DEG 
ufe in clearing the bafe line, which-was done by a party of 
foldiers. who removed the brefh wood and other les 
of the foil, in a traét of about three yards in breadth, 
Various methods were fuggefted, aud tried for the mea- 
furement of the bafe, as on the accuracy of ea delicate 
operation all the fubfequent dedu€tions would depe Deal 
rods were firft employed, and afterwards hard ae as they 
were found to be affected by the moilture and drynefs of the 
in a manner not eafily reducible to rule. Glafs rods 
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mities of the bafe were marked by two pipes, and afterwards 
by cannon. The line joining them was meafured through 
the air, and not by a fucceffion of level lines of unequal alti- 
tudes, as had been ufually pratifed in other countries. The 
inclination of this hypothenufal line to the horizon was 
meafured by a proper inftrument, and the requifite correc- 
tion applied. 
The firft meafurement of the bafe with the glafs rods gave 
for its total length 27404.08 feet 
n the year 1791, this bafe was meafured a fecond time, 
a a fteel chain of exquifite workmanfhip, conftrudted b 
Ramfden, and this meafurement a 27404.315 feet, differ. 
ing but 23 inches from the form The ftandard meafure 
ufed on this occafion was taken on a brafs {cale 42 inches 
in length, in the poffeffion of the royal fociety. A bafe 
of verification was likewife meafured on Romney-marhh, in 
the fame manner as had been done on Hounflow-heath, except 
that the fteel chain was fubftituted for the glafs rods. 
e length of this bafe, after ali the requifite reduction, 
was 28,535 feet, 8.128 inches. 
The ae ner deftined to meafure the angles furpaffed, 
as we have before mentioned, in its dimenfions and elabora 
workmanthip, every inftrumerit of the kind that had been 
feenin Europe; it meafured angles with fuch precifion, that 
it became neceflary, in the calculation of the triangles, to take 
into coniderscon the excefs of three {pherical angles above 
two right angles, a quantity that had hitherto been too mi- 
fo that the angles dgalooe?| mu 
table obfervatory, an portable fe 
vey, fo that the centre of 
ought into the very point, 
which was to conttitute the Genal from the next ftation; by 
eae means all the troublefome correGtions, which we fhall 
e occafion to notice in the French furvey, were avoided, 
ae “all caufe of error arifing from them entirely removed. 
As the fituation of Paris, with refpe€t to Dunkirk, had 
been fettled by former operations, the only part of this pro- 
ration of French 
was 
wes undertaken by Caffint, Mechain, and Le Gendre, men of 
Slane celebrity, and members of the academy of fciences. 
y them that the firft trial was made with the repeat- 
ing on The one they ufed on this occafion was only 
twelve inches diameter, arid the firft that had been cubes 
yet it is beyond a doubt, but that the angles taken with it 
differed but little ‘ — from thofe ee with the 
theodolite. The er was certainly the fuperior inftru- 
ment, but it was large and of difficult tranfport; whereas 
REE. 
the French inftrument -went into a fm a bee which the 
oe carried them in their voit 
um ca ae io twenty-three 
different. ree Sy a ae ere diftinguithed into two fets; 
the firft were permanently cae by pipes funk in the earth, 
the others were thofe where the inftrument was elevated to 
the top of fome tower, fteeple, or other building. 
The ftations = a firft fet marked with pipes are four- 
teen in number, 
Hampton poor. “houte, i extremities of Hounflow- 
~ King’s-Arbo 
St. Raw s- ‘hill, chau the ae ne eaft edge. 
Hundred-acres, near the weft end of the ga ur aeDe 
Norwood, towards the Croydon end of the hei 
Botley- hill, in a field belonging to Simps sfeld- Teeiee 
arm 
Wro tham-hill, in a field belonging to Mr. Johnifton. 
Hollingborn-bill, ina aie a to Mr. Duppa. 
Fairlight-down, 347 fee 
whi ch makes with Fait light piles an angle of 105° 53/ 
Ruc kin nge, 
High-Nook, t 
Allington-knell, an artificial mount belonging to fir John 
od. 
Paddlefworth, eaftward from ae church in the broom- 
field belonging to Mr. Broc 
Folkitone turnpike, weftward ee the public honfe. 
e ftations of the fecond fet, where the inftrument 
was ea on bu ea are nine in number 
Hanger-hill to 
Tranfit room a Caen royal obfervatory. 
North-weft turret of Sevendroog caftle, on Shoot 
r’s-hill. 
Swingfield church fteeple. 
North adi of the feep of Dover caftle. 
Lydd ftee 
Tenterden i ple 
Goodhurft fteeple. 
Frant fteeple. 
The ftations on the French fide were, 
Montlambert 
Calais. 
the extremities of the bafe of verification. 
Blancnez. 
ey 
Wat 
Mount Caffel. 
Dunkirk. 
The direGtion of the fides of the triangles was determined 
by placing the theodolite over the tranfit inftrument at Green- 
h, and obferving the angle which the meridian mark 
made _ the next station 
thefe were eornee: ed, they found 37 triangles, ag 
fecvelented in Plate VIIL., ar ee and angles of which were 
obferved and calculated as 
coeuow hee ead with 
gla 27404.01 feet. 
By eae with a feel Be in 
17gI . - “ 27404 32 
Mean length = 2 27404.2 
