make no clefts, or separations, in those parts where the minerals and mountainous rocks part from the light mold and clay. If an hundred barrels of gun-powder could be fixed in some cave, a thousand yards under ground; allowing the force of the explosion sufficient to raise all the weight of earth incumbent on the cavern; it would certainly break the loose mold from any large solid rock we may conceive adjacent, and leave at least some clefts behind it. But we seldom or never hear of such clefts, made in such places, when earthquakes happen."
Again, he writes thus: "I cannot apprehend, (if all earthquakes must be made by explosions in subterraneous caverns) why sometimes a large country, or whole continent, should be thereby shook all at once; why there should be no eruptions in the neighbourhood?"
From all circumstances consider'd, he concludes, that the abstruse, effective cause of them comes from the air; and that a calm is necessary before an earthquake. And these two particulars are likewise Dr. Hales's positions: "The earth-lightning, as he calls it, is first kindled on the surface, and not at great depths, as has been thought; whose explosion is the immediate cause of an earthquake. He says long, dry, hot sea