cold ones of our present world, some proof that its poles have been changed? Is not the supposition that the poles have been changed, the easiest way of accounting for the deluge, by getting rid of the old difficulty how to dispose of its waters after it was over? Since, if the poles were again to be changed, and placed in the present equator, the sea would fall there about fifteen miles in height, and rise as much in the present polar regions; and the effect would be proportionable if the new poles were placed anywhere between the present and the equator.

Does not the apparent wreck of the surface of this globe, thrown up into long ridges of mountains, with strata in various positions, make it probable that its internal mass is a fluid, but a fluid so dense as to float the heaviest of our substances? Do we know the limit of condensation air is capable of? Supposing it to grow denser within the surface in the same proportion nearly as it does without, at what depth may it be equal in density with gold?

Can we easily conceive how the strata of the earth could have been so deranged, if it had not been a mere shell supported by a heavier fluid? Would not such a supposed internal fluid globe be immediately sensible of a change in the situation of the earth's axis, alter its form, and thereby burst the shell and throw up parts of it above the rest? As if we would alter the position of the fluid contained in the shell of an egg, and place its longest diameter where the shortest now is, the shell must break; but would be much harder to break if the whole internal substance were as solid and as hard as the shell.

Might not a wave, by any means raised in this supposed internal ocean of extremely dense fluid, raise, in some degree as it passes, the present shell of incumbent earth, and break it in some places, as in earthquakes. And may not the progress of such wave, and the disorders it occasions among the sol

ids of the shell, account for the rumbling sound being first heard at a distance, augmenting as it approaches, and gradually dying away as it proceeds? A circumstance observed by the inhabitants of South America, in their last great earthquake, that noise coming from a place some degrees north of Lima, and being traced by inquiry quite down to Buenos Ayres, proceeded regularly from north to south at the rate of leagues per minute, as I was informed by a very ingenious Peruvian whom I met with at Paris. B. FRANKLIN.

To M. Dubourg.

ON THE NATURE OF SEACOAL.

I am persuaded, as well as you, that the seacoal has a vegetable origin, and that it has been formed near the surface of the earth; but, as preceding convulsions of nature had served to bring it very deep in many places, and covered it with many different strata, we are indebted to subsequent convulsions for having brought within our view the extremities of its veins, so as to lead us to penetrate the earth in search of it. I visited last summer a large coalmine at Whitehaven, in Cumberland; and in following the vein, and descending by degrees towards the sea, I penetrated below. the ocean where the level of its surface was more than eight hundred fathoms above my head, and the miners assured me that their works extended some miles beyond the place where I then was, continually and gradually descending under the sea. The slate, which forms the roof of this coalmine, is impressed in many places with the figures of leaves and branches of fern, which undoubtedly grew at the surface when the slate was in the state of sand on the banks of the sea. Thus it appears that this vein of coal has suffered a prodigious settlement. B. FRANKLIN.

CAUSES OF EARTHQUAKES.

The late earthquake felt here, and probably in all the neighbouring provinces, having made many people desirous to know what may be the natural cause of such violent concussions, we shall endeavour to gratify their curiosity by giving them the various opinions of the learned on that head.

Here naturalists are divided. Some ascribe them to water, others to fire, and others to air, and all of them with some appearance of reason. To conceive which, it is to be observed that the earth everywhere abounds in huge subterraneous caverns, veins, and canals, particularly about the roots of mountains; that of these cavities, veins, &c., some are full of water, whence are composed gulfs, abysses, springs, rivulets; and others full of exhalations; and that some parts of the earth are replete with nitre, sulphur, bitumen, vitriol, &c. This premised,

1. The earth itself may sometimes be the cause of its own shaking; when the roots or basis of some large mass being dissolved, or worn away by a fluid underneath, it sinks into the same, and, with its weight, occasions a tremour of the adjacent parts, produces a noise, and frequently an inundation of

water.

2. The subterraneous waters may occasion earthquakes by their overflowing, cutting out new courses, &c. Add that the water, being heated and rarefied by the subterraneous fires, may emit fumes, blasts, &c., which, by their action either on the water or immediately on the earth itself, may occasion great succussions.

3. The air may be the cause of earthquakes; for

the air being a collection of fumes and vapours raised from the earth and water, if it be pent up in too narrow viscera of the earth, the subterraneous or its own native heat rarefying and expanding it, the force wherewith it endeavours to escape may shake the earth; hence there arises divers species of earthquakes, according to the different position, quantity, &c., of the imprisoned aura.

Lastly, fire is a principal cause of earthquakes; both as it produces the aforesaid subterraneous aura or vapours, and as this aura or spirit, from the different matter and composition whereof arise sulphur, bitumen, and other inflammable matters, takes fire, either from other fire it meets withal, or from its collision against hard bodies, or its intermixture with other fluids; by which means, bursting out into a greater compass, the place becomes too narrow for it, so that, pressing against it on all sides, the adjoining parts are shaken, till, having made itself a passage, it spends itself in a volcano or burning mountain.

But to come nearer to the point. Dr. Lister is of opinion that the material cause of thunder, lightning, and earthquakes, is one and the same, viz., the inflammable breath of the pyrites, which is a substantial sulphur, and takes fire of itself.

The difference between these three terrible phenomena he takes only to consist in this: that the sulphur in the former is fired in the air, and in the latter under ground. Which is a notion Pliny had long before him: "Quid enim," says he," aliud est in terrá tremor, quam in nube tonitru ?" For wherein does the trembling of the earth differ from that occasioned by thunder in the clouds ?

This he thinks abundantly indicated by the same sulphurous smell being found in anything burned with lightning, and in the waters, &c., cast up in earthquakes, and even in the air before and after them.

Add that they agree in the manner of the noise. which is carried on, as in a train fired; the one, rolling and rattling through the air, takes fire as the vapours chance to drive; as the other, fired under ground in like manner, moves with a desultory noise.

Thunder, which is the effect of the trembling of the air, caused by the same vapours dispersed through it, has force enough to shake our houses; and why there may not be thunder and lightning under ground, in some vast repositories there, I see no reason; especially if we reflect that the matter which composes the noisy vapour above us is in much larger quantities under ground.

That the earth abounds in cavities everybody allows; and that these subterraneous cavities are, at certain times and in certain seasons, full of inflammable vapours, the damps in mines sufficiently witness, which, fired, do everything as in an earthquake, save in a lesser degree.

Add that the pyrites alone, of all the known minerals, yields this inflammable vapour, is highly probable; for that no mineral or ore whatsoever is sulphurous, but as it is wholly or in part a pyrites, and that there is but one species of brimstone which the pyrites naturally and only yields. The sulphur vive, or natural brimstone, which is found in and about the burning mountains, is certainly the effects of sublimation, and those great quantities of it said to be found about the skirts of volcanoes is only an argument of the long duration and vehemence of those fires. Possibly the pyrites of the volcanoes, or burning mountains, may be more sulphurous than ours; and, indeed, it is plain that some of ours in England are very lean, and hold but little sulphur; others again very much, which may be some reason why England is so little troubled with earthquakes, and Italy, and almost all round the Mediterranean Sea, so much; though