Page:Cyclopaedia, Chambers - Volume 2.djvu/333

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ORVIETAN, an Antidote, or Counter-poifon ; fo call'd, becaufe invented and propagated by an Operator from O- vleta in Italy 5 who made Experiments thereof in his own Perfon, on the public Stage, after taking feveral Dofes of Poifons. See Antidote and Poison.

In Cbarras's Pharmacopeia is a Method of making Orvietan 5 where it appears that Treacle is one of the principal Ingre- dients. SeeTHERiACA.

OS, in Anatomy, fee Bone.

Os Pubis, Os Sacrum,

Os Jfchium, Os HyoideSy

Os Femoris, &c. ■

-PUBIS. kSiCRTJM.

^Ischium. JHyoides. .Femur, &c.

OSCHEOCELE, in Medicine, a kind of Hernia, where- in the Inteftines defcend into thcScrotum. See Hernia.

The Word isform'dfrom the Greek o Scrotum, and joU;<, Tumor. ' m

OSCHOFHORIA, in Antiquity, Feafts mftituted by Tbejeus, in acknowledgment for his having deftroy'd the Minotaur, and by that means freed his Country, 4tbens t itom the Tribute of feven young Men, which were to be fent every Yearinto Crete, to be devoured by thatMonfter. See Minotaur. .,

Some fay the Ofchofboria were mftituted in honour ot Minerva ^nd Bacchus, who had afftfted Tbefeus in his Enter- prize. Others, that they were in honour of Bacchus and Ariadne.

To celebrate the Ofcbopboria, the young People who had Fathers and Mothers alive, run to rhe Temple of Bacchus and that of Minerva, with Grapes in their Hands. He who arrived there firft, was the Conqueror ; and was to perform the Sacrifice by pouring out of a Phial a Mixture of Wine, Honey, Cheefe, Flower, and Oil.

The'Wordis form'd from the Greek eo-jg, Branch of a Vine loaden with Grapes, and ?«p«, I bear. Plutarch fays, the Ofebofboria were fo named, becaufe mftituted by Tbe- feus when on his Return to Athens 5 and the Feaft celebra- ted after the Vintage.

OSCILLATION, in Mechanics, Vibration, or the recipro- cal Afcent, and Defcent of a Pendulum. See Pendulum.

Axis of Oscillation, is a right Line, perpendicular to the apparent horizontal one, and paffing thro' the Centre of the Earth ; about which the Pendulum ofcillates. See

If a fimple Pendulum be fufpended between two Semi- cycloids, whofe generating Circles have their Diameter e- qual to half the Length of the Thread ; all the Ofcillations, luowfpever unequal, will be lfochronal, ot Equi-diurnal. See Isochronal.

The Time of the entire OfciUation thro any Arch of a Cycloid, is to the Time of the perpendicular Defcent thro* the Diameter of the generating Circle, as the Periphery of the Circle to the Diameter. See Cycloid.

If two Pendulums move in fimilar Arches, the Times of Ojcilhuotts are in a fubduplicate Ratio of their Lengths.

The Numbers of lfochronal Ofcillations, perform'd by two Pendulums in the fame Time, are reciprocally as the Times wherein the feveral Ofcillations are perform'd. See

Clock.

M.Huygens's whole Doclrine of OfciUation, is founded on this Hypothefis ; That the common Centre of Gravity of feveral Bodies, connected together, muft return precifely to the fame height whence it fell 5 whether thofe Weights return conjointly, or, whether after their Fall, they return feparately j each with the Velocity it had then acquired. See Centre of Gratify.

This Suppofition was oppos d by feveral, and very much fufpected by others. And others who inclined to believe it true, yet thought it too daring to be admitted into a Science', which demonstrates every thing.

At length Hi. Bernoulli demonstrated it by ftricT: Geome- try ; by referring the Weights to a Lever. After his Death, a more eafy and natural Demonstration of the Centre of OfciUation was advane'd by his Brother. The Subflance whereof may be conceiv'd as follows.

A fimple Pendulum of a determinate Length and Weight, raifed to a determinate Height, whence it is to fall till it recover its vertical Line; employs, in that Fall or Demi- Vibration, a determinate Space of Time, which cannot be either greater or lefs. Which Time is neceffarily fuch, becaufe the agitative Force, i. e, the Force which produ.es the Motion of the Pendulum, is determined in every thing that concurs to the Formation thereof: fo that it can only caufe one certain Effect. .

The agitative Force of the Pendulum is torm'd of three Things: i°. Of the Power or Moment of the Weight. a ° Of the Mafs or Body tied to the end of the inflexible Rod. 5°. Of the Diftance of that Body from the Point of Sufpenfion, or, which is the fame, of the Length of the Rod or the Pendulum.

