594 



NA TURE 



[April 21, 1892 



which the explanation of the general circulation of the atmo- 

 sphere has undergone ; little or nothing more will then be 

 wanting in order to understand the answer to the question at 



issue. 



Dove was the first person who advanced a theory of the gene- 

 ral circulation of the atmosphere after meteorology took a place 

 among the exact sciences. He considered the question from a 

 high and correct stand-point, because he considered the atmo- 

 spheric envelope of the earth as a whole, which received its 

 motion from the sun, the universal motor. According to his 

 theory, the explanation of the general circulation of the atmosphere 

 took the following form : — 



The heat of the sun is not uniformly distributed over the 

 earth's surface, but decreases from the equator to the poles. 

 The greatest heat occurs at the equator, and the least at the 

 poles. The air which is greatly heated at ihe equator must 

 consequently rise, an ascending current must be developed there 

 (the celebrated cotirant ascendant). The zone of calms is the 

 region of the ascending air current. The air carried upwards 

 must flow away above towards the poles, while, owing to the 

 rotation of the earth, it endeavours to deviate to the right ; the 

 equatorial current is originated as an upper south-west wind, in 

 the higher strata of the atmosphere. ^The flow of the upper air 

 towards the poles is compensated below by the flow of the air 

 towards the equator, and this polar current seems to be turned 

 away towards the west owing to the rotation of the earth ; it 

 appears on the earth's surface as a north-west wind, and blows 

 as far as the zone of calms, where it ceases, and the air brought 

 with it is carried upwards in the ascending air current, where 

 the circulation then begins afresh. 



You see that this theory leads to a circulation of <he 

 air between pole and equator. The air rises at the equator 

 and flows towards the poles, in order to descend there 

 and to flow again towards the equator. But Dove states 

 expressly that the equatorial current partially descends, even 

 in the temperate latitudes, down to the bed of the polar 

 current. If this happens, and according to Dove this continually 

 occurs, then both winds, the equatorial and polar currents, begin 

 the conflict for the mastery. The variable winds of the tem- 

 perate zone arise out of this conflict, and Dove deduced there- 

 from his law of wind gyration. The description which he gave 

 of this conflict of the opposing equatorial and polar currents is 

 a thoroughly masterly picture, and it is in a great measure | 

 owing to this elaborate completion of his exposition that his 

 theory of the general circulation of the atmosphere and his law 

 of wind gyration were accepted by scientific men. The reader 

 is carried away by it as if he were present at the conflict of the 

 winds in vEolus's mythological cave, and he is inclined to believe 

 that science verifies the ancient mythology so far as regards the 

 lower latitudes. Dove's theory found an actual support in the 

 trade-winds, and in the law of wind gyration which generally 

 obtains in Central Europe. In fact, in our regions, the wind 

 rotates very frequently from east through south to west and north. 

 Dove's explanation of this, the conflict of the equatorial and polar 

 currents must have appeared to be correct as long as no more 

 serious investigations existed. 



Since 1863, weather telegraphy and synoptic weather charts 

 have led to the discovery of the baric wind law (Buys Ballot's 

 law), and the position was at once changed. We learned that 

 the winds depend upon the differences of pressure which prevail 

 over the region under consideration, and that they circulate round 

 the place of lowest pressure ; i.e., with us, they flow to the place 

 of lowest pressure, in spirals opposite to the movement of watch 

 hands. 



It was now said that the cause of the winds in our latitudes 

 was not the great heating of the air at the equator, but the dif- 

 ferences of pressure which were formed in the temperate zone. 

 The law of wind gyration also became untenable. It was found 

 that the swirls which arose about the place of lowest pressure do 

 not stand still, but move, and it was seen that Dove's law of 

 wind gyration for places which lie northwards of the tracks of 

 these swirls does not hold good, in fact that the rotation is exactly 

 the reverse. 



The reason why Dove's law of wind gyration mostly holds 

 good in Germany and Central Europe, was found to be that the 

 tracks of the swirls almost always lie to the northwards of Central 

 Europe, and therefore the law must hold good there. But as 

 often as a swirl moves more southwards all places in Central 

 Europe which lie to the north of it have a wind rotation which is 

 opposed to Dove's law. 



