418 



HOEAEY VARIATIONS OF THE BAROMETER. 



of winds whose course approximates to the north and northwest, the differences 

 of the means generally much exceed the three-tenths of a millimetre yielded 

 by the supposition of a stratum of water two millimetres in height. In ob- 

 serving with what regularity the phases of the phenomenon of the diurnal period 

 present themselves even in the latitude of Paris, we can feel no surprise at the 

 course, worthy of being styled chronometric, which the same phenomenon ex- 

 hibits in tropical regions, and which led Hnmboldt to sa^; that "no atmospheric 

 circumstance— neither rain, nor fair weather, nor wind, nor tempest — affects the 

 perfect regularity of these oscillations under the tropics; but that they subsist 

 alike at all times and in all seasons." 



Humboldt has given the law of these variations under the torrid zone; desig- 

 nating by Z the mean height of the mercury for the whole day, or, what is the. 

 same thing, the height at mid-day, we have: • 



-The oscillation at 9 o'clock in the morning, equal to Z -\- 1™™. 1280 



That of mid-day Z 



That of 4 o'clock in the evening Z — . 9024 



That of 1 1 o'clock in the evening Z + . 2256 



And, finally, that of 4 in the morning Z — . 4512 



For, as Humboldt tells us, he had observed that the ascensional movement, 

 which is manifested from five o'clock in the evening, and which attains its ap- 

 parent maximum towards ciine, ten, or even eleven o'clock in the evening, ceases 

 towards this last hour, and is replaced by a very slow fall of the mercury, a fall 

 which is continued until about four o'clock in the morning, when it is at 0"^™.4512 

 in relation to the mean height of the day. 



The explanation which we have given of the diurnal period very readily 

 accounts for this last phase of tbe phenomenon. There must, in effect, be a 

 moment when the column of air, which is condensed on approaching the surface 

 of the earth, has lost almost all its velocity of descent, and when, consequently 

 it exerts on the barometric cistern no other action than that wliich results from 

 its weight. There is no reason, on the other hand, why dew should cease fro^ 

 forming and from replacing the watery vapor which pressed upon the barome- 

 ter, by liquid water, which exerts no pressure on that instrument. There 

 may well, therefore, be a subsidence of the mercury, having its point of de- 

 parture in the neighborhood of midnight, and continuing slowly until the mo- 

 ment when the morning sun begins to warm the surface of the earth and convert 

 the dew into vapor. 



In a very remarkable memoir, in which Ramond has treated of the question 

 of the horary variations of the barometer, we find a table which gives the results 

 of 2,065 observations made at Clermont, a table which we here present, still 

 retaining the notation Z for the height of the barometer at noon, or the mean 

 height of the day : 



Morning. 



Noon. 



Afternoon. 



Evening. 



Depression. 



Ascension. 



Spring .... 

 Summer. .. 

 Autumn. .. 



Winter 



Z + 0™°!. 39 

 Z + .32 

 Z + .33 

 Z + .37 



Z 

 Z 



z 

 z 



Z — ©mm. 72 



Z — .56 

 Z — .41 

 Z — .36 



Z -f .33 



Z -f .47 

 Z -f .36 



imm. Jl 



.88 

 .74 

 .73 



1™™. 05 

 .89, 

 .88 

 .72 



It w^oiTld result from this table that, in our climates, the depression of the day 

 is reduced to the half of that which is observed at the equator. Another con- 

 sequence to be drawn from the same table would be this : that, with us, the 

 ascension of the evening is very neai-ly equal to the depression which preceded 

 it, while under the tropics these quantities differ from equal to double. 



But here it is proper that an important remark should be made: 'the above 

 table, as well as those which precede it, is the result of observations made iu 

 France at the same hours of the day, but at periods of the year wholly different. 



