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air; (3.) and principally, that it is very unequal, approaching one while increased in strength, and afterwards receding decreased, which is a thing of capital use to the generation of bodies, for it was justly observed by Aristotle, that a chief cause of the generations and corruptions upon the surface of the earth, is the oblique motion of the sun through the zodiac; whence the heat of the sun, partly by the alternate changes of day and night, and partly by the succession of summer and winter, becomes wonderfully unequal. And yet this philosopher immediately after, corrupts and spoils his own just position, for rashly presuming to judge of nature, as his manner is, he very magisterially assigns the cause of generation to the approach of the sun, and the cause of corruption to its retiring; whereas both the access and recess of the sun give occasion to generation, as well as to the corruption of things ; not respectively, but as it were indifferently ; for inequality of heat administers to their generation and corruption; but equality of heat to their conservation only. *.

There is a fourth difference, of very great moment, between the heat of the sun and of fire;

See the Sylva Sylvarum, under the articles Preserva. tion and Putrefaction.

viz, that the sun insinuates its operation for great lengths of time; whereas the operations of the fire, through the impatience of mankind, are hurried to a conclusion, in short intervals. But if any one were intent upon tempering the heat of fire, and reducing it to a moderate and gentle degree, which is easily practicable several ways, and would sometimes sprinkle and intermix a little moisture; and particularly if he would imitate the heat of the sun in point of inequality, and wait with patience somewhat longer than men usually do in chemical processes, he might get quite clear of that false notion of the heterogeneity of heat, and easily imitate, rival, or in some cases exceed the operations of the sun by the means of culinary fire*.

We have a like instance of alliance in butterAlies benumbed, and as it were become dead with cold; for these creatures are re-animated, or brought again to life, by means of a small warmth of fire; whence it easily appears, that fire can as well vivify animals, as ripen vegetables. Thus in the famous invention of Fracastorius, the metalline pan, strongly heated and applied near the head of a person in a dangerous fit of the apoplexy, expands the animal spirits,

* Here is a foundation laid for a kind of chemistry that seems to be very little known or practised.



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compressed and, as it were, suffocated by the humours, and obstructions of the brain, and thus excites them to motion, in the same manner as fire operates upon water or air, and at the same time consequently expands and quickens them.

Sometimes also eggs are hatched by the heat of a fire, which in this respect perfectly resem. bles animal heat. These instances therefore, with numerous instances of the like kind, render. it unquestionable, that the heat of fire may in many cases be modified, so as to resemble and imitate the celestial and animal heats,

Again; let the nature sought be motion and rest. Here it seems a settled division drawn from the depth of philosophy, that natural bodies either revolve, move in a straight line, or continue at rest; because motion is either without end, proceeding to an end, or stationary in the end. Now constant rotation seems proper to the heavenly bodies; station or rest to the terrestrial globe; and the other bodies called. heavy and light, being out of their natural places, are carried strait upwards or downwards to the masses or congregations of similar bodies; those that are light, towards the heavens, and those that are heavy towards the earth: and all this appears neat and plausible in discourse.

But we have an instance of alliance in some of the lower comets, which, though they de

scend below the celestial bodies, yet move irregularly through the various quarters of the heavens, as appears by experience and observation.

Another instance of alliance relating to this subject is the motion of the air, which between the tropics, where the circles of rotation are larger, seems itself to revolve from east to west.

The flux and reflux of the sea might be another instance of alliance, if the sea was observed to revolve, though but slowly and faintly, from east to west; yet so as to be driven back twice a day. Upon these suppositions, therefore, it is manifest, that this motion of rotation does not terminate in the celestial bodies, but is communicated also to the air and ocean*.

Again; that property of ascending upwards, found in light bodies, labours under a defect; and to this purpose an instance of alliance may be taken from a bubble of water, for if air be thrust under water, it hastily ascends to the surface, by the motion of impulsé, as Democritus calls it, wherewith the descending water impels and raises the air upwards, and not by the striving or endeavour of the air itself, but

See the Essay upon the ebbing and flowing of the sea.

when it comes to the surface of the water,

the air is kept from ascending farther, by a small resistance it meets with in the water* which will not presently yield to be discontinued or sepa, rated, so that the appetite of the air to rise upwards is exceeding weakt.

In like manner, let the nature sought be gravity. It is a received difference, that dense and solid bodies move towards the centre of the earth, but rare and light ones towards the heavens, as if, in each case, it were to their

proper places. But as to these places, though the notion of them prevails in the schools, yet it is perfectly idle and childish to imagine that place can have any effect. Whence it is trifling in philosophers to assert, that if the earth was perforated, heavy bodies, let fall in the perforation, would stop at the centre; for, in that case, a kind of nothing, or a mere mathematical point, would have a virtue and efficacy ; so as either

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The wates now throwing itself into a thin film, or spherical bubble, to avoid a solution of continuity.

Or, according to the late discoveries, none at all. See Mr. Boyle's Works, and Sir Isaac Newton's Principia, passim.

This is the general case of mathematical demonstrations applied to physics, where mere mental powers or forces are supposed, instead of those that really exist in nature. Hence, great caution is required in the reading of mathematical writers upon physical subjects, "lest ideal

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