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to affect other bodies, or other bodies to affect it; whereas body is never affected but by body.s

This appetite of ascent and descent is either in the structure of the body moved, or in a sympathy or consent with’some other body. But if any dense and solid body can be found, which does not move to the centre of the earth, this received difference will be falsified. And if the opinion of Gilbert be admitted, that the magñetic 'virtue of the earth, whereby it attracts heavy bodies, extends not beyond its own sphere of activity, which always operates to a certain distance, and no farther; and this be verified by any instance, such an instance will be an in

stance of alliance upon this subject; but at present there is no certain and manifest instance thereof*.

What seems to come nearest to it, is the cataracts or spouts which are met with in sailing through the Atlantic Ocean towards either the East or West Indies; for the quantity and bulk

powers should be inadvertently introduced into matter. And let it be well considered and remembered, that what is mathematically just and true, may be physically false and absurd. 70 * Sir Isaac Newton's doctrine and discoveries upon this bead are now generally known. But, perhaps, the physical

cause of gravity still remains unassigned; so far, we mean, Las it may be. Its WIE ER IST EN 1E2!T.Tim

of water suddenly discharged by these cataracts appears so great, that they seem to be collections of water made before, and to have rested and remained in these places, and afterwards to be thrown down by some violent cause, rather than to fall by the natural motion of gravity. Whence we may conjecture, that a dense and compact body of a large bulk may remain at a great distance from the earth, pendulous, like the globe of the earth itself, without falling, till it be vior lently precipitated or thrown down*, But with regard hereto we affirm nothing for certain.

Only in this and numerous other cases it may easily appear how deficient we are in natural history, when, instead of verified and assured instances, we are often obliged to bring bare suppositions in the way of examples t.

Again; let the nature sought be the reasoning faculty. A just distinction here seems made betwixt the human reason and the sagacity of brutes; yet there are some instances of actions which brutes perform, whereby they also seem to reason, Thus it is reported of a raven, that in a time of great drought, espying water in the hollow trunk of a tree, where the orifice was too small for her to enter, she continued to

See the account of spouts in the Philosophical Transactions.

+ See above, Part II. Aph. 14.

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drop small stones therein, till the water rose high enough for her to drink. Whence the reason of the raven afterwards became proverbial.

Lastly, let the nature sought be vision. Here it seems to be an extremely just and exact distinction betwixt light and colour, that light is an original visible thing, affording the primary means of sight, and that colour is a secondary visible thing, not to be seen without light, whence it

may seem no more than the image or modification of light, and yet there appear to be instances of alliance on both sides. Thus, for example, in large quantities of snow, there seems to be a somewhat original, lucid colour*; and in the flame of sulphur, a light tending to colourt.

36. In the fourteenth place come those wé entitle Crucial Instances, deriving the word from the crosses set up where two roads meet, to point and mark out their separation again. We otherwise call them Decisive, and Indicatory Inė" stances; and, in some cases, Oraculous and Commanding Instances.

For snow affords a considerable degree of light, by means whereof men travel by night in the northern regions. See Mr. Boyle's History of Cold, passim.

# Viz. Blueness.

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They are of this kind, that when in the search of any nature, the understanding comes to an equilibrium, as it were, or stands suspended -as to which of two or more natures the cause of the nature enquired after should be attributed, or assigned, by reason of the frequent and common concurrence of several natures, then these Crus cial instances shew the true and inviolable association of one of these natures to the nature sought, and the uncertain and separable alliance of the other, whereby the question is decided, the former nature admitted for the cause, and the other rejected.

These instances therefore afford great light, and have a kind of over-ruling authority, so that the course of interpretation will sometimes terminate in them, or be finished by them." Some times, indeed, these Crucial Instances occur, or are found, among those already set down, but in general they are new, and expressly and purposely sought and applied, or after due time and en deavours, discovered, not without great diligence and sagacity.'

For example, let the nature sought be the tide of the sea, which happens twice in the day, and is six hours. in coming in, and six in going out, with a certain difference coinciding with the motion of the moon. Now, the cross-way of this subject lies as follows.

This reciprocal motion must of necessity happen either, 1. from the waters going forward and backward, like water moved in a bason, which, when it rises on one side, forsakes the other; or, 2. from the rising and falling down of the waters, like water that rises in boiling, and again subsides; but to which of these causes the ebbing and flowing of the sea should be assigned, is the doubt. If the former assertion be admitted, when the sea flows on one shore, it must necessarily ebb, about the same time, somewhere on the opposite shore; the enquiry therefore is thus brought to a point.

Now Acosta, and some others, have found, by diligent observations, that on the coast of Florida, and the coasts of Spain and Africa, the sea flows and ebbs at the same times; not contrari. wise, that when it flows on the coast of Florida, it ebbs on the coasts of Spain and Africa ; and yet, when carefully considered, the rising mon, tion is not proved by this, and the progressive motion disproved; for it is possible that the waters may have a progressive motion, and yet overflow the opposite shores of the same channel at the same time; that is, if the waters be protruded and driven": from another quarter, which is the case of rivers ebbing and flowing on both shores at the same hours, though the mo

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