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L E V ( .97 1 L E V
th~t \ViII not eaf,ly , for more ~pdy Cmliog
the balaoce of che \Ve'gllt aod tdef,ope. The ifollru –
menl carries C\VO telercopes cloCe aod \'ery parallel 10
e. eh olher ; Ihe eye glars of Ihe ooe beiog againfl ¡he
o!Jjell-glafs of Ihe oll1er, ¡hat ooe may ree each way
wilhouc lurniog ¡he level. lo the focus of ¡heobjeél–
glars of each ¡e1efcope mull a lietle hair be firaioed ha–
rizoot,lIy, 10 be raifed . od lowered as occafioo re·
quires by a litlle fereIV.. If the tub, of the lelercope
be not fouod level \Vhee fupeoded, a ferril or riog,
4, is pUt 00 it, aod is to be f1id aloog till il fixes to
a level. The hook 00 whieh the ionrument is huog,
is fixed 10 a flat \Voodeo erofs; at the eods of eaeh
arm whereof there is a hook ferviog 10 ~eep the le–
lefcope from too much agitatioo_in uGog or carriage.
Tu Ihe faid ftal crofs is applied another hollow one,
Ihat ferves as a cafe for the ,of¡rument; but ¡he t\Vo
cnd, are left apeo, Ihat the Ielefcope may be feeured
from Ihe \Veather, aod always in a conditioo to be u·
Ced. The foo¡ of thisinllrumeot is a rouod brafs plate,
tO which are fallened three braCs ferilt, moveable by
means of joints whereie are put llaves, and on Ihis
fool is plaeed Ihe box.
N° 12. marked 1, is a bolance·level j which being
fu Cpended by the ring, the tWO fights, when in equili–
brio, .·ill be horizontal, or in a level.
LEVELLING, lhe art of fioding a line paralle! to Ihe
horizon at one or more llations, in order 10 determine
the height of one place Wilh regard to another. See
Ihe preceding anicle.
A truly Im l furface is a fegment of a fpherieal fur–
face, whieh is concentric 10 theglobe of the eanh. A
true lioe of leve! is ao areh of a great cirele, which is
imagioed to be defcribed upon a truly level furface.
The apparent leve! is a llraight lioe drawn tangenl tO
In m h or lioe of lrue level. Every point of the ap–
parent leve!, except lhe poinl of eootaél, is higher than
Ihe !rue level: thus let EAG (Plate CIV. hg. 2.
nO],) be an areh of a great cireJe dra. n upon the
emh j tOa perfon ",ho fiaods opon tbe eanh at A,
lhdine HD is lhe ~pparent level pmllello his raliooal
borizon RR ; but lhis line, lhe f,nher it is eXlended
{rom his llatioo A, the fafl her il reeedesfrom theceno
ler; for BC is longer litan AC-.. and OC is longer lhan
BC, &c. The common melhods oílevelling are fuf·
heieot for laying pavemenls of walks, for conveying
wat~r 10 fmall dillanees, for placing horizootal dials,
or allronomical iollrumeol! i bUl in levelliog lhe boto
lnm! of eanals which are 10 coovey waler 10 lhe dif'
lance of many miles, the dilf.rence betwecn the ap'
pareot and true leve! mull be takeo inlo lite aceount :
Ihos let IAL (ihid. nO 2.) beanarch of a grea! eirele
"pon the ea,lh : let it be required tO cut a.canal whofe
Itottom /h~1! bea true level[rom A10 B, of lhelenglh
of 5078 feet : the common melhod il tO place the
level!ing inllrumeot io lhe hotlom of lhe caoal at A,
and looking through Ihe figh ls plaecd horizomally at
e ltick Cet op perpendicular al S, to amarkwhere
the vifoal ray or point of Ihe al'p"reot leve! poiols at
E, and Ihen 10 fink the bottom of lhe eaoal at B al
lIlueh btlow E as Ai$ below D. llUI lhis will not
give the lroe level : for aecording to Camoi's calclllation,
a¡ the diflaoce of 5°78 feet lhe apparent level-is feven
inehes above lhe lrue; and lherefore, 10 make a lruelevel,
B mull be fo nk feveo in,hes lower lhanIhe apparent level
diretb; fo that if A be four feel below O, B moll be
four feel Ceven inche; below lhe mark E. We have here
mentioned lhe error which wil! ariCe from placing the
level at one end of Ihe line lO be Ievel!ed, aod /hewn how
to correél it j bUI in mol! caf" il is better 10 cake afia.
