now measvring on the Continent 183 



signals were made on each night, but five only were seen from 

 both stations on the night of the 7th : — 



Sept. 6 . . .6 signals ♦ . . .10' 18",90 

 Sept. 7 . . .6 signals . . . .10' 18 ',81 



Mean ofthe two series, 11 signals . . . .10' 18',86 

 Reduction to the station of Isson . . . . + 3 ",85 



Difference of lon2:itude between the station at 



Isson and the observatory on Mont Colombier . 10' 22'',71 



At the same period that the difference of longitude was thus 

 ascertained between Isson and Mont Colombier, signals made 

 on Mont Colombier, and having the time of their apparition 

 noted by M. Carlini, were also observed at Geneva by MM. 

 Pictet and Gautier, giving the difference between the meridians 

 of Mont Colombier and the observatory of Geneva: whilst other 

 signals made on Mont Tabor, and observed by M. Carlini on 

 Mont Colombier, and by M. Plana at a temporary observatory 

 at the Hospice on Mont Cenis, gave the difference between the 

 meridians of the temporary observatories of Mont Colombier and 

 Mont Cenis, at which those gentlemen were respectively esta- 

 blished. The following are the results of these several obser- 

 vations : — 



Between Mont Colombier and Geneva. 

 Sept. 4 . . .6 signals .... 1' 35",27 

 Sept. 5 . . .6 signals .... 1' 35'',60 

 Sept. 6 . . . 6 signals . . . . l' 35",10 

 Sept. 7 . . .6 signals .... 1' 35",12 



Between Mont Colombier and Mont Cenis. 

 Sept. 5 . . .6 signals .... 4' 43'', 96 

 Sept. 7 . . .6 signals . ... 4' 44' ,28 

 Sept. 11 . . . 6 signals .... 4' 43 ',01 



Mean of three series, 18 signals .... 4' 43",75 

 The unfavourable circumstances of the weather having caused 

 a more than ordinary uncertainty to attend the determination of 

 the time at the observatory on Mont Colombier, elevated 1434 

 metres above the sea, MM. Brousseaud and Nicollet have deemed 

 it preferable to substitute Geneva as the eastern extremity of the 

 arc, of which the western is the trigonometrical station at Isson, 

 and to consider Mont Colombier as a station of transmission only. 



