April 2j, 1892] 



NA TURE 



595 



north-westerly current prevails. The heat of the sun, or dif- 

 ference of temperature between equator and pole, is the cause of 

 this general circulation of the atmosphere, as Dove maintained; 

 the deviation of the wind from the direction of the meridians 

 arises from the rotation of the earth. 



This whole theory of the circulation of the atmosphere, which 

 Ferrel deduced by mathematical means, differs considerably 

 from Dove's, but shows that Dove's fundamental idea was sound 

 and worthy of respect. 



Ferrel's investigation certainly remained long unknown in 

 Europe, and when at length it became known, it was only 

 received with a purely theoretical interest. It seemed to have 

 no connection with the present almost exclusively interesting 

 question of the origin, development, and propagation of cyclones, 

 and so it was in fact set aside ; although from that time people 

 frequently referred to it, and began to pay more attention to the 

 currents of the general circulation. But it was only in 1885 that 

 Sprung in his treatise on meteorology could write : " And so the 

 conviction is now often expressed that we have gone too far in 

 accrediting the individual systems (cyclones and anticyclones) 

 with the sole control over the motions of the air in higher lati- 

 tudes " ; and he then sets up a system of general circulation based 

 on the same principles as those of Ferrel. Sprung's system cor- 

 responds tolerably well to Ferrel's, but he also finds in the trade- 

 wind zone that the wind direction at a mean height deviates from 

 the direction of the trade, namely south- east, over the north-east 

 trade-wind. At great heights \he westerly winds prevail from 

 the equator to the pole, without passing to calms in the horse- 

 latitudes. At the equator there exists an actual conical calm- 

 zone pointing upwards, in which only ascending air-currents 

 prevail up to the highest altitudes. 



Although, owing to the esteem which Sprung's treatise every- 

 where found, this theory of the general circulation of the atmo- 

 sphere continued to spread, it did not arouse great interest, 

 because it did not take special account of the influence of these 

 general currents on the formation and propagation of cyclones, 

 which justly continued to attract the most general attention. 



This influence of the general atmospheric circulation was first 

 insisted upon by the celebrated physicist and mechanician, 

 Werner Siemens, in 1886. He considered the origin of the 

 genera! circulation of the atmosphere from the great and fruitful 

 principle of the conservation of energy. 



Siemens introduced his far-seeing considerations with the 

 remark that, even if Dove's theory of the general circulation of 

 the atmosphere, which consisted of the ascending air- current at 

 the equator, and the development therefrom of the equatorial 

 and polar current with its conflicts for mastery in the temperate 

 zones, were defective, yet this explanation was at all events more 

 satisfactory than the "present almost exclusive reference of the 

 motions of the air in higher latitudes to minima and maxima of 

 pressure." He rightly asks that the seat of the forces — which 

 accumulate, in a manner that is not at present evident, powerful 

 energy in the cyclones and anticyclones which produce storms 

 and whirlwinds — and the point of their attack may be indicated. 

 Siemens makes no new assertion when he attributes all the 

 energy which occurs in cyclones and anticyclones to the heat 

 of the sun, but he discusses in a new and very noteworthy 

 manner the way in which the sun's heat produces the storms of 

 our latitudes. 



Siemens explains the general circulation of the atmosphere 

 according to the following principles : — (i) Without the heat of 

 the sun the earth's atmosphere would be in a state of relative 

 repose, i.e. it would everywhere rotate with the angular velocity 

 of the earth's rotation ; we should have no winds. (2) In reality, 

 the earth and its atmosphere are unequally heated, most at the 

 equator and least at the poles ; consequently, air currents must 

 arise from the equator towards the poles. (3) The energy which 

 is accumulated by the rotation of the atmosphere about the 

 earth's axis must, however, remain constant and unchanged ; the 

 theory of the conservation of energy requires this. If, therefore, I 

 a continual change of the geographical position of masses of air ' 

 takes place through equatorial and polar air currents, it must 

 take place so that the velocity of rotation of the whole atmo- 

 sphere remains unchanged. This is only the case when the 

 velocity of rotation of the whole atmosphere over lower latitudes 

 lags behind that of the earth, but in higher latitudes out- 

 strips it. 



