Earthquakes.
At the commencement of this chapter I mentioned that I had never been consciously sensible of the occurrence of an earthquake. I think it may perhaps be useful to state that on a recent occasion I really perceived the effects of an earthquake, although at the time I assigned them to a different cause.
On the 6th of last October, about half-past three, a.m., {224} most of the inhabitants of London who were awake at that hour perceived several shocks of an earthquake. I also was awake, although not conscious of the shocks of an earthquake.
As soon as I read of the event in the morning papers, I was forcibly struck by its coincidence with my own observations, although I had attributed to them an entirely different cause. In order to explain this, it is necessary to premise that I had on a former occasion instituted some experiments for the purpose of ascertaining how far off the passing of a cart or carriage would affect the steadiness of a star observed by reflection. Amongst other methods, I had fixed a looking-glass of about 12 by 16 inches, by a pair of hinges, to the front wall of my bedroom. It was usually so placed that, as I lay in bed, at the distance of about 10 or 12 feet, I could see by reflection a small gas-light burner, which was placed on my left hand.
By this arrangement any tremors propagated through the earth from passing carriages would be communicated to the looking-glass by means of the front wall of the house, which rose about 40 feet from the surface. The image of the small gas-burner reflected in the looking-glass would be proportionally disturbed. In this state of things, at about half-past three o’clock of the morning in question, I observed the reflected image of the gas-light move downwards and upwards two or three times. I then listened attentively, expecting to hear the sound of a distant carriage or cart. Hearing nothing of the kind, I concluded that the earth wave had travelled beyond the limit of the sound wave, arising from the carriage which produced it. Presently the image of the gaslight again vibrated up and down, and then suddenly fell about four or five inches lower down in the glass, where it remained fixed for a time. Still thinking the observation of no consequence, {225} I shut my eyes, and after perhaps another minute, again saw the image in its lower position. It then rose to its former position, vibrated, and shortly again descended: it remained down for some time and then resumed its first position.