happen more than once or twice in a month, and are always accompanied (so far as he had been able to observe) with an aurora borealis.

For the sake of those who may be desirous of examining the diurnal variations of the needle very minutely, Mr. C. annexed a complete year's observations; and deduced from the regular variations during that time the mean diurnal variation belonging to each month: whence it appears that the diurnal variation increases from January to June, and decreases from June to December.

The mean diurnal Variation for each Month in the Year 1759.

Conjectures concerning the Cause, and Observations on the Phenomena, of Earthquakes; particularly of that great Earthquake of Nov. 1. 1755, which proved so fatal to the City of Lisbon. By the Rev. JOHN MICHELL, M.A.

Ir has been the general opinion of philosophers, that earthquakes owe their origin to some sudden explosion in the interior of the earth. This opinion is agreeable to the phenomena which seem to point out something of that kind. That these concussions should owe their origin to something in the air seems very ill to correspond with the phenomena. This will sufficiently appear, as those phenomena are hereafter recounted; nor does there appear to be any such certain and regular connection between earthquakes and the state of the air, when they happen, as is supposed by those who hold this opinion.

Let us, then, rejecting this hypothesis, suppose that earthquakes have their origin under ground, and we need not go tar in search of a cause, whose real existence in nature we have evidence of, and which is capable of producing all the appearances of these extraordinary motions. The cause I mean, says Mr. M., is subterraneous fires. These fires, if a large quantity of water should be let out upon them suddenly, may produce a vapour, whose quantity and elastic force may be fully sufficient for that purpose. The principal

facts, from which I would prove, that these fires are the real cause of earthquakes, are as follow:

The same places are subject to returns of earthquakes, not only at small intervals, for some time after any considerable one has happened, but also at greater intervals of some ages. The returns of earthquakes in the same places, at longer distances of time, are confirmed by all history. Constantinople, and many parts of Asia Minor, have suffered by them, in many different ages: Sicily has been subjected to them, as far back as the remains even of fabulous history can inform us of: Lisbon did not feel the effects of them for the first time in 1755: Jamaica has frequently been troubled with them, since the English first settled there; and the Spaniards, who were there before, used to build their houses of wood, and only one story high, for fear of them: Lima, Callao, and the parts adjacent, were almost totally destroyed by them twice, within the compass of about 60 years: nor were these the only instances of the like kind which happened there; for, from the year 1582 to 1746, they have had no less than 16 very violent earthquakes, besides an infinity of less considerable ones; and the Spaniards, at their first settling there, were told by the old inhabitants, when they saw them building high houses, that they were building their own sepulchres.

Those places that are in the neighbourhood of burning mountains are always subject to frequent earthquakes; and the eruptions of those mountains, when violent, are generally attended with them. Asia Minor and Constantinople may be considered as in the neighbourhood of Santerini. The countries also about Etna, Vesuvius, Mount Hecla, &c. afford us sufficient proofs to the same purpose. But of all the places in the known world, probably no countries are so subject to earthquakes as Peru, Chili, and all the western parts of South America; nor is there any country in the known world so full of volcanoes: for, throughout all that long range of mountains, known by the name of the Andes, from 45° south latitude to several degrees north of the line, as also throughout all Mexico, being about 5000 miles in extent, there is a continued chain of them.

The motion of the earth in earthquakes is partly tremulous, and partly propagated by waves, which succeed one another sometimes at larger and sometimes at smaller distances; and this latter motion is generally propagated much farther than the former. The former part of this proposition wants no confirmation for the proof of the latter, viz. the wave-like motion of the earth, we may appeal to many accounts of

:

earthquakes: it was very remarkable in the two which happened at Jamaica in 1687-8 and 1692. In an account of the former, it is said, that a gentleman there saw the ground rise like the sea in a wave, as the earthquake passed along, and that he could distinguish the effects of it to some miles distance, by the motion of the tops of the trees on the hills. Again, in an account of the latter, it is said, "the ground heaved and swelled like a rolling swelling sea," insomuch that people could hardly stand on their legs by reason of it. The same has been observed in the earthquakes of New England, where it has been very remarkable. A gentleman giving an account of one that happened there, Nov. 18. 1755, says, the earth rose in a wave, which made the tops of the trees vibrate 10 feet, and that he was forced to support himself, to avoid falling while it was passing. The same also was observed at Lisbon, in the earthquake of the 1st Nov. 1755, as may be. plainly collected from many of the accounts that have been published concerning it, some of which affirm it expressly; and this wave-like motion was propagated to far greater distances than the other tremulous one, being perceived by the motion of waters, and the hanging branches in churches, through all Germany, among the Alps, in Denmark, Sweden Norway, and all over the British isles.

It is observed in places which are subject to frequent earthquakes, that they generally come to one and the same place from the same point of the compass. It may be added, also, that the velocity with which they proceed (as far as one can collect it from the accounts of them) is the same; but the velocity of the earthquakes of different countries is very different.

