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ble exhalations. This last hypothesis has been adopted by manyof the most celebrated moderns, as Gassendns, K ire her, Schottus,Varenius, Des Cartes , Du llamel, Uonorius, Fabri, &:c. 'I'hephilosopher last mentioned, indeed supposed, that waters prodigi-ously rarefied by heat, might sometimes occasion earthquakes.The others supposed, that there are many and vast cavities underground, which have a communication with one another: some ofwhich abound with waters; others with vapours and exhalations,arising from inflammable substances, as nitre, bitumen, sulphur,Kce. These combustible exhalations thev supposed to be kindledby a subterraneous spark, or by some active flame gliding througha narrow fissure from without, or bv the fermentation of somemixture; and when this happens, that they may necessarily pro-duce pul es, tremors, and ruptures at the surface, according to thenumber and diversity of the cavities, and the quantity and activityof tiie inikunmable matter, ’i bis hypothesis they illustrated by avariety of experiments, such as nrixtures of iron tilings and brim-stone buried in the earth, gun-powder confined in pits, &'c. by allwhich a slinking of the earth will be produced. Dr. Woodwardsuggests another hypothesis. lie supposes that the subterraneousheat or lire, which is continually elevating water out of the abyss,which, according to him, occupies the centre of the earth, to fur-nish rain, dew, springs, and rivers, may he stopped in some parti-cular part. When this obstruction happens, the heat causes agreat swelling and commotion in the waters of the abyss; and atthe same time, making the like effort against the superincumbentearth, that agitation and concussion of it is occasioned, which wecall an earthquake. M. Amontons suggests a hypothesis entirelydifferent from all these. According to the philosophical princi-ples, which suppose the atmosphere to be about 45 miles high,and that the density of the air increases in proportion to the abso-lute height of the superincumbent column of fluid; it is shewn,that at the depth of 43,528 fathoms below the surface of the earth,air is but one-fourth lighter than mercury. Now, this depth of43,528 lathoms is only a 74th part of the. semi-diameter of theearth; and the vast sphere beyond this depth, in diameter(i,431,538 fathoms, may probably be only filled with air; whichwill be here greatly condensed, and much heavier than the hea-viest bodies we know in nature. But it is found by experiment,that the more air is compressed, the more does the same degree ofheat increase its spring, and the more capable does it render it of aviolent effect; and that, for instance, the degree of heat of boilingw ater increases the spring of the air above what it has in its naturalstate, in our climate, by a quantity equal to a third of the weightwherewith it is pressed. Whence we may conclude, that a de-gree of heal, which on the surface of the earth will only have amoderate effect, may be capable of a very violent one below.And, as we are certain, that there are in nature, degrees of heatmuch greater than that of boiling water, it is possible there may-be some, whose violence, further increased by the immense weightof the air, may be sufficient to break and overturn this solidorb of 43,528 fathoms; whose weight, compared to that of theincluded air, would be but a trifle. Though none of thesehypotheses were sufficient for explaining the phenomena ofearthquakes in a satisfactory manner, one or other of them con-tinued to be adopted by almost all philosophers till 1749. InMarch 1749, an earthquake was felt at London and severalother places in Britain . Dr. Slukely, who had been much engagedin electrical experiments, began to suspect, that pha-nomena of thiskind ought to he attributed not to vapours or fermentations ge-nerated in the bowels of the earth, but to electricity. In a paperpublished by him on this subject, lie rejects all the above hypo-theses for the following reasons:—1. That there is no evidence ofany remarkable cavernous structure of the earth ; but that, on thecontrary, there is reason to presume, that it is in a great measuresolid, so as to leave little room for internal changes and fermen-tations within its substance; nor do coal-pits, when on fire, everproduce any thing resembling an earthquake. 2. In the earth-quake at London , in March 1749, there was no such tiling as lire,vapour, smoke, smell, or an eruption of any kind observed, thoughthe shock affected a circuit of 30 miles in diameter. This con-sideration alone of the extent of surface shaken by an earthquake,lie thought sufficient to overthrow the supposition of its beingowing to the expansion of any subterraneous vapours. For, assmall fire halls bursting in the air propagate a sulphureous smell tothe distance of several miles, it cannot be supposed, that so ini-5

