Page:Cyclopaedia, Chambers - Volume 1.djvu/554

 DES

[ i? Q ]

DES

OM/'fMDESCENSioN, isa Point, or Arch of the Equator, which defccnds at the fame Time with a Star, or Sign be- low the Horizon, in an oblique Sphere. See oblique Sphere.

Defcenfions, both Right, and Oblique, are accounted from the firft Point of yfrre.c, or the Vernal Interaction ; ac- cording to the Order of the Signs ; that is, from Weft to Eaft. And as they are unequal, when it happens that they anfwer to equal Arches of the Ecliptick, as e.gr. to the 12 Signs of the Zodiack, it follows, that fometimes a greater Part of theEquator rifes, or defiends with aSign, in which Cafe the Sign is faid to afcend, or defcend rightly : And fometimes again a lefs Part of the Equator rifes, or fets with the fame sTgn ■ in which Cafe it is faid to afcend, and defcend ob- liquely. See Ascension.

Refraliion of the Descension, iSc. See Refrac- tion.

DESCENSIONAL 'Difference, is the Difference be- tween the Right, and Oblique Defcenfimi of the fame Star, ££r. See Difference.

DESCENT, orDlSCENT, in Law, an Order, or Man- ner wherein Lands and Tenements are derived to any Man from his Anceftors.

Thus, to make his Defcent from his Anceftors, is to /hew how, and by what particular Degrees, the Land in Queftion came to him from his Anceftors.

Defcent is either Lineal, or Collateral.

Lineal Descent, is that convey'd down in a right Line from the Grand-Father to the Father, and from the Father to the Son, from the Son to the Grand-Son, iSc.

Collateral Descent, is that fpringing out of the Side of the Line, or Blood, as from a Man to his Brother, Ne- phew, or the like. See Collateral, and Degree.

If one dies feifed of Land, wherein another has Right to enter, and it defiends to hisHeir, rachDefcent fhall take away' the other's Right of Entry, and put him to his Action for Recovery thereof. Stat. 32. Hen. VIII. ;

Descent, in Genealogy, and Heraldry, the Order, or Succeffion of Defendants in aLir.e, or Family. See De- scendant.

One Defcent, two Defcents, &c. A Gentleman is of perfect Blood, who has four Defcents of Gentility, both by his Father, and Mother's Side, i. e. whofe Great-Grand- Father, Grand-Father, and Father, on both Sides, were all Gentlemen. Sec Gentleman.

Descent, is alfo ufed in Heraldry, to exprefs the coming down of any Thing from above.

Thus a Lyon en Defcent, is a Lion with his Head to one of the bafe Points, and his Heels towards one of the Corners of the Chief; as if he were leaping from fome high Place.

Descent, Fall, in Mechanicks, 2*c. is the Motion, or Tendency of a Body towards theCenter of theEarrh, either directly, or obliquely. See Center, and Motion.

It is to be here obferv'd, Firft, That a Body cannot de- fcend, unlefs it can divide, and feparate the Medium 5 which it cannot do, unlefs it be fpecifically heavier than the Medium. For lince Bodies cannot penetrate each other, one muff give Way 'ere the other can move : And again, Tho' '& Medium, e. gr. Water ae divifible, yet if it be fpeci- fically heavier than another, e.gr. Wood, it is only heavier as it contains more Particles of Matter in the fame Bulk ; all which have an Impulfe downwards, and confequently in Water there is a greater Impulfe than in the fame Bulk ofWood. See Specific Gravity.

Secondly, The Defcending Body lofes as much of its Weight, as is the Weight of the Medium, with the Force of its Cohefion. See Medium.

Thirdly, The lefs the Bulk of the Defcending Body is, the more of its Gravity does it lofe in the fame Medium. For tho' the Proportion of the fpecific Gravity of the one to the other be ftill the fame in a greater, or lefs Bulk, yer the lefs the Body, the more the Surface, in Proportion to the Mafs ; and the more the Surface, rhe more the Friction, or Refiftance. See Friction, and Resistance.

The Caufe of this Defcent, or Tendency downwards, has been greatly controverted. There are two oppofite Hypo- thefes advanced ; the one holding it to proceed from^an in- terna], and the other from an exrernal Principle. The firft maintained by the Peripateticks, &niFpicureans, ar\& New- tonians ; and the latter by the Cartefians, and Gajfendifls.

The Peripateticks teach, that heavy Bodies fall by cer- tain motive Powers, flowing from their Forms towards the Centre of the Earrh, as their proper Elemenr, or Place in theUniverfe. Thcfe Powers rhey call Impetus's ; and will have them to have been imprefs'd on the f'everal Parts of the Earth, at the Creation ; being a Tendency of the Parts to the whole. And they fuppofe, that it is by means hereof, that the whole is ftill preferved in its Integral State. Adding, that if there were a perfect Vacuity in any Part of the Air round us, a Stone placed therein would not at all fall, or tend towards the Earth, as being no longer a Part there- of, nor having the natural Impetus.

