450 
NATURE 
| April 10, 1873 
CAPTAIN PERRIER’S GEODETIC OPERA- 
TIONS IN ALGERIA 
ip idea of prolonging the French arc of meridian to 
Sahara by the direct trigonometrical junction of 
Spain and Algeria, an idea of undoubted scientific value, 
presented itself to the mind of Captain Perrier when he 
was collecting the preliminary materials for the survey of 
Algeria. That survey was begun at the same time as the 
conquest, in the middle of military operations. 
The design was to calculate two great lines conforming 
to parallels, and transversely cut by three meridians ; 
quadilaterals would be thus formed, completed by triangles 
of the first order. Only a linear chain was however drawn, 
except in the mountainous regions where the operations 
would have been attended with too many difficulties, 
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ALGERIA 
Map showing prolongaticn of Meridians trom Spain to Algeria 
This chain, connected with the sea by three excellent 
bases, would serve the purpose of adjusting and arranging 
the detailed operations. French geodesy thus measured 
an arc oflatitude cutting the Paris meridian and extending 
from Morocco to Tunis, with a length of 990 kilometres. 
The chain of first-order triangles may be divided into 
two parts, the first from Blidah to Tunis, measured by 
Captain Versigny, the other more recent measured by 
Captain Perrier, 
_The admirable choice of triangles, stations, and 
signals is noteworthy. Those signals have been built by 
the observers themselves, as there were no steeples. 
The precision obtained is remarkable. In the sum of 
the three angles of any triangle, the error is about 3/12 
(centesimal seconds) * in M, Versigny’s operations (who 
mace use of Gambey’s repeating circles); the error is 
= = Ce > i 
One centesimal second = 0’ "33 ordinary second, 
about 3/07 in M, Perrier’s operations, who made use 
of Brunner’s excellent asimuthal circles. In order to 
measure the bases, the system of M. Porro, an Italian 
engineer, has been employed, in preference to the old 
method of Borda, and it has been followed by the best 
results. 
Colonel Levret proved (in 1869), by very exact calcu- 
lations, that the passage by Gibraltar could be dispensed 
with, and that it would be possible to communicate 
between Spain and Algeria, in spite of the immense 
distance between the two continents. The entire certainty 
of that possibility has been proved by Captain Perrier, 
who has pointed out in a precise manner the names and 
positions of the visible summits and the length of the 
sides of the new chain. 
It was only on October 18, 1868, that he managed to 
perceive the Spanish shore: he saw it from Seba Chioukh, 
near the mouth of the Tafna, very distinctly and with- 
out the glass. A serrated ridge was to be seen in 
the distance, toward the north-west, with five promi- 
nent summits. The distinctness was so perfect, that he 
could discern with his naked eyes the different parts of 
those mountains, those which were in the shade and 
those illuminated by-the sun. He thus measured azi- 
muths with the summit of the Tessala, the zenithal dis- 
tances of the two highest points of the ridge, and the 
zenithal distance of the horizon of the sea. 
After his return to France, he compared his meastire- 
ments with the survey of Spain, made by Colonel Cuello, 
and concluded that he had observed the Mulahacen of 
the Sierra Nevada, and the peak of Sagra of the Sierra 
Sagra, the highest points of the Sierras of the Province of 
Granada. 
In Spain the peaks of Sagra can be seen from Mula- 
hacen ; those mountains belong to the primordial geodetic 
chains of the Iberic peninsula. In Africa the points of 
the quadrilateral (Bem Saabia, Tessala, Filhaoussen, and 
Nador) are reciprocally visible, and the three last are 
situated in the primordial chain of Algeria. 
With these points, Captain Perrier was enabled to form 
a chain common to Europe and Africa. Leaving the 
station of Seba Chioukh as superfluous, as well as 
the direction of Nador-Sagra as being too close to the 
horizon of the sea, he delineated that immense pentagon 
formed by the five summits of Mulahacen, Sagra, Bem 
Saabia, Filhaoussen, and Nador, every side and diagonal 
of which, except one, are the directions that are to be 
observed. He has even calculated and valued in round 
numbers the length of the sides of this geodetic chain, 
He has found— 
Metres, 
Mulahacen-Filhaoussén 273,400 
rp Nador 314,500 
ft Bem Saabia 272,200 
Sagra-Filhaoussen . 313,300 
»» Bem Saabia . 271,000 
The length of the terrestrial sides are— 
Metres. 
Bem Saabia-Filhaoussen 109,800 
45 Nador 104, 800 
Mulahacen-Sagra 113,900 
With those approximate numbers (and valuing at o'o8 
the coefficient of refraction) he has calculated the altitudes 
of the two Spanish mountains. The measures thus ob- 
tained differ little from the real numbers— 
Metres, 
Mulahagen.. « « 3 « » @ 004 
Sagrais wy 2,398 
given by Colonel Cuello, and thus furnish a new veri- 
fication. 
As the geodetic operations are being continued with 
great activity in Spain and in Algeria, we may hope that 
in a few years the geodetic bases of Great Britain, of 
France, Spain, and Algeria, reduced to the same unit of 