mense a force, acting instantaneously, on that compass of ground,should never break the surface ot it, nor become discoverableeither to the sight or the smell, besides that such fermentationwould require a long time. That such an effect, therefore, shouldbe produced instantaneously, can be accounted for by electricity'only, which acknowledges no sensible transition of time, lior anybounds. 3. If vapours and subterraneous fermentations, explo-sions, and eruptions, were the cause of earthquakes, they wouldabsolutely ruin the whole system of springs and fountains, where-ever they had once been; which is contrary to fact, even whenthey have been frequently repeated. In the earthquake in AsiaMinor , A. D. 17, which destroyed 13 great cities, and shook amass of earth 300 miles in diameter, nothing suffered but the ci-ties ; neither the springs nor the face of the country being injured.4. That any subterraneous power, sufficient to move 30 miles indiameter, must be lodged at least 15 or 20 miles below the sur-face ; and therefore must move an inverted cone of solid earth,the base of which is 30 miles in diameter, and the axis 15 or 20 ;an effect impossible to any natural power whatever, except elec-tricity. So in Asia Minor , such a cone must have been 300 milesin the diameter of the base, and 200 in the axis; which not allthe gunpowder that has been made since the invention of it, muchless any vapours generated so far below the surface, could possiblyeffect.” 3. A subterraneous explosion will not account for themanner in which isbips, far from land, and even fish, are affectedduripg an earthquake. A subterraneous explosion would onlyproduce a gradual swell, and not give so quick an impulse to thewater as would make it feel like a stone. From these circum-stances the Doctor concluded; that an earthquake was a shock ofthe same kind as those in electrical experiments. And this hy-pothesis was confirmed by the pha'iiomena attending earthquakes;particularly those of 1749 and 1750, which gave rise to his publi-cation. From these phsennmena, he inferred, that if a non-electriccloud discharge its contents upon any- part of the earth, in a highlyelectrical state, an earthquake must necessarily ensue. As thedischarge from an excited tube produces a commotion in thehuman body, so the discharge of electric matter from many milesof solid earth must needs be an earthquake ; and the snap fromtlie contact, the horrid uncouth noise attending it. Dr. Stukelydoes not attempt to account for the manner in which the earth andair are put into a stale of electricity proper for this purpose. Thesame hypothesis was advanced by Signior Beccaria, without know-ing any thing of Dr. Stukely’s discoveries. But this learnedItalian'imagined the electric matter to be lodged deep in thebowels of the earth, agreeably to his hypothesis concerning light-ning. Now, as the quantity of electric matter in the simplestthunder-storms is so inconceivably great, that it is impossible tobe contained by any cloud or number of clouds ; and as, duringthe progress of a thunder-storm which lie observed, though thelightning frequently struck to the earth, the same clouds were thenext moment ready to make a still greater discharge; it was evi-dent, that they must have received at one place, the moment adischarge was made from them in another. Let ns suppose theseclouds ever so great, if the lightning proceeded only from them,the quantity must be lessened by every discharge; and no re-cruits, that any new clouds might bring, can bear any proportionto the discharge which must ensue, from the collision of so greata number as combine to form a thunder-storm. It seems there-fore most likely, that the electric matter is continually dartingfrom the clouds in one place, while it is discharged from the earthin another ; and, consequently, that the clouds serve as conductorsof the electric fluid from those places of the earth which are over-loaded with it, to those which are exhausted. This theory beingadmitted, he thinks earthquakes may be ascribed to the samecause. For if the equilibrium of the electric matter be lost in thebowels of the earth; so that the best method of restoring it shallhe by the fluid bursting into the air, and traversing several milesof the atmosphere, to come at the place where it is wanted ; itmay be easily imagined, that violent concussions will be given i#the earth by the sudden passage of so powerful an agent. This, hesays, was confirmed by the flashes of light, resembling lightning,which have been frequently seen to rush from the top of MountVesuvius , when ashes, &c. have been carried out of it into the air,and scattered over a large tract of country. And it is well known thatvolcanoes have a near connection with earthquakes. Dr. Priestley,in his History of Electricity , observes upon these theories, that a

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