In the earthquake of Nov. 1. 1755, we are told that both smoke and light flames were seen on the coast of Portugal, near Colares; and that on occasion of some of the succeeding shocks, a slight smell of sulphur was perceived to accompany a "fog, which came from the sea, from the same quarter whence the smoke appeared." In an account of an earthquake in New England, it is said that at Newbury, 40 miles from Boston, the earth opened, and threw up several cartloads of sand and ashes; and that the sand was also slightly impregnated with sulphur, emitting a blue flame when laid on burning coals. One of the relaters of the earthquake in Jamaica, in 1692, has these words: "In Port-Royal, and in many places all over the island, much sulphureous combustible matter has been found (supposed to have been thrown out on the opening of the earth), which on the first touch of

fire would flame and burn like a candle. St. Christopher's was heretofore much troubled with earthquakes, which, on the eruption there of a great mountain of combustible matter, which still continues, wholly ceased, and have never been felt there since."

The earth (as far as one can judge from the appearances) is not composed of heaps of matter casually thrown together, but of regular and uniform strata. These strata, though they frequently do not exceed a few feet, or perhaps a few inches in thickness, yet often extend in length and breadth for many miles, and this without varying their thickness considerably. Beside the horizontal division of the earth into strata, these strata are again divided and shattered by many perpendicular fissures, which are in some places few and narrow, but oftentimes many, and of considerable width.

The returns of earthquakes in the same places, either at small or large intervals of time, are consistent with the cause assigned: subterraneous fires, from their analogy to volcanoes, might reasonably be supposed to subsist for many ages, though we had not those instances already mentioned which put the matter out of doubt. And as it frequently happens that volcanoes rage for a time, and then are quiet again for a number of years, so we see earthquakes also frequently repeated for some small time, and then ceasing again for a long term, excepting, perhaps, now and then some slight shock. And this analogy between earthquakes and the effects of volcanoes is so great, that he thinks it cannot but appear striking to any one who will read the accounts of both, and compare them together.

The frequency of earthquakes in the neighbourhood of burning mountains is a strong argument of their proceeding from a cause of the same kind; and the analogy of several volcanoes lying together in the same tract of country, as well as new ones breaking out in the neighbourhood of old ones, tends greatly to confirm this opinion; but what makes it still the more probable is, that peculiarity in the structure of the earth before mentioned. It has been already observed, that the same strata are generally very extensive, and that they commonly lie more inclining from the mountainous countries than the countries themselves: these circumstances make it probable that those strata of combustible materials, which break out in volcanoes on the tops of the hills, are to be found at a considerable depth under ground in the level and low countries near them.

If we suppose that these vapours, when pent up, are the

cause of earthquakes, we must naturally expect that the most extensive earthquakes should take their rise from the level and low countries; but more especially from the sea, which is nothing else than waters covering such countries. Accordingly, we find that the great earthquake of Nov. 1. 1755, which was felt at places near 3000 miles distant from each other, took its rise from under the sea; as is manifest from that wave which accompanied it. The same thing is to be understood of the earthquake that destroyed Lima, in the year 1746, which, it has been said, was felt as far as Jamaica; and as it was more violent than the Lisbon earthquake, so if this be true, it must probably have been more extensive also. There have been many other very extensive earthquakes in South America: Acosta says that they have been often known to extend themselves 100, 200, or 300, and some even 500 leagues along the coast. These have been generally, if not always, attended with waves from the sea.

Of artificial Cold produced at Petersburgh., By Dr. HIMSEL.

The

ON December 14, 1759, they had at Petersburgh the most excessive cold weather that ever was known, even to 205° of De Lisle's thermometer, or 34° below zero in Fahrenheit. At that time Professor Braun repeated Fahrenheit's experiments in order to produce excessive cold, by means of spirit of nitre combined with snow. He saw, with surprize, the quicksilver fall considerably in the thermometer, and descend even to 470° at last: there the quicksilver remained fixed in the open air for the space of a quarter of an hour, and did not begin to rise till it was carried into a warm room. immobility of the quicksilver made him conjecture that it might be frozen, or become a solid body. - Dec. 25. in the morning, between nine and ten, De Lisle's thermometer was at the 199th degree of cold; and Mr. Braun, as well as Professor pinus, then repeated this experiment. As soon as the former had observed the quicksilver immovable in the thermometer, he broke the glass; and he found the quicksilver frozen, but not entirely; for in the middle of the glass ball there was a small portion yet remaining fluid. Mr. Epinus's thermometer fell with extreme rapidity almost to the 500th degree, and in breaking the glass from below, he found the quicksilver contained in it absolutely frozen, Both these gentlemen found that the quicksilver, thus rendered solid, bore hammering and extension, like other metals; but, being afterwards exposed to the open air, it soon recovered