182 On the Arcs perpendicular to the Meridian^ 



them Is consequently determined by the mean of not more 

 than ten comparisons : but as all the circumstances attendant 

 on the observations of that night were very favourable, and as 

 the extreme discordance of the particular results amounts only 

 to 0",33, the determination may deserve to be received with 

 the satisfaction which is expressed respecting it by the obser- 

 vers themselves. The difference of meridians thus deduced is 

 G' 28",341, which, diminished by 5",247, the reduction of the 

 observatory at La Jonchere to the neighbouring trigonometrical 

 station on thePuy de Sauvagnac, makes the difference of longi- 

 tude between the observatory of St. Preuil and the station at 

 Sauvagnac, 6' 23",094. 



In August, 1823, the same observers determined the differ- 

 ence of longitude between the observatory of La Jonchere and 

 a temporary observatory at the village of Solignat, by signals 

 made, on the 19th and 22d, on the summit of the Mont d'Or, ■ 

 ten signals on each night : the results were as follow : — 



August 19, mean of 10 signals . * . . 6' 49",808 



22, 10 signals . . . . 6' 50",17l 



Mean of 20 signals .... 6' 49",9895 

 Reduction to the Trigonometrical Station on the Puy 



d'Isson, adjoining the observatory of Solignat . — 3'',846 



Reductionto the Trigonometrical Station of Sauvagnac + 5',247 



Difference between the meridians of the Trigonome- 

 trical Station of Isson and Sauvagnac . . 6' 51",391 



On the 6th and 7th of September, 1822, signals made on the 

 mountain Pierre-sur-Autre, and observed at Solignat by MM. 

 Brousseaud and Nicollet, and at a temporary observatory on 

 Mont Colombier by M. Carlini, one of the members of the 

 Austro-Sardinian Commission, gave the following results: six 



summit would be reciprocally visible without difficulty from the mill. 

 It appeared, however, subsequently, that this was very rarely the case, 

 and only in certain states of the atmosphere ; as, during the forty-eight 

 days which M. Nicollet remained at St. Preuil, the Puy Cogneux was 

 only once seen, and that for an instant, at six in the morning, the 

 sky being slightly clouded. On the night on which the signals were ob- 

 served, the sky was also clouded, and the horizon appeared by no means 

 favourable ; nevertheless the signals were very distinctly visible, although 

 . made with only a quai'ter of a pound of powder, being half the quantity 

 that had been tried before. 



