184 On the Arcs perpendicular to the Meridian^ 



The observations of the 6th and 7th Sept. had made the difference 

 of longitude between Isson and Mont Colombier 10' 22",7i ; the 

 signals given on Mont Colombier on the same nights as the 

 observations between Isson and Mont Colombier, and observed 

 simultaneously there and at Geneva, made the difference of 

 longitude between those two places V 35",11. The sum, 

 10'22",71 + r35"ll = ir 57",82, will therefore be the dif- 

 ference of longitude between Isson and Geneva, independently 

 of any error which may have existed in the comparison of the 

 clock at Mont Colombier with sidereal time. There then 

 remains to be elicited from the preceding observations the arc 

 between Geneva and Mont Cenis, in which Mont Colombier 

 serves again as a station of transmission ; and taking, as before, 

 the observations of those nights only when the communication 

 with Mont Colombier was effected both from Geneva and from 

 Mont Cenis ; and when, by such means, the sidereal time at 

 Geneva became immediately compared with the sidereal time at 

 Mont Cenis, we have, by the observations of the 5th and 7th of 

 September, between Geneva and Mont Colombier, 1' 35",36, and 

 between Mont Colombier and Mont Cenis, 4' 44",12, whence 

 the difference of longitude between the observatories of Geneva 

 and Mont Cenis appears 3' 08", 76. 



In September, 1821, MM. Plana and Carlini ascertained 

 the amplitude of the arc between the Hospice on Mont Cenis 

 and the observatory at Milan, by means of thirty signals made 

 on the Roche Melon, of which ten were on the night of the 1st, 

 ten on that of the 2d, and ten on that of the 3d September ; 

 the results were as follow : — 



Mean of . .30 signals .... 



Reduction to the Dome of Milan .... 



Difference of longitude between the observatory on 



Mont Cenis and the Dome of Milan . . . 9' 00",806 



For reasons that are not given, but probably on account of 

 the small extent of the arc between Geneva and Mont Cenis, 

 the determination of its amplitude, accomplished, as we have 



