to the furface DI; because the mediums A I, DL, are uniform, they will both transmit the light which falls upon them, in the same proportion, that is, because both these mediums are by fuppofition of the fame nature, equally denfe, and of the same thickness, and the Rays which fall upon both, have the fame inclination, the quantity of Rays which fall upon the furface A C, will be to the quantity which paffes thro' the medium  I and comes to the furface DI as that quantity which comes to DI is to the quantity which paffes thro' DL, and comes to LC and by confequence if a reprefent the number of Rays which fall upon AC, a the number

mm

I

m

which fall upon DI, ma will represent the number of those that fall upon the furface GL. After the fame manner ma will be as the number of thofe Rays which come to MN, and m4a will be the number of those

I

which fall upon FB, that is, If the distances AD, AG, AM, AB, are in an Arithmetical progreffion, the Applicates AC, DE, GH, MO, FB, which reprefent the number of Rays that come to the points A, D, G, M, B, refpectively, will be in a Geometrical progreffion, which is the property of the Logarithmick curve,

Tho' this way of reafoning by a Calculus, feems to be plain and obvious enough to those who understand the common principles of Calculation, (which as I think ought to be unkown to none that pretend either to write or defend a Theory, as the Theorist himself owns) yet the Defender does whatever he can to find fhifts and evafions for fuch arguments; and here he tells us, that we ought not to confider furfaces but pores. Well, that he may fee how ready I am to please and obey him, I have done this already in the 237th Page of this Treatife, where I have fhew'd, that if the Sun fhin'd upon any furface that is expofed openly to it, its heat on that furface would be 202500 times greater than its heat upon the furface of the Abyfs, when it shin'd only thro' the Pores of the Cruft. Which difproportion is great enough to fhew, that no great store of vapours could be rarify'd in the Abyss. But fays he, thofe that allow a Comet at its nearest approach to the Sun, to be pierced thro' and thro', fo as to become hotter than red hot iron, will not think it ftrange that at our distance, it should have fome proportional effect upon the inward parts of the Earth. Let us illuftrate his fimilitude by another; Thofe who allow that a ball of iron ten inches thick, when put in a good fire may be made red hot, and be peirced by the fire thro' and thro', will not think it strange, that this ball of iron remov'd 10 feet

10 feet from the fire, fhould receive fome proportionable heat even in its inward parts, as without doubt it would. But the queftion is, if this effect is any way fenfible, or if we fhould fuppofe fome water inclosed in the middle of this ball, whether the heat of the fire could raise it into vapour at fuch a diftance, fo that the force thofe vapours have to expand themselves, would break or burst the ball.

I thought that this Gentleman had known fo much of the new Experimental Philosophy, as not to be ignorant, that heat does not pafs into the interior parts of a folid of confiderable thickness, till it has quite diffolved the Exterior parts; and if the folid is combuftible, (as wood) it confumes the outward parts, before it has any fenfible effect upon the Interior; but if its parts are compact, (as Metals or Stone) it loofens and diffolves the frame and texture of the outward parts, and fo makes its way to the inward.

He con

But our Philofopher thinks he has found out one remarkable Phenomenon, by which he can prove effectually, that the heat of the Sun peirces deep into the Cruft, and that is in the cafe of the Earthquakes. fiders the cause of them, and their depths, and he fays, that all agree that they arise from the rarefaction of Vapours and Exhalations; This rarefaction (fays he) must be made by some heat, and no other is prov'd

to us yet by this Author, than the heat of the Sun.

Why fhould I be oblig'd to fatisfy him in all his difficulties in Philofophy? Did ever I fet up to be a Theorist, and give an account of all the Phenomena of Nature? Well; but it feems he expects it from me, and tho' I am no ways oblig'd to it, yet out of abundance of good nature, I will give my opinion in this matter.

I think then that the rarefaction of Vapours within the Earth, may arife from another heat than that of the Sun. We know that there is an actual fire which always burns in feveral places of it, which fometimes bursts out and makes an horrible eruption, as in all Vulcano's and Fiery Mountains; and why may not this fire be the cause of the rarefaction? This appears to be more probable, becaufe Earthquakes are most common in thofe places where these Vulcanos are, as in the Kingdom of Naples and Sicily.

But fuppofing there were no actual fire under ground, yet I am of the opinion, that heat may arife from other caufes than that of the Sun. It is obferv'd, that from a due mixture of fome particles of matter with those of another fort, there will arise a very confiderable heat: Some places underground are obferv'd to be exceeding hot, as fome Mines, (where there is a mixture of Sulphureous, Nitrous, and Mineral principles) the heat is

fo

fo confiderable, that a Man cannot eafily endure it, even in the extremity of Winter. There are other places fo warm, that the waters that run thro' them will fcald a Mans hand. And may we not fuppofe, that there are fome Cavities deeper underground, where the Earth is of the fame frame and texture of parts? Now if fome Sulphureous and Nitrous Exhalations, fhould be gathered together within any of thefe Cavities, and by motion, or any other accident, they fhould happen to be kindled, it is plain they will expand themfelves, rarify the Air, and make that Concuffion of the ground we ufually observe in Earthquakes.

Now it is plain, that these Exhalations may be kindled without the heat of the Sun, from obfervations that are daily to be made in our Atmosphere; where the Sulphureous Exhalations that are the caufe of Thunder and Lightning are kindled in the Air, when the action of the Sun is not ftrong. This is alfo obferv'd of Meteors, (which we commonly call falling Stars) whofe matter is kindled in the night-time, when the immediate heat of the Sun can have as little effect as it has within the bowels of the Earth: and I hope this will be fufficient to fatisfy him, that Earthquakes may arife from other caufes than the heat of the Sun.

Tho' the arguments I have already given, clearly prove, that there was no rarefaction of