mence and rapidity; and that the determinations of the insensible corpuscles thus agitated be also very various; and that likewise the variously agitated particles be so small, as generally speaking to be singly insensible: for unless they be exceeding fine, they cannot penetrate readily into the pores of contiguous bodies, and so warm or burn them.

The operation of heat upon our senses, the result of which we commonly call heat, is usually estimated by its relation to the organs of our feeling. If the motion of the small parts be more languid in the object, than it is in the sentient, we pronounce the body to be cold; but if it be more violent in the object than in the sentient, we say the body is hot.

The intenseness of heat, as of light, always is as the density of the rays, or particles of fire, that occasion it; and this density is as the distance from the radiating point reciprocally.

Dr. Slare has published surprising experiments of producing fire and flame from the bare mixture of two liquors actually cold; a vegetable oil, and a compound spirit of nitre..

The incredible force of burning glasses. A burning concave, made at Lusace in Germany, near three Leipsic ells in diameter, and of a copperplate, scarce twice as thick as the back of a common knife, makes wood in the focus (which is two ells off) to flame in a moment; and water in an earthen pot boil immediately:' tin three inches thick, to be melted quite through in three minutes; a plate of iron to be presently hot, and very quickly perforated: it will run in five or six minutes; tiles, slates, and earthern pots, melt in a little time,

and run into glass; a clod of earth turns into a greenish glass.

Mr. Tschirnhaus makes convex burning glasses of three or four feet diameter, the focus at the distance of twelve feet diameter; which in a moment vitrify tiles, slates, pumice stones and earthen vessels; melt all resinous things under water; melt all metals in a moment, and gold itself is turned into glass of a purple colour: of such efficacy are the rays, when stripped of an unctious matter, with which we may suppose them generally clothed.

"The ancient Persians were the worshippers of fire: but I abhor their fire places. The Indians of my country, while unchristianized, concluded from the strange effects of fire, it must be a god. I will adore the glorious God who made the fire. Great God, I bless thee for the benefits, which thy creatures, and I among them, receive by the fire, which is brought from heaven to us. May my zeal for thy service be always kept boiling in the heat proper for it.

"Since fire is thus irresistible, and heat so insupportable, surely I should beware of that impiety, which will expose me to the vengeance of God. Who can dwell with such a devouring fire, such everlasting burnings? My God, be not thou unto me a consuming fire. My God, who can abide the heat of thine anger!" I have seen a book of devotion, entitled, a Disciple warming himself, and owning his Lord. It is there actually evident, and performed, that this one object, the fire on the hearth, will afford volumes of profitable contemplations.

ESSAY XI. Of the Moon.

WE are now coming down to our terraqueous globe. The Moon, a sort of satellite to this globe, salutes us in our way. Paying a homage to none but her glorious Maker, we will now behold her walking in her brightness.

What shall we think of the protuberant parts observed on that celestial body? What of the round hollows, like pits or wells of several magnitudes, which have been formerly mistaken for mountains? The periodical revolution of the Moon, in reference to the fixed stars, according to Mr. Flamstead, is 27 days, 7 hours, 43 minutes, 7 seconds.

In the same space, with a strange correspondence of the two motions, it revolves the same way about its own axis; by which the same side is always exposed to our sight. But because in the space of a periodical month, the Earth is also with this her satellite, moved on almost an entire sign, the Moon cannot come to a new conjunction with the Sun, but wants 2 days and 5 hours of it; which must be passed before the entire lunation will be over, and before the Moon has exhibited all her phases. These 2 days and 5 hours, added to the periodical month, make the synodical one; which is 29 days, 12 hours and 45 minutes.

Those librations of the Moon's body, which occasion that the hemisphere exposed to our sight is not always exactly and precisely the same, arise from the eccentricity of the Moon's orbit, and from the perturbations it suffers by the Sun's attraction, and from the obliquity of the axis of the diurnal rotation of the Moon's own orbit. Without the knowledge of these things, the phenomena of the

Moon would be inexplicable: but upon the consideration of these, they are very demonstrable.

It is very sure, that although it be almost the same face which the Moon turns to the Earth, yet it is not entirely so. There is a libratory motion, whence it comes to pass, that sometimes the more eastern and western parts of it, sometimes the more northern and southern appear alternately.

According to Sir Isaac Newton, the mean distance of the Moon from the Earth, is about 60 semidiameters of the Earth; or about 24,000 English miles. The mean diameter of the Moon is 32 minutes, 12 seconds; as the Sun's is 31 minutes, 27 seconds. The density of the Moon, to that of the Earth, he concludes to be nearly as 9 to 5. And the mass of matter in the Moon, to that of the Earth, to be nearly as 1 to 26.

The Moon hath properly no atmosphere, such as belongs to our earth, of clouds, winds, thunders; her face is always clear, and by our telescopes we can see the Sun's light pass regularly and uniformly, from one mountainous place to another.

The light of the Moon reflected on us, is of such a weakness, that even in the full moon, it can be brought by no burning glass to afford the least degree of heat. The rays have their force decreased, at least as the square of their distance. The force of the sun's rays reflected to us from the Moon, to those that come to us directly, is decreased, at least in proportion of the square of the Moon's distance from the earth, to the square of the Moon's semidiameter. And by calculation it will be found, that the light of the Moon brought hither, will be in force but the fifty thousandth

part of what comes hither directly from the

sun.

Dr. Hook finds, that the quantity of light which falls upon the hemisphere of the full Moon is rarefied into a sphere about 288 greater in diameter than the Moon, before it arrive to us. Consequently, the moon's light is 104,368 times weaker than the sun's; and it would require 104,368 full moons to give a light equal to that of the

sun at noon.

There is a secondary light of the Moon; that is to say, the obscure part of the Moon appears like to kindled ashes, just before and after the change. This is the sun's rays reflected from the bright hemisphere of the earth, to the dark parts of the Moon; and thence again reflected to the earth, destitute of the light of the sun. This is by Tacquet and Zucchius more largely discoursed on. When the Moon is at change to us, the earth is at full to the moon; and the light of the earth is about fifteen times greater than that of the Moon. The Moon also being so little, as not to obscure above a twentieth part of the earth, it may be supposed that the light from the earth may render her a little visible to us even in solar eclipses.

The Moon is almost one semidiameter of the earth nearer to us, when she is in the meridian, than when she is nigh the horizon. But why doth she then appear bigger to our sight when she is nigh the horizon, than when she is in the meridian? Dr. Wallis agrees with Des Cartes in the solution: the horizontal Moon is capable of being compared with many intervening objects, hills, trees, and the like; but the meridian Moon hath nothing to be compared with.

Though the Moon, as well as the earth, and