LEV 97 LEV
bet 1 * en tbe otdler > as before, if a coincidence takes place^ the thread and the line, the level is adjusted ; but if> the operation must be repeated till it come true.also most convenient class of levels is the spirit level, calleda nd • ' air level, which is more accurate than any other kind,Ote t 8 , most extensively used. The invention of this instru-att ... has been ascribed to M. Thevenot. .Others haveThe ■ U * ;e ^ tL is application of a bubble of air to Dr. Hooke,instrument consists of a cylindrical glass tube filled withits' * j w * ne > except leaving in it a small bubble of air ;hubhl be . in S hermetically sealed to keep in the fluid. Thisby t j e > being the lightest of the contents of the tube, will,th e , le , aws of hydrostatics, always run towards that end ofiectl U l 'T^mh is most elevated ; but when the tube is per-e ithe '° r ' zont;a l> the bubble will have no tendency towardsth 0ll ^, e nd. The tube is not strictly cylindrical witliinside,c 0tlv ° 1 ? hears that appearance, but is slightly curved, thetest * X Slt ^ e being upwards, and by this means the bubble willa Phr lQ middle of the tube when it is horizontal, butsioj ° ac hes either end if elevated above the other. Thelioriz 1 ^° rm a s P‘ l 'it level for fixing any plane trulyCa Ued ^ ’ cons ’ s t s °f a glass tube of the above description,figure 1 tube, fixed into a block of wood, as at ab,
and n ddie lower surface, d e, of the block is made flat ;marked 6n tbe bubble, c, stands between two scratchesMethod °r tbe ?* as3 at a b, the line D E is horizontal. Theinto th re ma hing it correct is this : the tube is first fittedbench the lower edge, d e, of which is placed on a
that tl ° r , ta bi e as nearly horizontal as can be determined, sois n le bubble stands between the scratches a b. The levelfirst T reve . rsed > that is, the end d is put where E was at
tn this . 1 — —'i_n-
Prov,
position, if the bubble stands in the middle, it
if the level to be correct, and the table horizontal ; butto 0 m Uns t° either end of the tube, it shows that end to bethe tub* ® leva . ted 5 . suppose it b, for instance ; this end ofsn r f ace e must therefore be let deeper into the wood, or theerr 0r m ° E rect *h ed to produce the same effect: one-half theby reofp 3 !' be compensated by this means, and the other halfn°\ v D n g the table or support ; for d e, the level, mustthey ar leversed again to verify these corrections ; and whenthe ley* 3 , 80 .made that the bubble stands at ab, either way,^*9Ure 2 ^ . correct;< To illustrate this more plainly, seefor e j . ’ "’.bicli represents a section of the bubble tube ; but,ar e ever atlon > * s shown as if curved much more than theyl'>be to / nade ' Suppose the convex or upper surface of thehyd„ e a . se gment of a large circle, b c d ; from the lawsfigbtest'h^ 03 ’ ^ ' 3 plain that the bubble of air, being thePoint of *. n Lie tube, will certainly occupy the highest
dually d' 16 c * rcde at C ; and the two points, B, D, being? E t>/ lat ant therefrom, will be in the same horizontal lineev el i )e j, e larger the radius of the circle d b, so will the|j m more sensible of any deviation from the hori-^ sta n'ce a ] Cauae tb e bubble will have to traverse a greater°f cither ° tbe tube, in proportion to any partial elevation
, the art or act of finding a line parallel to1 . bdo-hf 1 ’ ^ one or more stations, in order to determinea ying g ro °ne place with respect to another ; for the0t Jfi ac tina. Un ^ S even > regulating descents, draining morasses,o bltV£j^ WiUers fi° r the irrigation of land, &c.
Verh / ’ or Leaver (from the French levier, formed oftt ‘ e cL-i- • ever ~ A ~-' ■ ■
, . -, derived from the Latin , levare to a s e
a n inflexible straight bar, supported, a s S f*4- a fulcrum, or prop, and used for the imsmg
S!le lf!Ver L the first of those called ^ nica [P^;Z,C Inach ines, as being, of all others, the most P
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and is chiefly applied to the raising of weights to smallheights.
In a lever three things are to be considered: the weightto be raised, or upheld ; the power by which it is to beraised, or sustained; and the fulcrum, or prop, by which thelever is supported, or rather on which it moves round, thefulcrum remaining fixed.
Levers are of three kinds : sometimes the fulcrum, orcentre of motion, is placed between the weight and thepower. This is called a lever of the first kind, or vectisheterodromus ; to which may be reduced scissars, pincers,snuffers, &c. : sometimes the weight is between the fulcrumand the power, which is called a lever of the second kind;such are the oars and rudder of a boat, the masts of ships,cutting knives fixed at one end, and doors whose hinges areas the fixed point : and sometimes the power acts betweenthe weight and the fulcrum, which is the lever of the thirdkind; such is a ladder lifted by the middle to rear it upagainst a wall: these two are called vectes homodromi.
In this last, the power must exceed the weight in propor-tion as its distance from the centre of motion is less than thedistance of the centre from the weight. And as the first twokinds of lever serve for producing a slow motion by a swiftone, so the last serves for producing a swift motion of theweight by a slow motion of the power. It is by this kind oflever that the muscular motions of animals are performed, themuscles being inserted much nearer to the centre of motionthan the point where the centre of gravity of the weight tobe raised is applied ; so that the power of the muscle is manytimes greater than the weight which it is able to sustain.Though this may appear at first a disadvantage to animals,because it makes their strength less ; it is, however, the effectof excellent contrivance ; for if the power were, in this case,applied at a greater distance than the weight, the figure ofanimals would be not only awkward and ugly, but altogetherunfit for motion; as Borelli has shown in his treatise DeMotu Animalium.
The knowledge of the properties of the lever is of theutmost use in ascertaining the laws of the resistance of tim-ber ; we shall therefore begin with the first principles ofmotion, from which the properties of the lever are obtained ;and also the principles of the centre of gravity of one, or of asystem of bodies.
1. Force is the power exerted on a body to move it.
2. Direction of motion or tendency is the effort which onebody makes to move another towards a given point.
3. Line of direction is the straight line in which a bodymoves, or has a tendency to move, without having any regardto the point to which it tends.
4. Angle of direction is the angle contained between twolines of direction.
5. When two or more bodies act against each other withoutany of them being overcome by the rest, this state of quies-cence is called equilibrium.
6. Opposite directions, or opposite tendencies, are wheneach of two bodies move, or have a tendency to move, to adifferent point in the same line of direction.
7. Opposite forces are those that act upon each other in thesame line of direction, but have a tendency to contrary pointsin the line, by which tendency an equilibrium is produced, orotherwise a change of motion.
8. Contrary directions are when two bodies move, or havea tendency to move, in lines parallel to two opposite planes.
Axiom 1.—Every body endeavours to preserve its presentstate, whether of rest or of moving uniformly in a right line,till it is compelled to change that state by some externalforce.