partial degree, not at all equivalent to the compass of the shock; and is very far from being the constant concomitant of an earthquake. Quite the contrary. Innumerable such happen, when there is no breach of the surface; and of these three or four which we have now felt, nothing of it has appeared. But the immensity of the vibration of the earth which shook every house in London with impunity, and for twenty miles round, can never, in my apprehension, be owing to so unbridled a cause, as any subterraneous vapours, fermentations, rarefactions, and the like; the vulgar solution. Nor does the kind of motion, which I discern in an earthquake, in any sort agree with what we should expect from explosions.
In order then to proceed with some degree of certainty, in our inquiry after the cause of earthquakes, it will be useful, in the first place, to set in one view, the general appearances remarkable therein: the most usual concomitants: As we can collect them from our own observation, or from the relations and writings of others.
I. That earthquakes always happen in calm seasons, in warm, dry, sultry weather; or after dry, frosty air.