734 Prof. W. M. Thornton on the Least Energy 



halves were joined, and it is probable that these irregu- 

 larities were largely responsible for such inconsistencies as 

 were observed. In one series of experiments a steel bicycle 

 ball was used ("8 cm. diameter), and the results obtained 

 were much more consistent than those for the celluloid balls 

 and the line of fall was much more truly vertical. Another 

 source of inconsistency is probably to be found in the fact 

 that the centre of gravity of a loaded celluloid ball must in 

 general have been somewhat out of the vertical through the 

 centre of the sphere at the moment of release, and so 

 rotation was probably produced to varying extents in 

 different falls of the same ball. 



Conclusions, 



For spheres of diameters varying between about 2 cm. 

 and 7 cm., moving with velocities between about 500 and 

 2000 cm./sec, the resistance of the air (R) is nearly 

 proportional to the square of the velocity (V) and to the 

 area of the diametric plane of the sphere (S), but in the 

 usually accepted equation R = K8V 2 , K, instead of being- 

 constant, increases both with S and with V ; its value is 

 about one-fifth of the density of air. 



Experiments are in progress to investigate the relation 

 between the density of the air and the resistance to the fall 

 of spheres. 



In conclusion I wish to thank my assistant, Mr. E. Simpson, 

 for valuable help in the experiments. 



The University, 

 Birmingham. 



LXXVI. The Least Energy required to start a Gaseous Ex- 

 plosion. By Professor W. M. Thoknton, D.Sc, D.Eng., 

 Armstrong College, IS eiccastle-on-Tyne* '. 



[Plate XIII.] 



1. A LTHOUGH it is known from motor-car experience 



JlJL. that certain sparks fail to cause ignition, there do 



not appear to be any measurements of the critical energy 



required to start self-ignition in a gaseous mixture. The 



* Communicated by the Author. 



