180 On the Arcs perpendicular to the Meridian, 



Fiume on the Adriatic, by an uninterrupted series of one hun* 

 dred and six triangles, of which ninety were executed by the 

 French officers, and sixteen by the Austro-Sardinian commis- 

 sion. The excellence of the instruments that are now em- 

 ployed in geodesical operations, and the great experience that 

 the French officers in particular have had in them, might be a 

 sufficient warrant that the terrestrial measurement has been 

 executed with the requisite exactness. MM. Brousseaud and 

 Nicollet have, however, placed this beyond question, by employ- 

 ing as bases of verification the sides of the triangles of the great 

 chain of the meridian, which are common also to the triangula- 

 tion of the parallel resting on the base near Milan *. Contemplat- 

 ing the prolongation of the arc from Fiume to Orsowa, it has been 

 considered preferable to calculate from the triangles the length 

 of the arc of the parallel of 45° 43' 12", instead of that of forty- 

 five degrees exactly, as by that means a portion of the Turkish 

 territory between the thirteenth and seventeenth degrees of 

 longitude east of Paris is avoided : the parallel of 45" 43' 12" 

 is also that which intersects the greatest number of triangles. 



The operations for the measurement of the celestial arc 

 were not commenced until 1821 ; the method adopted for this 

 purpose was, by the explosion of small quantities of powder, 

 on summits of such elevation, that the flash might be simulta- 

 neously seen at two stations, including between them consi- 

 able differences of longitude ; when, the instant of the appa- 

 rition being registered at each station in its own sidereal time 

 accurately known, the comparison of the registers might show 

 the difference of time, or, in other words, the celestial arc, 

 included between the two stations. That this might be accom- 

 plished with the greater certainty as well as exactness, several 

 explosions were made in the same night, at nearly equal and 

 appointed intervals of time, and the signals were also repeated 

 on several successive nights. By this method of proceeding, 

 the total arc, the amplitude of which it is desired to measure, 

 becomes divided into several portions, each of which is suc- 



♦ Since the above was written, we have learnt that a base of above 

 14,000 metres was measured in the last summer by Colonel Brousseaud 

 near Bourdeaux, and is to be corrected in the next summer by a chain of 

 triangles with the western extremity of the arc at Marennes, as an addi- 

 tional verification. 



