Page:Cyclopaedia, Chambers - Volume 2.djvu/94

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perpendicular Afcent, when the Mercury is at 30 Inche Dr> ftattey allows of T £ of an Inch for every 50 Yards 5 which, considering how accurately the Barometers are now made, an Inch in fome of 'em being divided into an hundred or more Tarts, all very fenfible, he thinks this Method fufficiently exact to take the Levels for the Con- veyance of Water, and lefs liable to Errors than the common Levels. The fame Author found a Difference of 3 Inches 8 Tenths between the Height of the Mercury at the top and bottom of Snowdon Hill in Wales*

LEVER, in Mechanics, an inflexible Right Line fuppor- tedby afingle Point on a Fulcrum or Prop; andufed for the raifing of Weights; being either void of Weight itfelf,orat If ait having fuch a Weight as may be balanced. The Lever is the firft of thofe call'd Mechanical Towers or Simple Ma- cb"tnes,a& being of all others the moilfimple, and ischiefly applied for railing Weights to fmall Heights. It has its iName Lever froltn its QSxtel&vare or lever, toraife.

In a Lever there are three things to be principally con- fide red : 1. The Weight to be rais'd or fuitain 'd, as O, (Plate Necbanich 3 Fig.j.') 2. The Power by which it is to be rais'd or fuitain'd, as B. 3. The Fulcrum or Prop D, whereon the Lever is fuftain'd, or rather on which it moves round, the Fulcrum remaining fixed.

r l he Lever is threefold : Sometimes the Fulcrum is placed between the Weight and the Power, as in Fig. I. Plate Hechamch b and this we call a Lever of the firft kind. Sometimes the Weight is between the Fulcrum and the Power, which is the lecond kind ; as in Fig. 2. And fame- times the Power acts between the Weight and the Ful- crum j fig. 3. the third kind.

The Power of the Lever is founded on the following Theorem, viz-. That the Space or Arch defcribed by each Point of a Lever, and consequently the Velocity of each Point of a. Lever is as its Diitance from the Fulcrum or Prop.

From hence it follows, that the Aclion of a Power and the Reiiilance of the Weight increafe in proportion to their Diitance from the Fulcrum. And hence alfo it fol- lows, that a Power will be able to fuitain a Weight, if the Diftance of the Point in the Lever to which it is ap- plied, be to the Diflance of the Weight, as the Weight to the Intcnlity of the Power ; which if it be ever fo little increafed, mufl raife the Weight. See this Doctrine de- monit rated under the Word Mechanic Powers, and further illuitrated under the Word Balance: between which and the Lever there is a great Analogy ; a Lever of the firft kind being a fort of Steel-yard to raife Weights.

The Power and Action of the Lever will be fully illuf- trated by the following Propofitions.

I. If the Power applied to a Lever of any kind fuftain a Weight, the Power is to the Weight in a reciprocal Ra- tio of the Diilanccs from the Fulcrum- This is the Con- verfe of that demonftrated under the Head Mechanic Towers.

II. The Weight of a£ectfrof the firft andfecond kind AB, the Diitance of the Centre of Gravity from the Fulcrum C V, and the Diitances of the Weight and the Power AC and CB, being all given; to find the Power that will fuiUin it.

Suppofc the Lever void of Gravity, but in lieu thereof a Weight hung at V ; if then A C be made to C V as the Gravity of the Lever to a fourth Number, we fhall have the Weight which the Lever is able to fuitain, and this fub- itracted from the given Weight, the Remainder will be the Weight to be fuitain'd by the Power. Let CB then be to C A as the remaining Weight to a fourth Weight 5 and we fhall have the Power to be applied in B, in order to fuilain the given Weight with the given Lever.

III. The Gravity of a Lever of the firit or fecond kind A B, theDiilance of its Centre of Gravity from the Ful- trum CV, theDillancesof the Power and the Weight BC and CA being all given, to find the Weight to be fuf- tain'd.

Find the Part of the Weight fuitain'd by the Lever alone, as in the former Problem ; in the fame manner find the other Part of the Weight which the Power ap- plied in B is able to fuitain: Add the two Numbers to- gether, the Sum is the Weight required.

IV. The Gravity and Centre of Gravity F of a Lever of the fecond kind CB, with the Weight G, its Diitance from the Fulcrum C A, and from the Powrr C B, being given j to find the Power capable to fuilain the Weight.

Suppofe the Lever void of Gravity, but in lieu thereof a Weight equal thereto hung in F, the Power required to fuitain the Lever alone. Then find ihe Power requifite to fuilain the given Weight G j add the Powers together, the Sum will be the Power required.

V. If a Power applied to a Lever of any kind lift a Weight, the Space of the firft is to that of the laft, as this lait to a Power able to fuitain the fame Weight 5

LEV

whence it follows, that the Gain of Force' Is -Ws at- tended with the Lofs of Time, and vice verfa. '-\.

