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259 JOURNEY ,OF MAPERTUIS.
greateft heat and cold that are experienced at Paris; bringing into twelve hours an
equality of viciflitudes with what are felt by the inhabitants of the temperate zones in the
{pace of a year.
We chofe to feruple the meridional direétion of our heptagon. ‘This, as has been
noticed before, had been determined upon Kittis, by a number of obfervations on the
aflage of the fun over the verticals of Niemi and Pullingi; and it was not to be ap-
prehended that our figure fhould have received any alteration in its direction, feeing the
fmall number of triangles of which it confifted, and the clofenefs with which the fum of
the angles of our heptagon approached to goo’. Neverthelefs we were inclined to
sake the trial of the direction at Torneo.
For this purpofe a different method to that which had been ufed at Kittis was adopted ;
this cenfifted in obferving the angle between the fun when on the horizon, and fome
one of the fignals, with the hour at which the angle was taken. ‘The three obfervations
that we made, gave for a mean of the diretion by this procefs, within 34” of what we
had determined by the obfervations from Kittis.
Every part of our work having thus been repeated, nothing remained but to examine
the primitive conftruétion and divifions of our fextant. Although we had no ground to
doubt its correétnefs, we undertook to try it in the interval between the time at which
the weather would allow of our departure ; and this trial, from its fingularity, from its
demonttrating how much our inftrument was to be relied upon, as well as to fhew the
means to be ufed for difcovering its being out of order, ifit fhould be defirous that I
fhould mention it.
On the fourth of May upon theice of the river we meafured a diftance of 380 toifes
1 foot 3 inches which was to ferve asa radius. And upon meafuring this twice over we
found no variation. Two firm ftakes with two fights, in a line drawn perpendicularly at
the extremity of this diftance, were fixed ; and having meafured the diftance between the
centres of the two fights, this diftance was 36 toifes, 6 inches, 6 lines 4, which was to
ferve as a tangent.
The fextant was placed horizontally ina room upon two firm ftocks, fupported by an
arch in fuch manner that its centre was exactly at the extremity of the radius of 380
toifes, 1 foot, 3 inches; and five different obfervers having taken the angle between the
two fights, among whom the greateft difference was not two feconds, the mean of
which being taken, the angle between the two fights was 5° 29/ 487 95. by calculation
the angle fhould have been 5° 29! 50”, that is to fay, it differed from the angle ob-
ferved by1’.%.-
It will no doubt be thought furprifing, that a fextant of 5° 29’ 563”, in a climate fo
temperate as that of London, and divided in a chamber which certainly was not cold,
fhould be found precifely of the fame angle at Torneo when we tried it. The parts
of the fextant were certainly contracted by the cold in this laft experiment; but one’s
furprize will vanifh on confideration that the inftrument being made wholly of the fame
matter, its parts would contraét proportionally, and confequently its figure remain the
fame, which was the cafe.
Having thus found fuch a wonderful exa€tnefs in the whole arc of our fextant, we
wifhed to know if the two degrees. of its limb which we ufed the one for J, the other
for a were perfeétly equal. M. Camus, whofe ingenuity had already been fo ufeful
to us on feveral occafions, procured us the means of making this comparifon with all
poflible exactitude; and/having compared together our two degrees, the mean of the
ebfervations taken by five obfervers gave 1” more for the degree of the limb ufed in
taking é than that ufed for @.
We
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