430 SHORT MEMOIRS ON METEOROLOGICAL SUBJECTS. 



our knowledge of storms an idea that he had already, in 1861, published 

 in more general form in Sillimau's American Journal of Science.* 



In reference to the currents and pressures of the air in cyclones in low 

 latitudes, we possess an excellent mathematical investigation by the 

 genial Danish engineer Colding, well known as one of the founders of 

 the mechanical theory of heat. Since, however, his work appeared in 

 the Banish language, in the Proceedings of the Academy of Copenhagen, 

 certainly but few meteorologists outside of Scandinavia will have become 

 acquainted with it, and we will, therefore, pretty fully reproduce it in a 

 later number of this journal. [See Paper I, which translation was made 

 in 1873 directly from the original, with but few corrections, suggested by 

 a subsequent comparison with Hann's German version. — C. A.] Colding 

 considers the tropical cyclones as true whirls, in which the air rotates 

 about a vertical axis, and the differences of pressure from the center to the 

 edge of the whirl computed by him under this assumption agree perfectly 

 with the observed values. 



Of course we cannot assume that there exists in our whirlwinds any 

 rotation of the mass of air in the sense that it several times or even once 

 revolves about the center: the air moves apparently in spiral paths to- 

 ward the center. This, however, is no reason why, even in the smallest 

 portions of the curved path, the centrifugal force should not have its influ- 

 ence according to the prevailing radius vector and velocity- [Buchan is 

 therefore in error in denying the influence of centrifugal force. HandBooTc 

 of Meteorology^ 2d edition, p. 281.] The analogous influence of the earth's 

 rotation is to be added thereto, and the consequence of both these in- 

 fluences is that the atmospheric pressure at the earth's surface dimin- 

 ishes toward the center of the storm-area. Especially does the influence 

 of the earth's rotation explain simply and naturally the frequently im- 

 mense extension of the area of much diminished pressure in the higher 

 latitudes, while it is impossible to explain barometric minima of such 

 extent by means of an ascending current of air or by precipitation.t 



In order to make somewhat clearer the influence of the rotation of the 

 earth upon the origin of barometric differences in a mass of air in mo- 

 tion, I will assume a definite case, and for simplicity assume a storm- 

 wind advancing in a straight liU'C. 



Let us assume a southwest storm, extending from Northern Scotland 

 to the foot of the Alps, or over about ten degrees of latitude, as often 

 occurs. The velocity of the wind at a short distance above the earth's 

 surface will, at 30 meters per second, not be assumed unusually great. 



* [Note by the Teaxslator.— See some account of Ferrel's works iu the note on 

 page 72.] 



t In reference to this see R. H. Scott's memoir " On Recent Progress in Weather Knowl- 

 edge." The magnitude of the depressions affords an argument against their being 

 simply due to the condensation of vapor, for on November 22, 1869, barometrical read- 

 ings were reduced to the extent of nearly an inch from what they had been on the 2l8t, 

 over an area of about 200,000 square miles. On January 20, 1873, the deficit of atmos- 

 pherical pressure amounted to about ^j- of its total amount over the United Kingdom. 