NO. I I 73, VOL. 45] 



The equatorial and polar currents, so far as regards the higher 

 latitudes, were thus driven off the field, and the conflict between 

 them, so beautifully described, was again relegated to the 

 mythical cave of yEolus. As it was feared that the wheat might 

 be thrown away with the chaff, it was wished that the equatorial 

 and polar current should be banished from the higher latitudes 

 altogether, but left, in a more limited sense, only in the regions 

 of the trade-winds — between the zone of calms and about 30° north 

 and south latitude ; for here it was clear that Dove's theory held 

 good. People unceremoniously denied the right of the polar 

 current to blow beyond the 35th parallel, for in this latitude, 

 as observations show, a band of high pressure encircles the whole 

 earth, while from there the pressure decreases both towards the 

 equator and the poles. But according to the baric wind law the 

 wind can only flow from a place of higher to one of lower pres- 

 sure ; the air can therefore only flow from the 35th parallel 

 towards the pole, and not the reverse. The polar current was 

 thereby banished without mercy from the higher latitudes. The 

 equatorial current fared somewhat better in the upper regions. 

 Many persons allowed it to remain in a small degree, especially 

 those who recognized the accuracy of the calculation according 

 to which in the higher atmosphere, about 4000 m. and over, 

 the pressure decreases from the equator to the pole. But very 

 few persons knew what to make of this equatorial current in the 

 upper regions, and so at all events it was little regarded. Gene- 

 rally speaking, the regime of cyclones and anticyclones was 

 established for the higher latitudes, and people were completely 

 absorbed in investigating the details of their qualities and tracks, 

 without having hitherto succeeded in obtaining a satisfactory 

 explanation of their origin and development. With regard to 

 the propagation of cyclones, it was observed that with few ex- 

 ceptions they advanced from the westward to the eastward. 

 Gradually, opinions became general that it was the south-west 

 and west wind of the upper regions which brought the cyclones 

 with it ; some persons explained this west wind in the upper 

 regions as the equatorial current. But, on the whole, the general 

 opinion of this period favoured the rejection of Dove's polar and 

 equatorial current, and the explanation of all winds by the pre- 

 valent differences of pressure, without being able to account for 

 the origin of the latter. 



Dove was of an excitable temperament, and he strongly com- 

 bated these new views, and laid too much weight on the defence 

 of the defective portion of his theory. His contest was fruitless, 

 and only caused him to be accused with some justice of hindering 

 the progress of meteorology by his great authority. Dove at 

 length remained silent, and the funeral anthem of the polar and 

 equatorial current was then chanted. 



Miihry, a well-known meteorologist of the Dove school, sur- 

 vived him, and constantly raised his voice in favour of Dove's 

 equatorial and polar current ; he attentively followed the pro- 

 gress of meteorology until his death, a few years ago, and the 

 way in which he frequently warned meteorologists against entirely 

 excluding the equatorial and polar current from their consider- 

 ations is quite stirring. He was continually trying to turn 

 attention to it, but with him disappeared the last defender of 

 Dove's theory. 



But while meteorologists were almost exclusively concerned 

 with the details of the phenomena offered by cyclones and anti- 

 cyclones, and almost lost sight of the great problems which the 

 considei-ation of the general circulation of the atmosphere afford^, 

 a revolution was being prepared, at first only individually, but 

 gradually more generally, which disposed minds to the more 

 favourable consideration of the general movements of the 

 atmosphere. 



Mor6 than thirty years ago, Ferrel, the great American 

 meteorologist, was occupied with the question of the general 

 circulation of the atmosphere. He developed its laws in a 

 mathematical form, and arrived at the following theory : — 



Three great zones of calms exist round the earth, one at the 

 equator (or near it), usually called the calm-belt, the second and 

 third 35° north and south of the equator, the so-called '"horse- 

 latitudes." Between the calm-belt and the horse-latitudes, the 

 north- east or south-east trade-wind prevails at the surface of the 

 earth, and in the higher regions above them the anti-trade 

 (south-west or north-west). In the calm-belt, in which no other 

 than an ascending movement of the air exists up to the greatest 

 heights, an upheaval of the air occurs, and in latitude 35° a 

 descent of the same. Northwards of 35° N., and southward 

 of 35° S., south-west and west winds prevail both at the earth's 

 surface and at great altitudes, while at a mean height a return 



