SHORT MEMOIES ON METEOROLOGICAL SUBJECTS. 429 



I have already remarked (vol. x, p. 12) that I believe that the greatest 

 part of the barometric depression in storms must be explained in some 

 mechanical way. This is a return to the older theory, and one could in 

 this respect consider me as reactionary. 



It is remarkable that the newer theory of storms no longer utilizes 

 the centrifugal force, and that it entirely overlooks a further influence 

 similar to centrifugal force, although it i)erfectly recognizes its necessity 

 in deriving the direction of the wind. The older theory utilized the 

 centrifugal force to explain the diminution of pressure in the center of 

 the whirl ; but as yet I have nowhere found, at least in German writings, 

 that any one has shown us, not even approximately, how great the baro- 

 metric difl'erences resulting from the centrifugal force of the rotating 

 masses of air can be, or whether it suffices to explain the greater part 

 of such depressions as occur in cyclones. 



It is here the place to promptly say that one should never attempt to 

 explain all barometric differences as consequence of the movement of the 

 air. This extreme is to be avoided precisely as is the other. Our view is 

 only that the larger part of the observed great differences of atmos- 

 pheric pressure in a whirlwind, or even in a directly progressing current 

 of air so far as any exist, is a consequence of the movement of the air. 

 This latter movement is, however, itself the result of an original slight 

 but more general variation of pressure. 



We have quite recently heard Reye defend the view that a con- 

 densation of aqueous vapor over an extended portion of the earth's 

 surface must cause a fall in the i^ressure of the air in consequence of 

 the increase of temperature that takes place at the place of condensa- 

 tion. From a theoretical point of view, but little objection can be 

 brought against this view. But it is impossible even to estimate, how 

 great the consequent actual fall in pressure may be even under any given 

 circumstances; and since we know from observation that where the most 

 rain falls the irregular variations in the barometer are not great enough 

 to disturb the regular diurnal period, whose amplitude in favorable cases 

 amounts to 3 millimeters, therefore we may conclude that the influence 

 under discussion must have a less eflect than this. On the other hand, 

 from known principles of mechanics, w-e must necessarily conclude that 

 the movement of the air once set up must of itself bring about differ- 

 ences of pressure that are ten or fifteen times greater. We can there- 

 fore scarcely be in doubt as to how we are to explain the great variations 

 of pressure observed in the extra- tropical regions. 



At the same time that I was writing out for this journal my thoughts 

 adverse to pushing Espy's theory too far. Professor William Ferrel, of 

 Washington, published a memoir (American Journal of Science, vol. 

 viii, Nov., 1874) in which he endeavored to express mathematically the 

 variations of pressure occurring in storms as a consequence of the centri- 

 fugal force and of the earth's rotation. In this memoir, however, he 

 merely especially elaborates and accommodates to the present state of 



