370 Summary of Meteorological Ohservathms at Paris in 1816. 



Barometer. 



The determination of the mean pressure o( the atmosphere 

 on efcry place of the earth is the most important object to be 

 attained by the observation of barometrical variations. In oc- 

 cupying ourselves with this inquiry we have followed the course 

 which M. Rainond has traced in the excellent work which he 

 published in ISll. All the heights have been corrected by the 

 effect of the temperature, and reduced to zero by employing 

 TTTV ^^ the coefficient of the dilatation corresponding to a de- 

 gree centigrade. Tiie observations made at the same hours 

 have been collected in vertical columns, and the averages rec- 

 koned by decades and by months. From these calculations it 

 appears, as M. Ramoiid has already deduced from his oljserva- 

 tions at Clermont, that the barometer is subject in our climate* 

 as well as at the equator to a periodical daily oscillation, the 

 operation of which, though often masked by accidental varia- 

 tions, becomes manifest when a sufficient number of observations 

 are combined to compensate the fortuitous eifects of disturbing 

 causes. It has also been found that the mercury attains its 

 greatest height at nine o'clock A.M. and that it continues after- 

 wards descending until three o'clock P.M.; when it remounts and 

 attains its second maximum at nine or ten o'clock at night, af- 

 ter which it redescends to repeat the following day the same 

 coTU'se of fluctuation. If we might be permitted to reckon from 

 the observations of one year the exact compensation of accidental 

 causes, it would seem to result from the averages of the different 

 hours exhibited in the subjoined table, that the diurnal oscillation 

 diminishes in proportion as the latitude increases. Thus under 

 the equator the e.Ktent of this sort of atmospheric tide reaches 

 to two millimetres, according to the researches of Humboldt ; 

 and three years of observations at Clermont-Ferrand (lat. 45" 47') 

 make it only about one millimetre ; while at Paris it has not 

 altogether exceeded seven-tenths of a millimetre. Ulterior ob- 

 servations will, perhaps, settle this point : those of which we 

 have here given a summary prove already, that the heights cor- 

 responding to the different hours of the day differ sufficiently 

 the one from the other, to show that the thing, as M. Ramond 

 lias remarked when proposing to determine the mean pressuie 

 of the atmosphere in a given place, cannot be arbitrary. 



Table 



