476 THEORY OF EARTHQUAKES—MAURY. 
steamer are overheated and the supply of water is low. A slight. 
lurch of the boat will bring a quantity of water suddenly into 
contact with the heated wall of the boiler. Superheated steam 
is generated and an explosion occurs. Now the temperature to. 
which such a boiler wall is raised does not of course compare 
with that of the molten steel and slag of the casting pot. And 
hence a similar result is perfectly natural, only that what. 
occurred at New Glasgow was probably even more violent. It 
is not surprising that when the explosion took place portions of 
the cireumjacent metal and slag were shot out of the casting pot 
high into the air. 
I observed to my informer that this was an extremely inter- 
esting fact. He therefore went on to tell me that according to 
his observation, molten slag, when brought into contact with 
moisture, is productive of much more violent explosions than 
molten steel. Some time ago a casting from a small pot of steel 
had been made. The slag which remained in the bottom of the 
pot was carefully poured upon the ground. This was done very 
near the wall of one of the buildings belonging to the Steel 
Works. The foreman (my informant) was in a distant part of 
the works. He heard a violent explosive report like that of a 
heavy cannon, and ran to the spot from which the sound came. 
He imagined that the boiler of the engine had burst. He found 
that the explosion had been occasioned simply by pouring out an 
inconsiderable quantity of slag upon ground where there was a 
little ice or snow. Such, however, was the violence of the con- 
cussion that portions of the roof of the building, which was about 
twenty feet high, were ripped off and blown away, although 
the explosion, be it remembered, had taken place in the open air, 
outside the building. 
Now let me ask your attention to what seem to me to be the 
weighty suggestions involved in these seemingly trifling facts. 
You are well aware that two prominent theories are offered 
for the explanation of earthquakes. According to one they are 
the result of the sliding and grinding of immense masses of rock 
one upon another,—such sliding being itself a consequence of the 
secular cooling and shrinking of the crust of the earth. 