It is evident from this that anywhere in middle latitudes, both 

 north and south, there must be a belt round the earth where 

 neither a retardation nor an acceleration occurs, i.e. where the 



NO. I 173, VOL. 45] 



air is in relative repose with regard to the earth. Siemens 

 calculates the position of this belt to be at latitude 35°. 



According to Siemens, therefore, we have the following system 

 of air circulation :— 



Between latitude 35° N. and 35^ S., the general movement of 

 the atmosphere is directed towards the west, i.e. east winds 

 constantly prevail there in all latitudes and at all heights. In 

 the vicinity of the equator, where north- east and south-east 

 trades meet, an interference occurs at the surface of the earth, 

 which produces the calm-zone, but it does not reach to any 

 considerable height. In the higher regions above the calm-zone, 

 an east wind must likewise prevail. 



In the higher latitudes, therefore, northward and westward of 

 the 35th parallels, the general movement of the atmosphere 

 must be directed towards the eastward, and west winds must 

 generally prevail in these latitudes. 



Now, what do direct observations say to this system of the 

 general circulation of the atmosphere ? 

 ! There is no doubt that between the 35th parallels an easterly 

 i air current prevails, viz. the trade-winds. It is also known that 

 j west winds prevail in the higher latitudes, and in the southern 

 [ hemisphere they blow almost uninterruptedly. But Siemens's 

 I theory is not to be considered as proved by these general facts, 

 I although they are fully borne out in Ferrel's and Sprung's 

 explanations ; we must examine what is new in it more closely. 

 Siemens's exposition requires an easterly air current at the 

 equator, and even in the upper regions above the calm-zone. 

 ' Does this exist ? Until a short time ago, it was undoubtedly 

 i the general opinion of meteorologists that the calm-belt was a 

 I zone of actual calms up to the highest altitudes, and that only 

 I the slowly ascending current prevailed there ; in the vicinity of 

 the calm-zone, the trade-wind blowing below ought to rise, and 

 with increasing height gradually assume a poleward direction, 

 to appear soon as an upper south-west trade-wind. This theory, 

 which had become rooted by custom and time, was upset by 

 Siemens : between the 35th parallels there are only easterly 

 currents — the strongest and most purely easterly over the calm- 

 belt, and decreasing continually towards latitude 35° ; in the 

 upper regions, on approaching latitude 35°, these currents come 

 continually more from the south, and at the earth's surface more 

 from the north. This was then a serious revolution in the 

 theory of the general circulation of the atmosphere, which even 

 the great authority of Siemens could not carry through without 

 further experimental proofs. He had not reduced his investiga- 

 tions to a mathematical form, and so his theory, notwithstanding 

 the great respect due to his name, would with difficulty have 

 obtained greater success, if facts had not recently become known, 

 which appeared to confirm it. 



It is known that after the Krakatab eruption, in the year 1883, 

 the opinion was expressed that the frequent coloured phenomena 

 of the sun in the tropics and the long evening glows were re- 

 garded as consequences of this eruption. The spread of these 

 phenomena in the first ten days after the eruption was such that 

 we were obliged to assume that the dust-haze thrown oat had 

 travelled round the earth in about twelve days from east to west ; 

 for the explanation of the diffusion of these phenomena, a 

 violent easterly wind was required in the upper regions of the 

 atmosphere in the vicinity of the equator. For a long time it 

 was this easterly wind which threw doubt upon the whole 

 hypothesis of the unusual appearances which were referred to 

 the Krakata~o eruption. But Siemen's theory of the general cir- 

 culation of the atmosphere was thereby confirmed. It appeared, 

 however, as if here two hypotheses happened to mutually support 

 each other, and it was a long time before on the one hand the 

 Krakatab hypothesis, and on the other Siemens's theory, were 

 regarded as established. 



This theory found however further support — on one hand in 

 the observations of the motion of high clouds by Abercromby, 

 and on the other in the mathematical establishment of Siemens's 

 statement by Oberbeck. 



In 1885, Abercromby, during a voyage from Aden to Australia, 

 had observed that in the neighbourhood of the equator the 

 cirrus moved from the east. He was much surprised at this, 

 and wrote: "The discovery of an easterly current over the 

 north-west monsoon is not only altogether new, but also quite 

 anomalous." He thought this so important, that he undertook 

 another voyage from Mauritius to Bombay, in order to clear up 

 the matter. The result of his further observations is couched in 

 the following terms : " I may point out another very important 

 result of these observations — namely, that the highest air 



