776 



Paris to Marseilles, Murviedro, Madrid, and 

 Corogne, before I embarked for Cumana, I 

 could have but little confidence in my deter- 

 mination. Fortunately, I can now substitute 

 another far more precise. MM. Boussingault 

 and Rivero, before they embarked for LaGuayra, 

 compared, conjointly with M. Arago, two excel- 

 lent barometers of Fortin, with that of the ob- 

 servatory of Paris. The two barometers have 

 preserved the same difference which they had 

 in Europe. M. Boussingault found, at the level 

 of the ocean at La Guayra, the mean of the 

 maxima and minima observed during twelve 

 days, to be 760.1 7 mm (at the temperature of 

 zero). M. Arago, from nine years of observa- 

 tions at Paris, estimates the mean barometric 

 height (reducing it to the temperature of zero, 

 and the level of the Ocean *) at 760.85 mra . 

 The difference of the two mean heights, deter- 

 mined as it were by the same instrument, rises 

 consequently to 0.68 mm . We must not forget 

 that in the torrid zone, accidental causes have 

 also an influence on the mean height. I have 

 tried to estimate carefully the probable limits 

 of those changes ; and it results from the ana- 



* Mean bar. height at Paris, (Royal Observatory), 

 755.43 mm . Difference between the Observatory and the 

 port of Havre, according to a year of correspondent obser- 

 vations made with compared instruments : 5.42 inm . 



