62 



NATURE 



\_May 1 6, 1878 



and of the Atlantic Ocean. '■■ I was early struck with the 

 incompleteness of a theory which implied the develop- 

 ment and movement of detached areas of high and low 

 barometer, especially as such areas must be traced with 

 more or less irregular but distinct outlines, I soon found 

 that in order to justify the appreciation of the term "area" 

 to such figures representing particular conditions, the 

 pressures over large spaces between them should be left 

 practically unaccounted for, and that the most singular 

 alignments of isobars which appear on the weather charts 

 of the United States Signal Service Bureau, and of the 

 European observatories, would not satisfy the want. I 

 became convinced that the lack of continuity in the 

 recognised systems of high and low pressures was a vital 

 fault which affected the whole fabric of weather predic- 

 tion. Following the^ investigation from day to day, I 

 found that these areas, so called, performed their pro- 

 cession across the weather chart with a very striking 

 regularity, and that they maintained relations toward 

 each other which gave me anridea of their general 

 arrangement and distribution over the field of observa- 

 tion. By a practical application I found that the zone 

 theory of pressures fulfilled all my expectations. By 

 means of the facilities for cabling daily observations 

 from Europe afforded by Mr. Bennett, and through the 

 agency of the Herald ship news department in collecting 

 meteorological data from the logs of vessels that were 

 making West Indian and Transatlantic voyages, I have 

 been able to establish the connection or rather the 

 continuation of the American zones across the Atlantic 

 Ocean and Europe. 



Some two months after the Herald storm predictions 

 had begun to attract attention in Europe, the late M. 

 Leverrier, as director of the Observatory of Paris, 

 requested that he should be informed fully on the 

 Herald's system. At the same time he expressed the 

 greatest interest in the work, and said that he regarded 

 it as so far successful. In reply to M. Leverrier' s 

 inquiries I addressed him a letter dated July 10, 1877, on 

 the subject of the Herald's weather warnings, from which 

 I extract the following paragraphs explanatory of the 

 zone law of atmospheric movements : — 



" From a system of observations and comparisons 

 which the Herald Meteorological Department has had in 

 operation for over two years, I have been drawn to the 

 conclusion that instead of forming a series of detached 

 areas of erratic movement, the high pressure encircles 

 the earth in a number of unbroken zones, the axes of 

 which alter in direction under the influence of inconstant 

 conditions. Also that between these zones of high pres- 

 sure lie the zones of low pressure along which the storms 

 take their courses. The normal direction of these zones 

 is nearly parallel with the equator, but they are some- 

 times so displaced by a combination of influences that 

 their axes form an angle of forty-five degrees and over 

 with that line. I am also satisfied that between the 

 southern extremity of Greenland and the equator there 

 are two zones of low, and perhaps two, but certainly one, 

 of high pressure. The approximate axis of the zone of 

 high pressure lies between the 30th and 45th parallels 

 of latitude, but, as I have already stated, this direction 

 is subject to extraordinary variations. 



" The zones of high pressure are defined by the isobars 

 of 30'o inches, or 762mm., and their margins, as well as 

 their axes are undulating constantly under the influence 

 of disturbances moving along the zones of low pressure. 



Storm areas or depressions are invariably found within 

 the concave curvatures of these margins, and roll, as it 

 were, along the lines which yield before them more or 

 less readily. I therefore call these the undulating zones 

 of high and low pressure, because of the constant wave 

 motion observable in their axes and margins. 



" The outlines of the zones of high and low pressure 

 are governed by the movements and development of the 

 storm centres or depressions. When disturbances of 

 ordinary and uniform energy succeed each other at regular 

 intervals on the northern margin, the axis of the zone of 

 high pressure lying between the 30th and 45th parallels 

 approaches in alignment to a regular undulation whose 

 general direction is almost parallel with the equator. 

 Modifications of this condition are caused by the relative 

 infrequency of disturbances along the southern margin of 

 the zone. When storms of unequal energy and develop- 

 ment occur, the northern undulations become corre- 

 spondingly irregular, and these reacting on those of the 

 southern margin produce the distortions of outline which 

 sometimes occur. The axis of the zone of high pressure 

 assumes a compound undulatory movement which pro- 

 duces extraordinary variations of the weather within the 

 range of its influence. You will readily perceive how the 

 direction of the course of a southern storm could be 

 changed by the influence of a northern disturbance, and 

 how we can account in some measure for the eccentric 

 movements of these meteors ? 



" Sometimes two, or even three storm centres, will 

 force their way into the same concavity of the northern 

 margin of the high-pressure zone. When it occurs, the 

 combined energy of the storms enlarges the area of the 

 general depression, absorbs or presses away the obstruct- 

 ing wave of dense atmosphere, or, may be, causes it to 

 temporarily accumulate, with its apex or crest, far into 

 the north. Then according to the resisting power of the 

 wave in front, the storm centre commences to ascend 

 toward the north along a gradually diminishing gra- 

 dient which has the effect of throwing it on the Nor- 

 wegian coast, or of giving it a curved course that will 

 bring it from the north-westward toward the British 

 Isles. Hence, although a storm centre may be leaving 

 the Newfoundland coast apparently en route to Ireland, 

 it is impossible to predict where it will reach Europe 

 unless the direction and character of the undulation of 

 high pressure in advance of it and the general trend of 

 the axis of the zone of high pressure are known. These 

 can only be ascertained by a series of daily observations 

 extending over a large area such as that of the United 

 States and Canada on this Continent, or through your 

 international system in Europe and Western Asia. . . . 



" In the centre, or nearly so, of each wave of high 

 pressure, there is a point where the atmosphere attains 

 its maximum density. This point is recognised ordi- 

 narily, and, indeed, properly, as the centre of the area of 

 highest pressure, or anti-cyclonic area. The centres 

 of highest pressure always alternate with the centres 

 of lowest pressure when crossing a given meridian, the 

 air movement around the former being with the hands of 

 the clock, while that around the latter is in the contrary 

 direction. I find that the general movement of the wind 

 along the southern margin of the zone of high pressure is 

 always westward, following the undulations, while that of 

 the northern margin is eastward, also following the wave- 

 lines. Now, taking this fact in connection with the geo- 

 graphical position of the zone of highest pressure be- 

 tween the 30th and 4Sth parallels, we have on the 

 former line a general westward movement of the wind,^ 

 corresponding with the trade winds, while on the 45th 

 parallel, and north of it for some distance, the pre- 

 vaihng winds are from the westward. Again, a series 

 of observations on our Atlantic and Pacific coasts, and in 

 the interior, go to prove beyond question that some 

 regions such as Lower California, and the south-eastern 



