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comes from them, when pounded, should hinder my sight: I found that the long sharp figures which might be likened to a weaver's shuttle, were now changed into figures, whose basis was oblong, rising up pyramidally, like a pointed diamond, as P. Others had their basis square, as Q. Others an irregular quadrilateral, as R. But R these last two figures, I supposed, were accidental, for want of sufficient matter to complete, and perfect them on all sides. The number of particles was so great, that in a gross computation I judged them to be 6000 in a drop about the size of two barley-corns.

I took some vinegar out of a glass, that had crabs' eyes in it, at a time before all the air-bubbles were ascended: but even then the basis of the salt particles was four-square, and not as in common vinegar. The liquor had quite lost its acidity. I took also white chalk, beaten to pieces, and put

it in vinegar, where it caused as great a commotion and rising of air-bubbles as the crabs' eyes had done : it produced also the same-figures of the salt, and the same insipidness.

Abstract of a Letter from Mr. LEUWENHOEK 'to the R. S., dated

Jan. 23. 1684-5, concerning the various Figures of the Salts contained in several Substances. — [1685.]

I took some of the salt of Carduus Benedictus, such as is commonly sold in the apothecaries' shops : it was rather moist, and its parts seemed to be so huddled up together, that they could not be distinguished from one another : I closed it up in a glass, to prevent the evaporation ; and when it had stood stopped for some days, many of the particles were run together, on the side of the glass, forming some flat longish figures, of different magnitudes, the largest in length about the diameter of a

G hair of my beard, as fig. É.

In another place, these salts lay so, that I could easily

discern their thickness, as fig. G. another place, the thin flat salts lay over one another, as fig. H. I put this salt

I

K

M

In

in water to dissolve it, and took of it about as much as two barley-corns, spreading it thin before me: and when it was in motion, I not only observed the above-mentioned figures, and shootings of the salt, but found several new figures which were thin and long, and sharp towards both ends, as fig. I. Others that lay near them were broader, but not so long, and their ends not so sharp, as fig. K. I saw also some perfect four-square figures, as fig. L; but they had no thickness that I could discover. Also there were some quadrilateral pyramidal figures, like those of common salt, as fig. M. These observations must be made before the water is evaporated, for when the water is almost gone, such a multitude of particles appear, and run together, that they cause a confusion. On a further examination, of a more genuine sort, I perceived very plainly a number of figures tapering towards both ends, as above mentioned in fig. I. After about a day's time, I saw several flat figures, as F and H. But having dissolved the salt in rain water, and viewed it as it lay thin upon my plate, I found all the above-mentioned figures; but those of K, L, and M, exceeded in number all the rest; insomuch, that I conceived I saw more in a quantity of water equal to the weight of a grain, than there are stars to be seen in the heavens by the naked

eye. Sal Ammoniac. -The figure of this salt dissolved in water generally appeared like the boughs of a tree, beset with irregular leaves, one larger than another, as is represented A E. In another place lay five or six branches like A, seeming to proceed from a common centre, as E. I saw also salt particles like B and C; and where there were no branches, the scattered salts looked like so many flints, differing from each other in size, but being never perfectly round, as fig. D.

A Discourse concerning Gravity and its Properties, and on the Laws of the Descent of heavy Bodies. By E. HALLEY

NATURE, amidst the great variety of problems wherewith she exercises the wits of philosophical men, scarcely affords any one wherein the effect is more visible, and the cause more concealed, than in those of the phænomena of gravity or weight. Before we can go alone, we must learn to defend ourselves from the violence of its impulse, by not trusting the centre of gravity of our bodies beyond our reach; and yet the acutest philosophers, and the subtilest enquirers into the original of this motion, have been so far from satisfying their readers, that they themselves seem little to have understood the consequences of their own hypotheses.

The notion of Descartes seems to be quite incomprehensible; he would have the particles of his celestial matter, by being reflected from the surface of the earth, and so ascending from it, to drive down into their places those terrestrial bodies they find above them : this is as near as I can gather the

scope of the 20, 21, 22, and 23 sections of the last book of his Principia Philosophiæ; yet neither he, nor any of his followers, can show how a body suspended in free ether shall be carried downwards by a continual impulse tending upwards.

Vossius and others assert the cause of the descent of heavy bodies to be the diurnal rotation of the earth upon its axis ; not considering that, according to the doctrine of motion, all bodies moved in a circle recede from the centre of their motion; by which an effect contrary to gravity would follow, and all loose bodies would be thrown into the air in a tangent to the parallel of latitude without the intervention of some other principle to keep them fast, such as that of gravity. Besides, the effect of this principle is found throughout the whole surface of the globe nearly equal; and certain experiments seem to argue it rather less near the equator than towards the poles ; which could not be the case, if the diurnal rotation of the earth on its axis were the cause of gravity; for where the motion is swiftest, there the effect would be most considerable.

