Barometric Pressure. 21 



" (170.) The great length of time in which the efficient 

 causes are acting in one direction, to produce the annual 

 oscillation in question, admits of a very considerable fraction of 

 the atmosphere to be transferred from hemisphere to hemisphere, 

 and to allow a range in the values of P, for instance, to the 

 large extent (as we have seen in the case of Benares), of nearly 

 an inch and a quarter of mercury, partially neutralised by a 

 fluctuation of more than half-an-inch of aqueous vapour. Thus 

 the effects are brought out into prominence, in both elements, by 

 the long-continued action of the causes ; and thus, by the study 

 in the iirst instance of the annual oscillation, we are led to any 

 easy understanding of the perfectly analogous phenomena in the 

 diurnal oscillations (or, as they have sometimes, though, in fact, 

 improperly, been called, ' atmospheric tides '), which have a 

 good deal perplexed meteorologists, but whose analysis into what 

 we have for convenience called wet and dry pressure, has happily 

 been suggested by M. Dove as affording a rational explanation. 



" (171.) To simplify our conception of the diurnal 

 oscillation, we will suppose the sun to have no declination, but 

 to remain constantly vertical over the equator. The surface of 

 the globe will then be divided into a day and a night hemisphere, 

 separated by a great circle passing through the poles, coincident 

 with the momentary horizon, and revolving with the sun from 

 east to west in twenty-four hours. The contrast of the two 

 hemispheres, both in respect of heat and evaporation, in this 

 case will evidently be much greater than in that of Art. 165, and 

 therefore the dynamical cause, the motive force, transferring both 

 air and vapour from the one to the other, will be much greater. 

 But, on the other hand, much less time is afforded for this 

 power to work out its full effect, and long before this can be 

 accomplished for any locality, the circumstances are reversed, 

 and a contrary action commences. The causes, then, and the 

 mode of their agency, are perfectly analogous, in the production 

 whether of the annual or diurnal oscillation ; but in the former 

 the feebler acting cause is aided by the very much greater length 

 of the period ; in the latter its superior intensity is in great 

 measure neutralised by the frequency of its reversal. 



" (172.) It ought to be observed, that the oscillations in 

 question are only in appearance analogous to those of the oceanic 

 tides. In the latter the tide wave is merely a circulating form 

 without any bodily transfer to any great distance. The sun's 

 heating action is not one which, destroying a portion of its 

 gravity, tends to alter its form of equilibrium, but one which, 

 leaving its gravity unaltered, tends to throw its strata by their 

 dilatation, and by the introduction of vapour from below, into 

 forms incompatible with equilibrium, and therefore necessarily 

 productive of lateral mo\ements. When anemometry is further 

 perfected, we may expect to trace the influence of this chain of 



