274 



Mr. Dugald Clerk 



[Jan. 29, 



diii(^ram not only is the pressure rise shown, but temperature rise at 

 different points is also indicated on the same dia^^ram. 



In all these cases the pressure rises slowlv at first and proceeds 

 more and more rapidly until maximum pressure is approached, when 

 it proceeds slowly again. 



In Petavel's experiments, Fig. 4, at 0*5 second after the start of 

 the ignition, the rate of rise suddenly increases 9 times and then slacks 

 off again before maximum pressure. 



[Fig. 4. — Fall of Peessuee aftee Explosion. (Petavel.) 



Spherical enclosure capacity, 551 • 9 c.c. Temperature of enclosure : 

 before firing, 21° C. ; after firing, 27" C. Initial pressure, 74 -38 atmos. 

 (1094 lbs. per sq. in.). Maximum explosion pressure, 654 atmos. 



(9618 lbs. per sq. in.). Rati 



Air 

 Gas 



5-71. -R^tio^^:^ ]^^^^ Vr:essuve 

 Initial pressure 



In Messrs. Bairstow and Alexanders diagrams, Fig. 5, mixture 

 10 '72 per cent gas, a rise of 50 lbs. of initial pressure takes about 

 0*6 second, while the next 100 lbs. is completed in 0*04 second at 

 the rate of 50 lbs. 0'02, that is, three times the original rate. 

 Hopkinson's Fig. 6 shows a rise in 0*1 second of only 3 lbs. ; the next 

 0*1 second causes a rise of 33 lbs., a rate of 11 times. 



At maximum pressure it will be observed that for a very short 

 period no fall occurs, and when fall does take place it is at first slow. 

 All gaseous explosions in closed vessels show similar characteristics. 



Now, what is happening on this rising line ? Reasoning from such 

 diagrams it was long ago recognized that the rapid rise was caused by 

 the spread of the flame, and it was believed that nothing could check 



