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NOTES AND ILLUSTRATIONS

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LETTER V.

(a) Page 436.

WITH respect to this experiment of Meffieurs de Sauffure and Pictet,

I perfectly agree with Dr. Hutton, in a differtation just published, that it could not be heat which was thus irradiated, reflected, and concentrated. We know from conftant experience, that heat is circularly and progreffively expansive in all directions, tending to put itself in equilibrium in whatever space it gradually fills, and to place all bodies which it meets in its progress in equilibrium with refpect to that quality. Light, on the contrary, we know from every other experiment to move in a direct line till repelled and diverged by fome intervening object, from which it again moves in fome other direct line, and is thus capable of being concentrated in a focus. Are we from this folitary experiment to conclude that heat in this instance has deviated from its general mode of propagation, and taken up the direct contrary one of light?-Is it not more reasonable to fuppofe, with Dr. Hutton, that it was not obfcure heat which was here reflected and concentrated, but the element of light ftill exifting in its active state on the furface of the cannon ball no longer incandefcent, but fo weakened as not to affect our vifion with the fenfation of light? As he obferves, light affects not every eye alike: to the perfon coming out of broad day-light, objects

3 M 2

objects in a darkened room are invifible, which to others who have been long in obfcurity, and to the fame perfon after ftaying fome time in it, are perfectly difcernible. Light is, in my opinion, the fole effentially active matter, as I fhall hereafter obferve, effluent and refluent from and to the great body of the fun. Abforbed and imprifoned in terreftrial matter, it becomes a conftituent part of all other fublances, to which it imparts the colouring principle, but is probably without gravity. Its laws, as well as thofe of elective attraction and repulfion and of gravity in all terrestrial matter, cannot be refolved but in the will of the Creator. Dr. Hutton judiciously obferves, that the different coloured rays into which light is divided are wifely endowed with different powers with refpect to exciting vifion or heat. The combination of all is the moft forcible on vifion, but has the leaft power to excite heat, whilft the properties of the separate rays are inverse as to thefe two purposes. Its power of exciting heat I take to proceed from its attraction of congenial particles abforbed by terrestrial fubftances, which by their revived activity give motion to fuch elastic bodies as are compounded of a confiderable proportion of its own principle. By the primary laws of nature which we can discover, but which we cannot further investigate, the pure element of light invariably moves in a direc line with inconceivable rapidity; but elaftic bodies, though containing the greatest quantity of its principles, obey a different law, whence the propagation of heat is circular and progreffive. It is with great pleasure that I find my general ideas on light fo confonant to, and fupported by, thofe of this author, whofe late publication will, I hope, further excite the attention of all philofophers to this important fubject.

(b) Page 447.

The human conception never appears fo finite, the proud lord of this little earth never feems fo truly to fhrink into an atom, as when he raises his thoughts to the immenfity of creation. From the microscopic infect to thofe innumerable luminaries rolling in that space to which we can affign no limits, all is wonder and aftonishment. The effence of even the minutest

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particle is beyond the utmost fagacity and penetration of man.

Struck

with fo much of the admirable ftructure both of the whole and of each part as he is permitted to fee into, he must confefs that more remains behind above his feeble comprehenfion, and his final exclamation must be: These are thy glorious works, Parent of Good!

Almighty, thine this universal frame,

Thus wondrous fair!-Thyfelf how wondrous then!

Those innumerable ftars which our weak fight can difcern, those which the curious aftronomer has been able to discover by the help of telescopes, and thofe, perhaps in ftill greater number, which even Mr. Herfchel's inftruments have not been able to draw within our ken, are all fo many funs, many of as great, and poffibly many of much greater, magnitude than ours. Reasonable and just analogy pronounces that each of these is probably accompanied by as many attendant planets as our fun. In our fyftem the fize of this our globe is not even a medium with that of other planets. Whilft the diameter of the Earth is about 6700 miles, that of Mercury is indeed only 2748, but that of Jupiter is 120,650 miles. Can it be fuppofed that bodies of fuch magnitude, and stars not inferior to our fun, were created merely to regulate our seasons, to guide our steps or our navigation, or to ornament the canopy of man's refidence as with fo many lamps? Much lefs bodies placed at fmaller distances would have equally answered these ends. No; fuch immense bodies, and fo many funs, many of which are imperceptible to the naked eye, muft have had more immediate and more important purposes. Reafon must affure us that the planets of our fyftem, and those attendant on other funs, muft have been destined for the habitation of other creatures, of whofe natures we are indeed and must remain for ever ignorant, but whofe exiftence feems not problematical. The fatellites of fome of our planets, fo wifely increased and diftributed to fupply light at their great distances from the fun, proclaim it beyond a doubt. If heat, however, is entirely dependant on proximity to the fun and on the intensity of its rays, we can have no conception that any living creature can exist in Mercury, from the violent heat occafioned by the

vicinity

vicinity of that body to the fun; or in Saturn, from the extreme cold arifing from its prodigious distance from that fuppofed fource of heat. But if heat is really more dependant on the nature of the materials of which animal or inanimate bodies are compounded, and the aptitude of their compofitions to be acted upon by the fun's rays, than on any real warmth conveyed by them, we can then conceive that the inhabitants of Mercury may not be more fcorched by their proximity to the fun, or thofe of Saturn more frozen by their distance from it, than we are on this earth. The density of all the planets exactly proportional to their distances from the fun fhews that this is really the cafe. If the rays of the fun are only mediate causes of heat, if they are only agents capable of bringing it forth, the difference of their intenfity at the feveral diftances of the planets may be fully compenfated by the more immediate caufes, the quantity of calorific particles contained in thofe planets and the force of their adhesion. The universal and all-foreseeing providence of the Supreme Being extends itfelf with equal bounty to all and every part of the univerfe. Under his powerful hand, one fingle agent operates all the wonders of his beneficent intentions; and at fuch unequal distances the fame fun animates the foil and inhabitants of Saturn without fcorching thofe of Mercury. Light alfo is weakened by diftance. The compofition of the moon is fuch as to reflect to us a moft benign light during the night, but it is probably invifible in Saturn. The compofition of that planet's moons and ring must be exceedingly different; for they not only reflect ample light on that planet, but, notwithstanding their immenfe distance, are visible, and make it more wifible to us.

THOUGHTS

THOUGHTS

ON THE

STRUCTURE

OF THIS GLOBE.

LETTER VI.

Abstract of the System of Profeffor Wallerius on the Formation and Structure of the Earth.

BEFORE I venture to expose to you, Sir, my own ideas on the

original formation and structure of this globe, and on the changes which have reduced it to its present ftate, you will give me leave to introduce you to the knowledge of the opinions of Mr. Wallerius, who not many years fince published a regular system on thofe fubjects. That learned profeffor has been ftyled by the Swedes the father of mineralogy, as they had before given to his countryman Linnæus the honourable name of father of botany, fully confirmed by the univerfal fuffrage of all Europe. This philofopher

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