Others assign the pressure of the atmosphere, as the cause of this tendency towards the centre of the earth ; but unhappily they have mistaken the effect for the cause, it being plain from undoubted principles, that the atmosphere has no other pressure but what it derives from its gravity ; and that the weight of the upper parts of the air, pressing on the lower, do so far bend the springs of that elastic body, as to give it a force equal to the weight that compressed it, having of itself no force at all: and supposing it had, it will be very hard to explain the modus, how that pressure should occasion the descent of a body circumscribed by it, and pressed equally above and below, without some other force to draw or push it downwards. But to demonstrate the con

trary of this opinion, an experiment was long since shown before the Royal Society, by which it appeared, that the atmosphere was so far from being the cause of gravity, that its effects are much more vigorous where the pressure of the atmosphere is removed; for a long glass receiver, having a light down-feather included, being evacuated of air, the feather, which in the air would hardly sink, did in vacuo descend with nearly the same velocity as a stone.

Some think to illustrate this descent of heavy bodies, by comparing it with the virtue of the loadstone. But, setting aside the difference in the manner of their attractions, the loadstone attracting only in and about its poles, but the earth almost equally in all parts of its surface, this comparison avails no more than to explain unknown things by another equally so.

Others assign as the cause, a certain sympathetical attraction between the earth and its parts; whereby they have, as it were, a desire to be united. But this is so far from explaining the modus, that it is little more than telling us in other terms, that heavy bodies descend, because they descend.

But though the efficient cause of gravity be so obscure, yet its final cause is clear enough ; for it is by this single principle, that the earth and all the celestial bodies are kept from dissolution ; the least of their particles not being suffered to recede far from their surfaces, without being immediately brought down again by virtue of this natural tendency; which, for their preservation, the infinite wisdom of their Creator has ordained to be towards each of their centres ; nor can the globes of the sun and planets be otherwise destroyed, than by depriving them of this power of keeping their parts united.

The affections or properties of gravity, and its manner of acting on falling bodies, have been in a great measure discovered, and most of them made out by mathematical demonstration, by the accurate diligence of Galileo, Torricelli, Huygens, and others, and now lately by our worthy countryman Mr. Isaac Newton, who has an incomparable Treatise on Motion almost ready for the press. Of these properties, the first is, that by this principle of gravitation all bodies descend towards a point, which either is, or else is very near to the centre of magnitude of the earth and sea, about which the sea forms itself exactly into a spherical surface, and the prominences of the land, considering the bulk of the whole, differ but insensibly from it. 2. This point, or centre of gravitation, is fixed within the earth, or at least has been so, ever since we have any authentic history : for a consequence of its change, though never so little, would be the overflowing of the low lands on that side of the globe towards which it approached, and the leaving new islands bare on the opposite side, from which it receded. 3. That in all parts of the surface of the earth, or rather in all points equidistant from its centre, the force of gravity is nearly equal ; so that the length of the pendulum, vibrating seconds of time, is found in all parts of the world to be nearly the same, 4. That gravity equally affects all bodies, without regard either to their matter, bulk, or figure; so that the resistance of the medium being removed, the most compact and the loosest, the greatest and smallest bodies, would descend the same spaces in equal times; the truth of which appears from the experiment before cited. In these last two particulars is shown the great difference between gravity and magnetism, the one affecting iron only, and that towards its poles, the other all bodies alike in every part.

From hence it will follow, as a corollary, that there is no such thing as positive levity; those things that appear light, being only comparatively so; and whereas several things rise and

float in fluids, it is because, bulk for bulk, they are not so heavy as those fluids; nor is there any reason why cork, for instance, should be said to be light because it swims on water, any more than iron because it swims on mercury. 5. That this power

increases as you descend to, and decreases as we ascend from, the centre, and that in the proportion of the squares of the distances from it reciprocally, so as at a double distance to have but a quarter of the force ; a principle on which Mr. Newton has made out all the phænomena of the celestial motions, in so easy and natural a manner, that its truth is past dispute.

Besides, it is highly rational, that the attractive or gravitating power should exert itself more vigorously in a small sphere, and weaker in a greater, in proportion as it is contracted or expanded; and if so, seeing that the surfaces of spheres are as the squares of their radii

, this power at several distances will be as the squares of those distances reciprocally; and then its whole action on each spherical surface, be it great or small, will be always equal. And this is evidently the rule of gravitation towards the centres of the Sun, Jupiter, Saturn, and the Earth; and thence is reasonably inferred, to be the general principle observed by nature in all the other celestial bodies.

These are the principal affections of gravity, from which the rules for the fall of bodies, and the motion of projects, are mathematically deducible. Mr. Isaac Newton has shown

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