 same order, the results being recorded by an instrument 

 of similar type to that usually employed for similar 

 purposes. 



The manometer is ingenious, and appears to work satis- 

 factorily, but takes no account of negative pressure. It 

 should be supplemented by adding another much more 

 delicate instrument for the latter purpose. 



No satisfactory instrument for reccM-ding temperature 

 instantaneously has yet been devised. On the other hand, 

 the contrivance adopted for collecting samples of after- 

 damp automatically is simple and efficient. 



When there are obstacles in the form of " props and 

 bars " (presumably of similar dimensions to the timbers 

 employed for supporting the roof in the roadways of 



The highest pressure recorded, viz. 100 lb. per square 

 inch, appears to have been in experiment 55, when a length 

 of 150 feet next the mouth of the gallery was free from 

 dust, the next following, 275 feet, strewn with dust, and' 

 the igniter was fired at the innermost end of the dust 

 zone, that is to say, at a distance of 325 feet from the 

 mouth, with presumably thirty-six sets of " props ancT 

 bars " forming obstacles in the path of the exptosion. 

 The manometers were in the same positions as in the 

 other experiments recorded above and below. 



When there are no obstacles the pressures are much- 

 smaller, and appear rather to decrease than to increase- 

 with distance of travel, as shown below : — 



B 



Number 

 of expe- 

 riment 



50 



1 10 



from Igniter 



ft. lb. 



125 ... II '9 

 150 ^ 9-8 

 27s ... 85 



Distance 



from igniter 



ft. 



225 



250 



• 375 



Pressure 



lb. 



8-75 



8-3 

 92 



Fig. 



-Flame bsuing from the Downcast. 



mines) fixed at a distance of 9 feet apart in the gallery, 

 the pressure and velocity of the explosion are found to 

 vary more or less directly with the length of gallery 

 strewn with coal dust, through which the flame has to 

 travel between the igniter and the mouth. In giving rise 

 to greater frictional resistance, these obstacles apparently 

 raise the pressure and temperature of the air advancing 

 towards and rushing past them, and thereby promote a 

 more rapid and intense combustion of the coal dust. 



Two manometers, \ and B, fixed at distances of 50 and 

 150 feet respectively from the mouth of the gallery, re- 

 corded the following maximum pressures per square inch 

 when the point of ignition was at the respective distances 

 from them shown in the following table : — 



Number 

 of expe- 

 riment 



Distance 



from Igniter 



ft. 



54 

 53 

 62 



Pressure 

 lb. 



... 125 ••• 35 

 ... 150 38 



... 275 ... 43 



NO. 2154, VOL. 85] 



Distance 



from igniter 



it. 



225 



250 



375 



Pressure 

 lb. 

 50 



65 

 92 



The records of velocity are as follows : — 



First, with Silkstone coal dust and with obstructions in« 

 the form of " props and bars " in the gallerj' (pp. 155-6), 

 it is stated that the velocit}, between one contact-breaker 

 175 feet from the point of ignition and a second contact- 

 breaker at manometer .\ (200 feet distant from the firsts 

 was 2014 feet per second in No. 62 experiment, and that. 

 between the point of ignition and a point 275 feet distant 

 it was 475 feet per second in No. 53 experiment. 



Secondlv, with the same coal dust and without obstruc- 

 tions, the velocities between six points — the first at the- 

 igniter, the second 59 feet, the third 109 feet from the 

 first, and so on, with an increase of 50 feet successively 

 up to the sixth— were 397, 2525, 72-5, 1190, and 222-S 

 feet per second respectively. 



Thirdly, with South African coal dust, and presumablv 

 with props and bars in the gallerj-, the velocities^ 

 between five points — the first 12 feet distant from the 

 igniter, the second 59 feet, the third 109 feet from the 

 first, and so on, with an increase of 50 feet, as in the 

 last case, up to the fifth — were as given below in feet per 

 second : — 