The ancient Epicureans, or Atomills fciA A, , J?eTO«»i«»PhiIofophers, hold, that all Bodies a modcrn tend downwards, and that the more ftrongly * ra, ? tate > or tain the more Matter ; confequently perfectly fol'H « j""" fuch as they fuppofe Atoms, or primirive CornnfM Bodl, es > themoft of all. See Gravity e ^ or PrfcIes to be,

The Cartefians afcribc the Caufe of Defcent of BnrV a circular Motion of the JEther, which they feign £' e V? veryPurpofe. According to them, all circular Motion S natural; and therefore is continually endeavouring to reced from its Center, and to change into a Recti-linear Motion confequently fuch as have the more Morion, recede the more' and in receding, deprefs, or thruft down fuch as have left Motion. And thus the Air being moved with the rao ft vehement Velocity, recedes more rhan folid Bodies, and bv thar means drives them towards the Center.

The Gaffendifts account for the Defcent of heavy Bodies by fuppoung certain magnetick Rays proceeding out of the Earth, and confifting of contiguous Particles, which by their continual Erniffion retain the Rigidity ofRods, or ftretched Cords. Thefe Rays being diftufed every way from the Earth, as a Center, ftrike, or imninge on what folid Bodies they meet withal in their Progrels, and penetrate rhe fame fome direftly, and others obliquely, and refractedly, after the Manner of Rays of Lighr. Thefe Rays, fay they, are a Kind of Arms; and the Defeflions, and Curvatures of their Extrcams, as it were Fingers, or Hooks, which pull or draw Bodies from above towards the Earth. And they add, that thefe Rays being the more rare, as they are more remote from the Center ; their Effect is the more feeble • and the Defcent of Bodies is the more flow, in Proportion as they are higher.

Laws of the Descent of "Bodies.

I. Heavy Bodies in an unrefifting Medium fall with, an uniformly accelerated Motion. See the Laws of uni- formly accelerated Motion under the Articles Accelera- tion, and Motion.

Hence, 1. The Spaces defcended are in duplicate Ratio of the Times, and Velocities; and increafe according to the un- even Numbers 1, 3, 5, 7,9, & c. 2. The Times, and Ve- locities, are in a fubduplicate Ratio of the Spaces. 3. The Velocities of defcending Bodies, are proportionate to the Times from the Beginning of their Falls. 4. The Spaces defcribed by a falling Body, are as the Squares of the Times from the Beginning of the Fall.

A Body fpecifically heavier defiends in a fluid Medium, fpecifically lighrer, {e.gr. rhe AirJ with a Force equal to the Excefs of the Weight of the Body above an equal Bulk of the Medium. For a Body only defcends in a Medium with the Force remaining, after a Part has been fpent in over- coming the Refinance of the Medium. And this Refinance is equal to the Weight of an equal Bulk of the Medium .- Coniequently the Body only faUs with the Excefs of its Weight above that of an equal Bulk of the Medium.

Hence, the Power that fuftains a Body in a fpecifical- ly lighter Medium, is equal to the Excefs of the ab- Iblute Weight of the Body above an equal Bulk of the Medium. Thus 47-1 Pound of Copper in Water lofe 5* of their Weight. A Power, therefore, of 42 Pounds is able to fuftain them.

In fuppofmg heavy Bodies to defcend in an unrefifting Me- dium, we conceive them free of all external Impediments, from what Caufe foever. So that we even fet a-fide that ob- lique Imj ulfe given to Bodies while in falling, by the Rota- tion of the Earrh, tho' this produce no fenfible Irregularity in a little Diftance.

Galliuus, who firft difcover'd the Law of the Defcent of heavy Bodies byReafoning, afterwards confirm'd the fame by Experiments; the Refult of which, repeated an hundred times over, was, that theDefceut was as the Square of the Times.

Grimaldo and Ricciolus made Experiments to the fame Effect, tho' in a different Manner, by letting fall aNumber of Balls, weighing 8 Ounces a Piece, from the Tops of fe- veral Towers, and meafuring the Times of falling by afPea-> dulum. Thefe Experiments are exhibited in the following Table.

Vibrations of

Time.

Space at the

Space defcen-

the 'Pendu-

End of the

ded each

lum.

Time.

Time.

" it

Rom. Foot.

Rom.Eoot. |

5

5°

10

10

10

1

40

40

30

15

2

3°

9°

5°

20

3

20

I6C1

7°

25

4

10

250

90

1 *

1

15

15

! 12

2

(To

45

18

?

13s

75

24

4

240

i°J ...

Since