The Meaflure of the Earth.



TreleliCope oflC ilC nftruiicnt with that of the Alidade. It is neceP
lar' hete that theV mufi le both well adjufed together at one and
the faPune far dillaint O1jecd. This being fuppofed, one obferves
lb ii v ith the Plumb, and with the TeleIcope faffned to the Inflru-
ment, the Meridional diflance betxxeen the Zenith and the Star pro.
poled, next one fixcs this InfIrulmnet in the plain of the Meridian,
as in the preceding mananer, but in fuch fort, that it may beqcoun-
terturnred, and dhat if tlhe Star Le tow ards the South, it returned as
'twcrc for obferving toiards the North,. and one ol)ferves exadly
the Degree and Minute ot the Limb wfihere the Plumb beats. After
thnis tile the Plumb Ibeit)g taken off, one applies the Alidade, with
which one ol.Acrves the NMeridional Diftance between the Zenith and
the Star, counting for this cfid the Degre and Minutes which are
found betwxev n the fiducial line of thc Ali'ade,and the part of the limb
where thc plumnb did beat before. Thlie firf diftance that was found
bein(g comnpared with this laft. fliall be too little if the Iniftrument
ele-
vates; and on the contrary, it Iflall be too big if it deprefres in fuch
fort
that the half of the diierence fhatl be the error of the Inftrument.
  After one has known the error of the Inftrument, and tha- one
is af(ured that it comes not but by the Telefcope, the lborteft and
eafieff way is to let it alone, and to have regard to it in thle Ob-
.fervations; but if one would corrre6t it , this may be done ei-
thier by difplacing the Filaments of the Telefcope, or by turning
the Objedt Glafs upon its Center; ro far as one knows by experi-
cnce it is neceffary for adjufting the Telefcope to the Degrees of the
Intfrument. An Alidade furnitht with its Telefcope may be of great
.help tt; make this corredion; for, this purpofe. one points to one
and the laime diffant Obje#Q  as well the Telefcope of the Alidade
as that of the Inairument.  Next, if the error is, for example, of
one Minute in elevating, one fets back the Alidade a Minute ; or
on thle contrary, one puts it nearer it, as much if the error be in de-
preffing; and having faftned it in this pofition, by removing the
Inftrument all togethcr, one maikes the, Telefcope of this Alidade to
bland pointed at the fame Obje6t as before; after which you mua
turn the Ojec6t Glafs of the Telefcope, which is falned to the In-
firument upon its Ccnter, till fuch time as it be found pointed to
the fame Objje t; and by this means one may be aflured, that a
right line which fliall be drawn from, the Obje6t by the Center of
the Inttrumient, comes to meet the point B, which we fuppofe to
have been eflablilhcd for the beginning of the decifion.
   But for avoiding as much as is poffible the refradtions of the Tele-
fcope, care muff be taken that the Obje&t Glafs be well centred,
which may be difcovered by making it refle6t the Rays of the Sun,
becaufe if it be well centred, the little focus which it, makes by re-
fledtion at a certain diftance, will be found exaffly in the middle
of a much greater round of light. Or elfe one may obferve that
the two Images which the Glafs refle6ts of the fame Objeic, comne
to unite in the middle of its furface.
                                 E z                         After



2.7