Page:Cyclopaedia, Chambers - Volume 2.djvu/388

 PER

and large Distances, where a great deal of Expedition, and not over much Accuracy is required. 5 Tis evident the driv- ing it along, and obferving the Hands ; has the fame Effect as dragging the Chain, and taking account of the Chains and Links. See therefore the Article Chain.

PERCEPTION, in Phiiofophy, a iimple Comprehension ; tir that Iimple idea of a Thing, which we conceive without making any Affirmation or Negation. See Idea.

If that Idea exhibit any Image to the Mind, it is call'd Imagination j if it exhibit none, it retains the general Name of (perception. See Imagination.

Thus when we hear the Word Tree; the Idea we then form in the Mind, is call'd an Imagination. But when we hear of a Thing, whereof no Image can be framed as of doubting^ the Idea we then have is a mere 'Perception. Sec Sensa- tion.

It may be obferv'd, that the Idea's we receive by Perce- ption, are often alter'd by the Judgement, without our taking notice of it; thus a Globe being fet before our Eyes, the Idea thereby imprinted, is a flat Circle, varioufly ihadow'd : But being accuitom'd to perceive what kind of Appearances convex Bodies are wont to make in us; the Judgement alters the Appearances into their Caufes; and from that Variety of Shadow or Colour iiames to itfelt the Perception of a convex Figure of one uniform Colour. See Judgement.

This in many Cafes, by a fettled Habit, is perform 'd fo rea- dily, that we take that for the 'perception of our Senfation, which is but an Idea torm'd by the Judgement ; fo that one ferves only to excite the other, and is fcarce taken notice of it- felf : as a Man who reads or hears with Attention, takes little notice of the Characters or Sounds; but of the Idea's which are excited in him thereby.

The Faculty of Perception, feems to be that which puts the Ditli nation between the animate and inanimate Parts of the Creation. Vegetables, fome of them, have fome degree of Morion, and upon different Application of other Bodies, alter their Figures and Motions ; and have hence obtained the Name of fenjitive Plants: Which, however, is the Refult of mere Mechanifm, and no otherwife produced, than the Ihortening of a Rope by the Amhion of Water. But per- ception is a Metaphyseal Principle, and found in fome De- gree in all Animals ; and in them alone.

PER Minima, in Pharmacy, an intimate and perfect Mixture of natural Bodies ; wherein their very Minima, i. e. their Atums, or firft component Particles are fuppofed to be accurately blended together. See Mixtion.

If Silver and Lead be melted together, they will mingle together per minima.

PER-SE, in Chymiflry. When a Body is diflill'd fingly, and without the ufual addition of any other Matter to raife it; it is faidto be dipWd Per-fe. See Distillation.

The genuine Spirits of Harts-horn, are thofe rais'd Per-fe, in oppontion to thofe diflill'd by the addition of Chalk.

Per-se, in Logic. A Thing is faid to be confidered Per-fe, when it is taken in the Abflracf. See Abstract.

PERCH, Pole or Rod, a long Meafure, much ufed in furveying and meafuring of Land. See Measure,

Among the old Romans, and ftill among Geometricians, the Pertica, Perch, is ten foot ; and they otherwife call it the Catena, funis, and P)ecempeda. See Decempeda.

In England, the Statute Perch contains 16" Foot and 4> and for Coppice-woods, £S?c. 18 Foot: 40 fquare Perches make a Rood, and 160 an Acre. See Rood and Acre.

The Cujlomary perch is various in various Counties : In Staffordshire 'tis 24 Foot; in the Foreft of Sherwood ai, the Foot there being 8 Inches, the Meafure whereof was mark'd on the Chancel Wall of Edonflow, and in the Church of St. Mary in Nottingham. See foot.

In Herefordpire, a Perch cf walling is 16 ' Foot ; a Perch ofDitching 21 Foot, &c.

In France, the perch is from 18 to 23, and even 27 of their Feet.

PERCHANT, among Fowlers a Decoy-bird, which the Fowler has faften'd by the Foot, and which flutters about the Place where 'tis tied, to draw other Birds to it, and give the Fowler an Occafion of catching 'em.

PERCOLATION, in Medicine, &c. the Action of Fil- trating. See Filtration.

PERCUSSION, inPhyficks, the Impreffion a Body makes in falling or finking upon another ; or the Shock or Colliflon of two Bodies, which concurring, alter each others Motion. See Motion and Collision.

'Percufficn is either director oblique.

tDiretl Perctifjion, is where the Impulfe is given in the Pire&ion of a right Line perpendicular to the point of Con- tact.

In Spheres, therefore, the Percuffion is diretl, when the Line of Direction paffes thro' both their Centres. See Di- rect.