tÍoo in Ihemiddle of lhe lioe 10 be levelled: thos, if lhe
points H and B are to be levelled, place the inflrument
in Ihe middle al A, aod fetting op l!iekl perpeodicular at
H aod 13, make marks opoo each fii, k where the appa–
reot level poinls, as E and F j thofe points are leve! :
and if you fink H as much below F, as B is below E,
HAB will be a true level.
The operation of levelling is as follolVs : fuppofe lho
height of the point A, (¡hid. nO 3.) on the 10p of a·
mountain above lhal of the point B, and at lhe foot
thereof, be reqoired. Place lhe level aboul Ibe middle
diflance between lhe two pOiOIS as in O, and llalfs in 'A
and B ; and let lhm be perfons inllroéled with úgoals
for raiGng and lowering, on lhe faid lIalfs, lilde ma.rks
of pafieboard or olher maner, lhe leve! being placed ho–
rizonlally by lhe bubble, &c. Look 10l\'ards lhe fialf
AE, and caufe lhe mark fo raifed to be lowered lil!lhe
middle, upper edge, or olher moll conrpicuous pan, ap–
pear io lhe vifual rayo Then meafuring exaélly lhe per–
pendicular heighl of Ihe POiOl E above lhe poinl A, which
fuppofe fix lee¡ four inches j fet that down io your ~ook:
then turo the leve! horizontal!yabout, lhat the eye-glafs
of lhe telcCcope may be fiill next Ihe eye when you look
the other wayi if you have only plain fights. the iollru–
meOl need nOl be lurned; and caufe the perfon at lhe
fialf S, 10 raife or lower his Olark. liJl fome confpieuous
pan of il fal! in lhe vi Cual ray, as al C: theo mearure
¡he perpendicular heighl of C above B, which fuppofe
fixleen feet ÚK inches : fet lhis alCo dow.o io !he book a·
bove Ihe other nomber oC lhe Grfi obfervatioo j fubtraél–
the one from lhe other, Ihe remainder will be ten feet
llVO inches, whieh is lhe d,lference of the level betweeo
A aod S, or lhe height of Ihepoint Aabove thepoinr B.
If lhe point D, where lhe inllrument is fixed, be in
the middle btl\Veeo lhe two points A and B, Ihere \IIill.
be no neceffilyfor redueing lhe apparent levello Inetrue
level j the vifual ray in Ihat cafe being raifed eqllally a–
bove Ihe true level If it be fUrlher required to knolV
whelher lhere be a fu!licient defeeO! for conveying water
from the fpring A (ihid. nO4') 10 lhe point B. Here,
in regard lhe dill.nee fróm A 10 B is cunfidcrable, il is
required lhal femal operations be made. Having lhen
choCeo a proper place for lhe r,rll fiation, as at 1. fel up
a llalf in lhe point A, near lhe fpring, \Vilh a proper
mark to Oíde up and down lhe Ib/f, as L, and OIealure
the di llanee fromA 10 J, wbich fuppofe I\VO Ihou(and
yards. Theo Ihe levcl being adjolled io lhe point 1, let
lhe mark L be raiCed and lo\Vered lil! fuch time as you
fpy (ome cooCpieoous par! of il th rough Ihe lclefcope or
fighl' of Ihr Im l, aod méaCure Ihe heighl AL, \\'hich
fu ppuC<Ihimeo (cet fiv( ine¡'cs. BUI in regard the die.
tiUlce Al j, IWO lhouf~Dd y¡rds, you muJl hore recourfe
10.'