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 a dry, frosty air.
II. That they are felt at sea, as well as land, even in the main ocean; and at that time, the sea is calm.
III. That earthquakes differ very much in magnitude. Some shake a very large tract of country, at the same instant of time; nay, sometime extend to very many countries, separated by mountains, seas, lakes, the ocean.
IV. That earthquakes differ very much in the quantity, of their vibratory motion: Whence in some, tho' largely extended, they are innocuous; in others, both small and great, they lay all in ruin and destruction.
V. That a hollow, thundering, unusual noise accompanies them, or rather seems to preceed the shock; which rolls in the air like the noise of cannon.
VI. That they are felt more sensibly in the upper story of houses than in the lower.
VII. That the shock is more violent upon more solid buildings, churches, castles, and stone-houses, than upon those of slighter materials.
VIII. That many people find themselves sick at stomach, with headake, and pains in their joints, and the like, which sometime lasts for the day after, or longer.
IX. That earthquakes generally happen to great towns, and cities, and more particularly to those that are situate on the sea.
X. That earthquakes do not cause any damage to springs and fountains; but the water in wells becomes foul for a short time.
XI. That they are more frequent in the neighboring countries of a vulcano.
This last circumstance, in my opinion, has led all inquirers in this question, out of the true path; therefore I propose in the ensuing paper.
I. To shew what it is not; the insufficiency of the vulgar opinion, of subterraneous fires and vapours.
II. To shew what it is in reality, as it appears to me.
III. I shall conclude with the moral use we ought to make of these prodigies of nature.