Page:Cyclopaedia, Chambers - Volume 2.djvu/690

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t l/fS were, like the Thomifts, Peripateticks; only di- flinguifll'd by this, That in each Being, as many different Qualities as it had, fo many different Formalities did they diftinguiih; all diflincl from the Body itfelf; and making, as it were, fb many different Entities : Only thole Metaphyfical, and, as it were, fuperadded to the Being. See Peripateticks.

SCOTOMY, in Medicine, a Dizzincfs, or Swimming in the Head, wherein the Animal Spirits are fb whirl'd about, that external Objects ieem to turn round. See Vertioo.

SCRATCHES, among Farriers, a Diftemper incident to Horfes, confiding of dry Scabs, Chops or Rifts; that breed between the Heel and the pattern Joint. There are various kinds of Scratches, diitinguifh'd by various jjames, as Crefances, Rats-Tails, Mules, Kibes, tains, Sec. which are all ft many Species of Scratches-, Engender'd from fome dry Humour falling on the Legs, or trom the Fumes of his own Dung lying under his Heels, or near them; or for Want of rubbing his Heels, efpeciatiy after a Journey, from over hard-riding; S?c. It begins firfl; with dry Scabs in the pattern Joint, in feveral Forms. 'Tis known by the flaring, dividing and curling of the Hair On the Spot.

SCRATCH-WORK, in Italian, Sgrafiti, was a Way of Painting in Frefco, by preparing a black Ground, on which was laid a white Plaifter; which White being taken off with an Iron Bodkin, the Black appear'd through the Holes, and ferv'd for Shadows : This kind of Work is lading; but being very rough, is unpleafant to the Sight.

SCREW, Cochlea; in Mechanicks, one of the Five mechanical Powers : Chiefly ufed in preffing or fqueez- ing Bodies clofe, though fbmetiirtes alio in railing Weights. The Screw is a right Cylinder, as A B, (Tab. Mechanicks Fig.l I.) furrow'd Spiral wife., 'Tis generated by the equa- ble Motion of a right Line F G, around the Surface of a Cylinder; while, at the fame Time, the Point I, defcends equably from F towards G. If the furrow'd Surface be convex, the Screw is faid to be Male; if concave, 'tis female. Where Motion is to be generated, the Male and Female Screw are always joined} that is, whenever the Screw is to be ufed, as a fimple Engine; or mechanical Power. When joyned with an Axis in 'Perirrochio, there is no Occafion for a Female \ but in that Cafe, it becomes part of a compound Engine.

Dciirine of the Screw.

1° If, as the Compafs defcribed by the Power in one Turn of the Screw, is to the Interval or Diflance between any two immediate Threads or fpiral Windings, as B I (meafiired according to the Length of the Screw) fo is the Weight or Refiflance to the Power; then the Power and the Refinance will be equivalent one to the other, and, confequently, the Power being a little increafed, will move the Refiflance.

For 'tis evident, that in one Turn of the Screw, the Weight is fo much lifted up, or the Refiflance fo much removed, or the Thing to be prefs'd, is fqueezed fo much dofer together, as is the Diflance between two immediate Spirals; and in the fame Time, theVowcr to be moved is fb much, as is the Compafs, defcrib'd by the faid Power in one Turn of the Screw. Wherefore the Velocity of the Weight (or whatfoever anlwcrs thereto) will be to the Velocity of the Power, as is the laid Diflance between the Spirals to the Compafs defcribed by the Power, in one Revolution or turning round of the Screw; So that jjJJ gaining in Power, is here recompenfed by the Lofs in Time.

1° As the Diflance between two Threads, B F, is «fi; the Power required to overcome the faid Refiflance is lei's; therefore the finer the Thread, the eafier the Motion.

3° If the Male Screw be turned in the Female^ at refl, alcftPower will be required to overcome the Refiflance, as 'lie Lever or Scytala, B D (Fig 12) is the longer. '

4° The Diflance of the Power from the Centre of the Screw, C D; the Diflance of two Threads I K, and the "wer to be applied in K, being given, to determine the Re- iiltanceit will overcome : Or, the Refiflance being given tofindthe Power neceffary to overcome it; rn^" 1 Peri pk**y of a GircIe defcribed by the Radius Jj~ L : Then to the Diflance between the two Threads, il J, eri P ner y jull found, and the given Power : Or, to ™ "nphery found, the Diflance of'the two Threads and ! ne given Refiflance, find a Fourth proportional. This "the former Cafe, will be the Refiflance that will be rcome by the given Power; and in the latter, the

°wer neceffary to overcome the given Refiflance. -, *j T^Suppofe the Diflance between the two Threads, 3, toe Diflance of the Power from the Centre of the Screw

u < 1J, and the Power 50 Pounds; The Periphery of

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the Circle to Be defcribed by the Power, will be foiinii

'V\ T D C rT' aS > '*?' 3°' '*7«> the Weight to which the Refiflance is equal.

