DEG 
neceflary to an operation of this kind, renders it probable 
offidonius was only foun aa oR 
an inveftigation of the labours of others. His 
€s of different 
atitudes probably 
ned in a moft inaccurate rer 3 fo that the ob- 
fervation we made on the degree of Eratofthenes applies, 
with equal propriety, to that of Poffidonius: and really 
thefe meafures would fcarcely deferve the notice of men of 
{cience, but for the importance which fome learned men 
have attached to them 
Ptolemy, who ine near 300 years after Eratofthenes, 
circumference of the earth 
y navigators, a 
ftadium being referred to 
is the more admiffible, as we know that the Egyptian cubit 
bas been divided at different ee into 32 and 24 digits. 
It is poffible, therefore, that the ftadium fuppofed to be 
derived from the cubit, might “ikewife, te lable to the 
fame a variatio 
along dark pe aad 7 nearly 1400 years, but 
one folitary aes occurs of any attempt either to verify 
or e thefe ancient ee About the year 814 
the caliph Almamon, the fon of Haroun al Rafchid, af- 
fembled his aftronomers on the plains of sa tamia, and 
other fouth, , tiil 
place of their depar 
shey suena their ic a nor how they meafured their 
diftance 
The degree refulting from this meafurement is reported 
to have been eftimated at 56 miles and a half; but fo great 
: the uncertainty relative to the ftandard employed, that 
is meafurement, like many others, may be confidered as 
lott to potterity. 
RAthe year 1528, Fernel undertook to meafure the length 
of a degree, and afcertained the diftance between Paris 
eftimated the degree at about 68,096 geo- 
paces. Ga is, likewife, fome uncertainty as to 
the value of thefe geometric pac 
Picard eftimates Fernel’s degree at 57,070 toifes, Ric- 
cioli at eee 
Thus, the labour of this ga gable is likewife ie 
m the pee mftance of feveral meafur — in t 
ination. We ee ot much t 
He perhaps, in th's particular cafe, but ee the oe 
n been ever fo exaét, we fhould equally have been de- 
Neverthelefs, M. de la 
confiders it as accurate, ave been accidental, 
confidering the imperfect nature of this method. He deter- 
mined the latitude of his extreme {tations by taking the 
altitude of the fun with | aa rules fimilar . thofe of 
Priolemy of eight feet ra 
Snellius, in the 
year 
meridian between ‘Alknaar method 
puting the degree very exactly, which, in 1 fome 
meafure, arofe from his having taken too fhort a bale, only 
631 toifes 
initrumenis fufficiently accurate for the purpo 
Norwood, a native of this country, in the year 1635, 
s; and, likewife, from not being pone with 
RE E. 
meafured the are of the ante omiaiaes between Londoa 
York, with confiderable 
e determined the ame cf the fun on the da ay of the 
folftice, in two different years, at each place, and found the 
difference of latitude 2° 28°; he meafured the diltances by 
chains, fometimes by paces,-eltimating, as well as he could, 
the various windings 1 in the direGtion of the road, and fond 
So 
mit che meaiurement of 
a method frit fuggefted by Kepler, 
which confifts in obferving the eluieate of two dittant 
objects below the horizon. It may be inferred, from the 
property of the fphere, that the ce of the depreffion of 
ae diftant objects 1 1S equal to the arc intercepted between 
Thig may be a underftood by referring to 
Pla iy Vi fe. 48. A i 
ee for either to be "vifible 
: Ac, Be, the refpective hoe In the — 
i "BC, A and B being mght angles, the angles C a 
oo 
= 
= 
ae 
oO 
co 
a 
fs) 
=) 
» 
re 
s) 
5 
a 
= 
ae 
2. 
0 
ene 
ao 
om 
ra 
} 
are together equal to two right ues Trorefore,. ne 
anglec A B, and ¢ BA, or the of the depreffions, is 
see to the angle C.  Riccioli neni the angle A 
the mountain of Patetno, near Bologna, formed by the 
perpendicu lar BC, and the tower of Mode He then ob- 
ferved the angle CAB at the oer of Modena, which the 
being taken from 180°, leave : 
e then bier ghee the included aru and found it 
equal to 20,016 paces of Bolog thefe data the 
value ae ee a be eaienictes by a a proportion, 
Riccioli appears to have taken a great deal of pains with 
this method, which, however, isvery defeCtive in practice, both 
e terre! ftrial refractions, and 
nd, 
therefore, it is not furpriing that the refult was inaccurate; 
e made the Me egree 63,000 toifes, ae is not withii one- 
tenth of the truth. 
Such was de itate of uncertainty, relative to this interett. 
ne problem, till the eitablifhment of the academy of f{ciences 
rance. One of the firft operations of that learned body 
was the determination of the magnitude of the — and 
Pica the execution of the projeG. 
But, nd the different opera. 
tions that fieccede thefe imperfe& ys it may be proper 
to ftate the nature of the problem a little more diftin@ly, 
and to oe a uA outline of the principle on which its 
ee is founded. 
Fig Let A and B be two places nearly north and 
fouth af each other 
m he objet is to determine the difference of 
latiiude between A ae x; to afcertain the diftance between 
them in terms of fome kee ftandard meafure ; and thew, 
by a fimple proportion, to find the value of a aie Thus, 
if the difference of latitude between x fhould be 
d its meafured dance 40 oco fathoms, 
000 fath. :: 60’ 
; 60,000 fath. 
For this purpofe, the moft ufual method is to meafure a 
bafe line, as a4, with all imaginable care: a number of in- 
termediate ftations are then to be felefted. as conveniently 
fituated for the purpole as poffible. Thefe confit of fuch 
fteeples, towers, or other confpicuous obje&s as are to be 
met 