LEVERET, a young Hare, fo call'd in" the firft Year of its Age. '•*».;.

LEVIGATION lignifies the Reduction of anv hard ponderous Bodies, as Coral, Tutty, Pieciois Stones, i$c. into a light fubtile Powder, by grinding upon Porphyry, a Sea-fhell, Marble, Stone, & c. as Painters do their Co- lours. This is rauchufed in Pharmacy and Chymiftry but unlefs the grinding Inftruments are extremely hard] they will wear away, lo asfomecimes to double the Weight of the Medicine thus managed.

LEV1TE, a Hebrew Prieft, or Sacrificer, fo call'd as being of the Tribe of Levi, or the Defendant of the Patriarch of that Name. In the Primitive Church they alfo gave the Name Levite to the Deacons, and Minitters of the Altar. The Urates of the Old Law had no fettled Lands allotted 'em for their Maintenance, but lived chief- ly of the Offerings made to God. They were dillributed through all the Tribes, each of which gave fome of their Cities to the Levites, with Grounds in their Neigh- bourhood for the Subfiftence of their Flocks. According to the Numeration made by Solomon, from the Age of Twenty there were Thirty-eight Thoufand capable of ferving : Twenty-four Thoufand of thefe he appointed for the daily MiniQery under the Prielts ; Six Thoufand to be inferior Judges in the Cities, and to decide Matters relating to Religion, and of no great confequence to the Staro } Four Thoufand ro be Door-keepers, and to take care of the Riches of the Temple ; and the reft to do the Office of Chanters or Singers.

The Word comes from the Latm Levita, which isform'd from the Greek h'-vmit ; the Root of which is the Name Lev,, which was given to this Patriarch by his Mother Leah, from the Hebrew n*7 lavah, to be tied or united ; Leah hoping, by the Birth of this Son, to be more clofely link'd to her Husband 3<jco4.

LEVITY, the Privation or Want of Weight in any Body, when compared with another that is heavier, in which fenfe it is oppofed to Gravity. The Schools main- tain that there is fuel; a thing as pofit'ive and abfolute Levity, and impute to this the Rife or Emergency of Bo- dies lighter in Specie than the Fluids wherein they rife. But befides that the common Senfe of Mankind difcovers that Levity is only a Relative Term, we find by Expe- rience that all Bodies tend toward the Earth, fome flower, and fome falter, in all Fluids or Mediums, whe- ther Water, Air, Jjc. Thus Cork is faid to be lighter than Gold, becaufe under equal Dimenfions the Gold will link in, and the Cork fwim upon Water. Archimedes hath demonstrated, That a folid Body will float any where in a Fluid of the fame fpecifick Gravity, and that a lighter Body will keep above a heavier. The Reafon of this is becaufe of Bodies falling towards the Earth, rhofe which have alike number of equal Parts have equal Gra- vity i fince the Gravity of the Whole is the Sum of the Gravity of all its Parts. Now two Bodies have an equal number of equal Parts, if under the fame Dimeniions there are no Intervals deftitute of Matter ; whence it fol- lows, that as no Portion of Matter is fo fmall, but that Body wherein it is contained may be wholly divided into Parts equally fmall, there can be no reafon for the De- fcent of thefe, which will not equally hold for the De- fcentofthat. Hence it maybe concluded, that thofe Bodies which do not equally gravitate under the fame Dimenfions, do not contain the fame equal portions of Matter ; and Therefore when we fee, that a Cube of Gold fubfides in Water, at the fame time that an equal Bulk of Cork fwims upon it, 'tis evident that the Gold mull have a greater number of equal Parts of Matter under the famo Bulk than the Cotk, or the Cork muft have a greater num- ber of Vacuities than the Gold 3 and that there are alio in the Water, a greater number of Vacuities than in Gold. Hence we have a clear Idea both of Denfity or Gravity, and of Levity 5 and know, that the latter cannot in a ftrict fenfe be accounted any thing pofitive, but a mere Negation or Abfence of Body, which determines that Body to be lighter than another which contains more Matter.

Dr. Hook, 'tis true, feems to maintain fomething like a pofitive Levity. This, if we millake not, is what he means by the Term Levitation ; viz. a Property of Bodies directly contrary to that of Gravitation towatds the Sun. This he thinks he has difcoverid in the Steams of Co- mets, which tho they had a Defcent from the Nucleus of the Comet towards the Sun, yet they quickly return'd, and went oppofite to the Sun, and that to a prodigious Ex- tent. In effecr, where the Power of Gravitation ceafes, it Ihould feem fome fuch contrary Force does begin, where- of we have Inftances in the Phenomena of Attraction. This is what Sir I. Newton calls the Vis Repellens, and ap- Yyyyy pears