French National Institute. 277 



results, obtained by processes so contrary, and drawn from 

 phaenomena so different, are proofs to which the most ob 

 stinate sceptic can have nothing reasonable to oppose. 



This important result is not the only merit of M. Ra- 

 mond's memoir. We there find means for distinguishing 

 circumstances which are favourable or contrary to this kind 

 of observation. The author ranges them under three dif- 

 ferent titles : the Influence of the Time of the Day ', the In- 

 fluence of Stations ; and the Influence of Meteors. That of 

 the time of the day occasions the altitudes observed in the 

 evening and morning to be always too small ; whence it 

 follows that it is always best to make observations about the 

 middle of the day : and this is easily attained. The influ- 

 ence of stations is not less real, but it is less easy to be 

 guarded against. The rule to be followed is, that the port- 

 able l)aromeler and the barometer of comparison should be 

 in stations where the local circumstances are perfectly simi- 

 lar. Great distance is not always an obstacle : thus M. Ra- 

 mond remarked that his observations at the Pyrenees, com- 

 pared with those continually made by M. Bouvard at the 

 Imperial Observatory, present a regular agreement, whilst 

 these same observations of M. Bouvard compared with 

 those of M. Ramond made at Marli-la-ville, indicate from 

 one day to another, differences of from ten to eleven metres 

 in the relative height of the two stations ; whence we con- 

 clude, that the use of the barometer for measuring height^ 

 that differ but little is rather uncertain, when the two st.ir- 

 tions are on a plain. 



As to the Influence of Meteors, it always acts in the same 

 sense ; it makes the heights appear too small, and we ought 

 to reject all observations made on a stormy day. From all 

 these considerations, it follows, that in order to measure the 

 height of a mountain more accurately, we must not indif- 

 ferently take a mean between observations taken at different 

 seasons, and at different times of the day, as by this means 

 we should evidently be liable to find the heights loo little. 



We shall not mention the grand work which Messrs. 



Biot and Arrago have just concluded, upon the aflinitirs 



S 4 between 