Oblique percuffwn, is where the Impulfe is given in the Direction of a Line Oblique to the Point of Contact. See Oblique.

C783)

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Doarim of Percussion, or the Laws of Motion, re- Jutting from the Percussion of Bodies.

■!? ?°n ie E, C n her F^fy hard > or peAaiy foft, and ft voidofall Elafticity; the Laws ot ' -Percuffion are eafily de- te ™ n i :. but &x f> oven the hardeft Bodies have their Thare of Elafticity; and in Elaftic Bodies, the Laws are very dif- ferent and much more intricate ; having been firft afcertain'd ' n / h e Philofoph. Tranf aa- by Sir Ch. Wren, Dr. fpUBs, and Mr. Huygens; we fhall lay down eacha-parr.

Laws of Percussion in Bodies not Elaftic.

I. If a Body in Motion as 4(Tab. Mechanicks Fig.40.,) ftrike directly againft another at reft B : the firft will lofe juft as much of its Morion as it communicates to the id; fo that the two will proceed thence with an equal Velocity, as if collected into one Mafs.

If ^therefore be triple of 3, it will lofe one fourth of its Motion; fo, that if betore, it moved thro' a Line of 24 Foot in a Minute, it will now only move 18.

II. If a moving Body, A, ftrike againft another already in Motion, 3; the firft will increafe the Velocity of the latter; but will lole lefs of its own Motion than if the latter had b-en at reft : Since all here requir'd, is that fome Degrees of Mo- tion be added to thofe it already has, to make 'em both pro- ceed with an equal Velocity.

Suppofe E. gr. the Body A with 12 Degrees of Motion, to ftrike againft the other B, lefs by half, and at reft: The firft will transfer 4 Degrees of its Motion to the latter, and re- tain S to itfelf : But if it ftrike with 12 Degrees of Motion oil the other already moving with three Degrees, ir will com- municate two Degrees ; for A being double of 2, this need only half the Motion to make it proceed with the fame Velocity.

III. If a moving Body A, ftrike on another 3, either at reft* or moving more (lowly, and either in the fame Direction, or in a contrary onej the Sum of the Momenta, if the Bodies move in the fame Direction ; or their Difference, if they move in a contrary one, will be the fame after the 'percujfon as before.

IV. If two equal Bodies Ami B meet each other with, equal Velocities; after the Congrefs, they well both remain at reft.

V. If a Body A, ftrike directly on another at reft B, its Celerity after the Stroke, is to its Celerity before it ; as the Weight A is to the Sum of the Weights A and B : if there- fore the Weights were equal, the Celerity after the Shock will be half of that before it.

VI. If a Body in Motion, A ftrike directly on another moving more ilowly, but in the fame Direction ; the Velocity after the Shock, will be equal to the Sum of the Momentaj divided by the Sum of the Weights.

VII. If two equal Bodies moving with different Velocities* ftrike directly againft each other ; after the Conflict, they will proceed with the Semi-difference of the Velocities* wherewith they were moved before it.

VIII. If two Bodies A and B meet directly with Veloci- ties that are reciprocally as their Weights ; after the Conflict, they will both remain at reft.

IX. If two Bodies A and B meet directly with the fame Velocity, the Celerity after the Impulfe will be to that be- fore it, as the Difference of rhe Weights to their Sum.

X. If two Bodies meet directly with any Velocity what- ever, the Celerity after the Stroak will be equal to the Semi- difference of the Momenta, divided by the Sum of the" Weights.

To determine the Momentum loft by the Conflict : multi- ply the Celerity which the Body had before the Conflict, into its Mafs : Thus have you the Momentum before the Conflict. In like manner, multiply the Celerity after the Conflict into the Mafs : Thus have you the Momentum after the Conflict. The latter Momenr therefore being fubtractcd from the for- mer Leaves the Lofs. Hence may the Magnitudes of the Strokes be eftimated.

XL A direct or perpendicular Stroke is to an Oblique one ; as the whole Sine is to the Sine of the Angle of Inci- dence.

Laws of Percussion in Elaftic Bodies.

In Bodies perfectly Elaftic, the Force of Elafticity is equal to the force wherewith they ate comprefs'd, i. e. the Colliibn. of two fuch Bodies on each other, is equivalent to the Motion which either of 'em wou'd acquire, or lofe, by mere iimple Impulfe. This Force exerting itfelf contrary ways ;_a Motion equivalent thereto muft be fubtracted from the Motion in the impelling Body, and added to that in the Body impell'd by mere Impulfe, to find their Velocities after 'Percuffion. See Elasticity.

XII. It