Of the Difference of Light and Heat. 203 is greater than the other Party, and therefore more Time will be requifite to pass the Summer half Year than the Winter, by about 8 Days; and hence alfo, the Sun will feem to move fomewhat flower in the Summer than in Winter.

A. All these Things evidently follow indeed, Sir, from the Confideration of the Earth's Orbit being an Ellipfis: But I am not fully fatisfied about the Sun's Heat being fo weak and faint in the Winter, when the Sun is really nearest to us; and fc very ftrong and intense when the Sun is really farther by much from us.

B. You will foon fee the Reason of that when you confider: 1. That it is not the Rays which fall on us, but those which are reflected back from the Earth's Surface, that chiefly heat us. 2. That those which fall on us moft directly, or nearest to the Perpendicular, are the most in Quantity, and act on us with the greatest Force: Thus, in Fig. XXXII. the Rays of the Sun, on the longest Day of Summer, fall on London, under the Angle TLO, in Spring, or Autumn, under the Angle ÆLO, and in the midst of Winter, under the Angle VLO, and the Quantity of those Angles are refpectively 65°, 38° 30', 15°; wherefore the Force of the Sun's Rays in each of thofe Cafes, fhall be proportioned to the Sines of thofe Angles, which are, as 88294,

62251,

62251, and 25881; that is, 10, 71, 3, nearly, for those Numbers are in proportion to the Sines TA, ÆB, and VC, of the aforefaid Angles: The Quantity of the Rays alfo falling on the fame Extent of Surface is greater or fmaller, as their Obliquity is leffer or greater, and fo the Intenfity of Heat is greater and fmaller; and therefore the Sun's Heat in Winter must be weakeft, because then the Sun's Rays fall moft obliquely on us. Befides, 3. That Rays of the Sun pafs through a much greater Part of the Atmosphere in the Winter than in Summer (as is evident from viewing the fame in the fecond and fourth Pofition of the Earth in the Scheme) and therefore muft be, when arrived to us at L, more weak and faint in the firft than in the latter Cafe.

A

I am

A. Sir, you need fay no more; fufficiently convinced there is abundant Reason for the Differences of Light and Heat, and all the Variety of Seasons through the Year, though I never was fo happy as to understand it before: We will now proceed to difcourfe of the internal Substance of the Globe, if you please; for I have several Matters of great Importance to enquire of you about, relating to its Conftitution, Texture, and various conftituent Parts.

B. Sir,

Of the internal Parts of the Earth. 205

B. Sir, none will more gladly take the Pleasure to refolve you, according to the Judgments of the best Philofophers of the Age, than your humble Servant: Pray therefore make a beginning.

CHA P. II.

Geography, or the Philofophy of the Conflitution, Texture, and conftituent Parts of the Earth, defcribing the various Stratas of Earths, Foffils, Minerals, Metals, Stones, and other fubterraneous Subftances.

A.

RAY what is the internal Make and Conftitution of the Globe of

PR

our Earth?

B. The Earth generally speaking is conftituted of the two Subftances of Earth and Water; the Water, as the lighter Part, poffeffing the greatest Part of its Superficies; and the Earth, being the heavier Body, making up the internal Compofition; about which it is, I prefume, you here folely enquire.

A. Yes,

A. Yes, what I would know is, of what the internal folid Body of the Earth doth confift?

B. To this I can only anfwer, that so far as it is within our Scrutiny near the Superficies, we find it to confift of different Strata, or Layers of Earth, Minerals, Metals, Oars, Stones, and various other compound Bodies both hard and foft: But what the more internal Parts, or Compofition of it may be, we can tell but very little concerning: However the deeper you go, the folider and more compact you find its Matter to be, and the more firmly and ftrongly does it cohere together; yet it is very certain that within the Entrails of the Earth, are many Hiatus's, Receffes, Windings, Conveyances, and vaft Receptacles of Water, fulphureous Substances, &c. which are often the Caufe of Earthquakes, and fupply Volcanoes with their fiery Eruptions, as Mount Etna, &c.

A. What do you fuppofe to be in the Middle of the Earth; that is, in and about its Center?

very

B. No Body can certainly tell; the Earth's Center is near 4000 Miles from us, and we can penetrate towards it but a few Fathoms; how very unlikely then is it, we fhould know any Thing at fuch a vast and impervious Diftance? However the learned Dr. Halley has made it very probable, that

a