5° The Refiflance to be overcome by a given Power being given; to determine the Diameter of the Screw, the pittance of the two Threads I K; and the length of the Scytala or Handle: The Diflance of the Th?ead Si and the Diameter of the Screw may be aflumedar pleafure if the Male be to be turned in the female by a Handle' lhen, as the given Power is to the Refiflance it is to overcome, fo is the Diflance of the Threads to i fourth Number, which will be the Periphery to be de fenbed by the Handle CD, in a Turn of the Screw Ihe Semi-diameter of this Periphery, therefore being fought we have the Length of the Handle CD. But it the Female Screw be to be turn'd about the Male with- out any Handle; then the Periphery and Semi-diameter found, will be very nearly thofe of the Screw required

Ejr. Suppofe the Weight 6ooo, the Power, 00 , and the Diflance of the Threads 2 Lines; for the Periphery to be palled over by the Power, fay, As; ,00, tjooo : 7; uojjlhe bemi-diametcr of which Periphery being ' of 1 20=40 Lines will be the Length of the Handle;

any be ufed; otherwift the Side of the Female Screw mutt be 40 Lines.

„£"f^fi? c ,^ w - If 3 Screw be fo fitted as to turn i Wheel Dl (Fig. 13) it is called an Endlefs, or perpetual Screw; in regard it may be turned for ever, without com. ing at any End. From the Scheme, 'tis evident enough; that while the Screw turns once round, the Wheel only advances the Diflance of one Tooth.

Ttoclrine of the Endlefs Screw;

1° J f the P<™« ^Plied to theLever.orHandle of an Eni- lejs Screw A B, be .0 the Weight, in a Ram compounded of the Periphery of the Axis of the Wheel E H, to the lenp iery defcribed by the Power, by turning the Handle 5 and of the Revolutions of the Wheel DF to toe Revolutions of the Screw C B; the Power will' be equivalent tc ihe Weight.

Hence, i° as the Motion of the Wheel is exceed ihgly flow; a fmatl Power may raife a vail Weight, by means, of an Endlejs Screw : For this Rcafbn, the great Ufe of tae Evalejs Screw, is either where a great Weight is to be railed through a little Space; or, where a very flow gentle Motion is required. On which Account 'lis very uieful in Clocks and Watches.

2° The Nu. .ber of Teeth; the Diflance of the Pow- er from the Centre of the Strew A B, the Radius of the Axis H f, and the Power, being given; To find the Weight it will raife; °

Multiply the Diflance of the Power from the Centre ottheScrew A B, into the Number of Teeth ■ The Produft is the Space of the Power pafs'd through, in the Time the Weight pafles through a Space equal to the Periphery of the Axis. Find a Fourth proportional to the Radius of the Axis; the Space of the Power now found, and the Power I his will be the Weight the Power is able to fuftain. Ihus, if AB-=j, the Radius of the Axis HF— 1 - ^ P " w « '0° Ipundsj Number of Teeth of the Wheel D F 48; the Weight will be found ,4400, whence it apV pears, that the Endlefs Screw exceeds all others in iri- creafing the Force of Powers.

Archimedes'a Screw, or the Spiral <Pmnp a Ma chine for the Railing of Water, invented by Archimedes. Its Struflure is as follows; A leaden Tube is wound round aCyhnder AB ( lab Hydraulicks F,g. ,J after the fame Manner as the fpiral Thread is drawn in the common Screw above defctibed. This Cylinder is inclined to the Horizon in an Angle of about 4S - Degrees, and the Orifice of the Tube B immerged under Water. If then, the Screw be Jurned about by the Handle I, againft the Water; the Water will rife up the Spiral, and at length be difcharged in A.

This Machine, with a very little Force, is able to raife a great Quantity of Water: Whence 'tis found of good Ufe, ,n the emptying of Lakes, fgc. If the Water be to be railed to any conliderable Height, one Screw wili not fuffice; but the Water drawn up by one is to be taken by another, and fo fiicceffively.

SCRIBE, a principal Officer in the Jewifh Law, whofe Bufinefs was to write and interpret Scripture. We find no mention of Scribes, in the Old Teftament, before Ef- dras ■ whence fome Learned Men have conftrued, That V"k n ,?-„ ms brou g ht Oom Cbaldea, and Myria, and firff eftabl.fhed by the Jews after their Return from this Babylomfo Captivity. The Scribes were in great Credit and Efteem among the yews, and had even the Prece dencyofthePriefts, and Sacrifices : Indeed, there were three Kinds of Scribes, whereof thofe juft mentioned pro- perly called Scribes of the Law, were the firft, and mod