Now, i p, The Power of the Weight, be the Caufe what it will, is that Power which makes a Body fall, and that, v. gr. at the rate of fourteen Foot, in the firft Second of Time. 'Tis vifible, then, that this Force is the Effect of a Quantity which determines thofe fourteen Feet; and that a heavy Body would pafs more or lefs Space in that fame firft Second, if the Force of the Weight were greater or lefs.

2 . As that Force is apply'd to each Point, or infinitely fmall Part of a Body, the greater this Body is, or the lar- ger its Mafs, the greater Quantity of Motion or Force it has.

5 . The Diftance of the moving Body from the Point of Sufpenfion, or the Rod, is always the Radius of a Circle, whereof the moving Body defcribes an Arch : And of con- fequence the greater the Radius is, eateris Paribus, the lar- ger Arch the Body defcribes. And at the fame time, the greater Height it falls from, the greater Velocity it ac- quires.

Now, the agitative Force of the Pendulum, is only that of the Body faften'd to the End of the Rod. So that it is the Product of the Force of the Weight, of the Mafs of the Body, and of its Diftance from the Point of Sufpenfion. The Force of the Weight therefore being always the fame 5 and a Body or Weight faften'd to the End of the Rod, al- ways the fame; 'tis impufiible that two fimple Pendulums of a different Length fhouid be lfochronal, or fhould make their Vibrations in the fame time: for by virtue of thofe different Lengths, the Velocities will be unequal, and of confequence, the Times of their Vibrations.

But if it be fuppos'd that there are in Nature different Forces of Weight ; it will then be poffible that two fimple Pendulums of different Lengths, fhouid be lfochronal j the one animated by the natural Weight, the other by the imaginary one. If the imaginary Weight be greater than the natural one, the Pendulum imagin'd lfochronal to the natural one, will neceffarily defenbe a larger Space or Arch in the fame time; and of confequence the Weight will be faften'd at a greater Diftance from the Point of Sufpenfion. Tho, to have an Ifochronifm, the two agita- tive Forces of the two Pendulums mult be equal ; yet of the three Things which compofe thefe Forces, there are already two greater in the imaginary, than the real Pen- dulum : the third, therefore, i. e. the Mafs of its Weight, muft be. diminifh'd in the necefTary Proportion. As the Space or Arch defcrib'd by the imaginary Pendulum, is greater than that by the natural Pendulum, in the fame Ratio as the imaginary Weight is great* r than the natural one ; and a Radius of that Arch, greater in the fame Ratio, are two Things infeparable: the two Weights will be al- ways to one another, as thofe two Radii, or the two Lengths of the two Pendulums; which always gives the Expreffion of the imaginary Weight, and by a neceffary Confequence, that of the diminifh'd Mafs of the Weight of the imaginary Pendulum. If the Weight be imagin'd lefs than that of the natural one, 'tis eafy to obferve how it is to be taken; but that were needlefs in our Defign.

If now there be a compound Pendulum, charg'd with two Weights faften'd to the fame Rod ; M. Bernoulli con- ceives each of thofe Weights removed to a greater Diftance from the Point of Sufpenfion, than it was before ; but both to the fame ; and, aiminifh'd in Mafs, in a due Proportion; fo as that both together only make one fimple Pendulum, animated with one Weight the Exprefllon whereof is had, and lfochronal to the natural compound Pendulum.

Thus we fhall have one fimple natural Pendulum lfo- chronal to the compound natural one, by having a fimple natural Pendulum lfochronal to the fimple imaginary Pen- dulum before found ; which is very eafy : fince as the ima- ginary Weight is to the natural, {o is the Length of the fimple imaginary Pendulum, to the Length of the fimple natural Pendulum ; and 'tis there is the Centre of OfciUation requir'd.

Centre of Oscillation, in a fufpended Body, is a cer- tain Point therein, each Vibration whereof is perform'd in the fame manner, as if that Point alone were fufpended at that Diftance from the Point of Sufpenfion.

Or, it is a Point, wherein, if the whole Length of a com- pound Pendulum be collected ; the feveral Ofcillations will be perform'd in the fame time as before. See Pendu- lum.

Its Diftance, therefore, from the Point of Sufpenfion, is equal to the Length of a fingle Pendulum, whofe Ofcilla- tions are lfochronal with thofe of the compound one. See Centre of OfciUation.

OSClTATtON, the Afl popularly call'd Tawivng.

It is perform'd by expanding almoft all the Mufcles of vo- luntary Motion at the fame time ; but moft considerably thofe of the Lungs: by infpiring a great Quantity of Air, very flowly, and after retaining it fome time, and ratifying it, by