Indian Meridional Arc, 
343 
1831 .] — ~ — — - ■— « 
course necessary that not only the measure of all three angles of each triangle 
shall be taken, but that the triangles shall be knit together by numerous cross sights 
as checks : wherever a hexagon or pentagon is formed by the meeting of five or six 
triangles, an additional check upon the whole is gained : the average angles should 
be as near to 60 degrees as the nature of the country will admit, (fig. 1.) 
The English and French methods of taking angles are distinguished from each 
other by the same peculiarity as was pointed out in their manner of measuring a 
base-line: the one may in fact be called the practical, and the othei the theoretica 
method: the circle of Troughton, always employed in English surveys, by con:ti c 
tion maintains a perfect horizontally, N and depends upon the beauty of its divist 
alone for the true measurement of the angle. The repeating circle o oua, u' 
by the French academicians, on the contrary, seeks to overcome eriorsc 
by bringing the whole circumference of the instrument in succession wi 
measured angle. It also necessitates a theoretical reduction of t.acli nicasu e fc 
to the plane of the horizon. Again the English surveyois geneially place t 
over the centres of their stations ; the French almost invariably had a ’ 
both for the eccentricity of the signals, and for that of the obsei v or, to say not nng 
thesmaller one, forthatof the underand upper telescope of their repeat^ g 
Delumbre, where it was possible, obviated the correction foi the o >ser J , 
placing himself in the direction of the tangent to a circle suppose o x* 
about the triangle under measurement, on the principle of ang es . 
ference, having the same base, being equal : A being the true centre o a * 
the observer placed himself at D in a tangent to the circle AB i ire 
the line AD being found by the equality of the angles CBA an 
of which had been previously determined, (fig. 2.) tnirpther 
Tl,c French e J,,Uy took twenty repetition, of each 
with ten measurements of the zenith distances, 01 P, of his circle, 
the plane of the horizon , whereas Colonel mg , e only . 
could rest well satisfied with four or five i radii 0 . of 1 or 2 seconds, 
With all the accuracy of both methods however, i ■ rejf i s tcrs of terrestrial 
and sometimes even 8 or 10, are observable in t e e ^ ^ ^ e j r accuracy 
an gles, and lateral refraction will always tend to oppo.' 
hardly short of the former amount. thc j mlian Surrey, 
The superiority of the new circle made by r0Up 1 &n g]e being read on 10 
consists in its having 5 microscopes, which allows of ^ prg j on 0 f the telescope; 
different parts of the divided circumference, with one tQ c i,ange the 
in the beautiful contrivance for lifting and turning t ^ ^ ^ j n jj en 0 f a plumb 
position of the zero ; and in the substitution of a nuc -pj, esc improvements 
!me, to adjust the instrument centrally over the 8tatl0 “ va i ua ble, in the saring 
Wl11 be acknowledged by the practical surveyor to e caU8e s of danger to 
^ time and trouble to himself, as well as the removal of many 
hls instrument. . 0 f triangles arc dialing™ h- 
The points determined for the positions of t ie serl ^ t j u . se ar e found to require 
from the theodolite station by means of signals, a “ ^ t tJirow . n up on them, 
many precautions in practice : first, as to the direct ^ ^ visible— sometimes the 
as it is sometimes only the illuminated portion w ^ ^ ^ ^ ^ne to ^bis cause. - s e- 
darkened : Mechain’s angles have differences of ^ or con es of straw work 
coridly, as to shape and size : In France, pyramids o ^ ^ nfl( atha nd ; night 
*ere used where natural objects, such as towers am the horrors of 
fi >gnals could not be employed, because of the revo ^ muc j 1 ^ intrepidity. 
wl *ich the labours of the academicians demanded cau 
