HORARY VARIATIONS OF THE BAROMETER. 42 J 



causes u-hicJi wflvencc, as icell the distrihtilion of the annual mean temperatures as 

 the distribution of the t,um (tfthe tLVii)cratures over the different hours of the day. 



Let us' go a step further, if you please, and state more precisely the effects 

 due to this cause, inasmuch as we know it : the greater the heat, (the habitual 

 heat due to the latitude,) the greater should be the extent of the oscilhitions ; 

 the warmer the season in comparison with other seasons of the same year, the 

 greater will be the amplitude of the oscillations during that warm season ; the 

 more rapid the ascension of the thermometer during the day, as, for example, 

 in certain countries of low latitude, where, in a few honrs, there is a tran3ition 

 from a temperature below zero to a heat of twenty-five or thirty degrees above, 

 the more considerable will be the oscillation ; the clearer the sky and more trans- 

 parent the air, the more rapidly will the earth be disposed to grow warm under 

 the action of the solar rays, and the greater and more rapid will be the oscillation. 



For the part of a continent having its general slope directed toward the rising 

 sun, the ascent of the mercury in the morning would be more rapid, and it would 

 attain a limit which it would not reach if that part of the continent Avere simply 

 level or nearly so. For the same tract of country, thus strongly incl ned to the 

 east, the fall of the mercury in the afternoon would be also much greater than 

 for a horizontal region, since what would be gained from the incLnation of the 

 surface in the morning would be lost in the evening. 



All these effects are influenced also by the alternation of the breezes of the 

 land or water, if the position is near the shore of an ocean, great lake, &c. 

 And since these breezes, according as they arrive from the east or west, counter- 

 act or favor the calorific action of the sun on the surface, they would differently 

 modify the oscillations of the mercury in the barometer. This your 77ionograph 

 conclusively shows. Nor would the direction of the trade-winds, on their part, 

 fail to exert an influence on the amplitude of the phenomenon ; not so much in- 

 deed as altogether to mask it, but sufficiently to extenuate certain details of time 

 and extent, which theory alone coiild then supply and place in relief. While 

 the theory here presented seems to account satisfactorily for the horary variations 

 in an extensive and level plain, it would lead us to expect that, since the degree 

 of caleftiction through ;,he presence of the sun is the cause of these variations, 

 the latter will be proportionately less as the former also is less and the plain more 

 elevated above the level of the sea ; in other words, the greater the altitude of 

 the country the smaller will be the oscillation of the mercury. This prevision 

 of the theory is confirmed by the observations reported in the ninth page of 

 your memoir. 



The horary oscfllation for a small island lost in the bosom of a vast ocean 

 must be difficult to recognize and measure, the effects of the heat communicated 

 to the soil being, as it were, absorbed and disappearing in the general movement 

 of the air on that ocean. An analogous result, though even less distinct, would 

 be produced in the case of a peak isolated in the midst of a great plain. 



In recapitulation, I would say : if the phenomenon of the horary variations 

 has its cause, as I am satisfied is the case, in the inertia of the air, that is to 

 say, in the reluctance with which it enters into motion, whether upward or down- 

 ward, when the surface oii which it rests begins to grow warm or cold, observa- 

 tion ought to lead to the verification of the following facts : 



The warmer a country in consequence of its latitude, the more decided will 

 be the oscillations uf the barometer, and the more feeble in proportion to the 

 elevation of the country above the sea Their regularity, that is, their sym- 

 metry in relation to the meridian, will be modified through the aspect of the 

 country as turned towards the east or west, by the manner in which it presents 

 itself to the general current of the trade-winds, the breezes of the land or sea, &c. 



Beyond the intertropical zone, the hours to be chot=en for the observations 

 should vary according to the seasons. The extreme hours should be more re- 

 mote from m.dday in spring and summer, and should be nearer to it in autumn, 

 but especially in winter. 



