The learned Dr. Childrey observes, treating on this subject, that earthquakes happen upon rain; a sudden shower of rain in the time of a great drought.
'Tis objected, that, if this was the case, nothing would be more frequent than earthquakes; but these two circumstances concurring, a shower and dry weather, must not necessarily cause it, any more than touching a tube before it is electrified causes a snap. The earth must be in a proper electrified state to produce it; and electricity has its fits; is remitted, intended, ceased and recommenced. It has its bounds. All causes must concur. And now, with us, all necessary causes did so apparently. Tho' a shower of rain falling upon the earth when electrified, may cause an earthquake, yet too much rain before, will prevent that state of electricity, necessary.
The day before the catastrophe of Port-Royal, the weather was remarkably serene and clear. In that most dreadful earthquake, 1692, of Sicily, where 54 cities and towns, beside a great number of villages were destroy'd; but especially the whole city of Catania: It was preceded by a most agreeable, serene and warm season, which was the more observable on account of its being unusual at that time of the year.
I have been inform'd, that in the morning of both earthquakes last past with us, the air was serene and calm; on the morning before that 8th of February, the air was observ'd to be remarkably calm; and that a little before, a black cloud appear'd over great part of the horizon. Dr. Hales, in his relation, says, the Centinels in St. James's Park, and others who were abroad in the morning of the last earthquake, observ'd a large black cloud, and some coruscations, just before the shock, and that it was very calm weather: And that, in the history of earthquakes, they generally begin in calm weather, with a black cloud.
This observation precludes the suspicion of earthquakes arising from tumults and commotions in the upper, or under region of the air. The remarkable clearness of the air before earthquakes, observ'd by all, shows evidently how free it is from vapours and the like.
Agreeable to our fifth position, Mr. Flamsted writes, "A hollow noise in the air always precedes an earthquake, so near that it rather seems to accompany them. He refers us to Philosophical Transactions, No 151. p. 311. The noise was heard by many that liv'd in the out-streets, and alleys of London, remote from the noise and tumult of the greater streets."
This he speaks of that felt in London 1692; but now the whole city heard the noise, on both these earthquakes of ours.
The gardener, who gave a relation to the Royal Society of what he observed in the Temple-garden, took notice, that first he heard the most dreadful noise imaginable, which he thought to be a great discharge of ship-guns, on the river: and that the noise rolled from the water-side towards Temple-bar, rather before the nodding of the houses.