September 3, 190SJ 



NATURE 



429 



analyser was installed in the Office in 1879, but_ subse- 

 quently numerical calculation was used instead. A con- 

 siderable amount of labour has been spent over the com- 

 putation of Fourier coefficients. Not many great general- 

 isations have flowed from this method up to the present 

 time. I have no doubt that there is much to be done in 

 the way of classifying temperature conditions, for climatic 

 purposes, by the analysis of the seasonal variations. A 

 beginning was made in a paper which was brought to the 

 notice of the Association at Glasgow. The most striking 

 result of the Fourier analysis we owe to Hann, who has 

 shown that, if we confine our attention to the second 

 Fourier coefficient of the diurnal variation of pressure — 

 that is, to the component of twelve-hour period — we get 

 a variation very marked in inter-tropical regions, and 

 gradually diminishing poleward in both hemispheres, but 

 synchronous in phase throughout the 360 degrees of a 

 meridian. The maximum occurs along all meridians in 

 turn about 10 a.m. and 10 p.m. local time. This semi- 

 diurnal variation with its regular recurrence is well known 

 to mariners, and we have recently detected it, true to its 

 proper phase, in the observations at the winter quarters 

 of the Discovery ; small in amplitude indeed — about a 

 thousandth of an inch of mercury — but certainly identifi- 

 able. 



The reality of this variation of pressure, common to the 

 whole earth, cannot be doubted, and, so far as it goes, we 

 may represent it (if indeed we may represent pressure dif- 

 ferences as differences in vertical heights of atmosphere) 

 as the deformation of a spherical atmosphere into an 

 ellipsoid, w^ith its longest axis in the Equator pointing per- 

 manently 30° to the west of the sun. Its shortest axis 

 would also be in the Equator, and its middle axis would be 

 along the polar axis of the earth. Somehow or other this 

 protuberance remains fixed in direction with regard to the 

 sun, while the solid earth revolves beneath it. Whatever 

 may be the cause of this effect, obviously cosmical, and 

 attributable to the sun, at which it indirectly points, its 

 existence has long been recognised, and further investi- 

 gation only confirms the generalisation. It is now 

 accepted as one of the fundamental general facts of 

 meteorology. 



Prof. Schuster, for whose absence from this meeting I may 

 venture to express a regret which will be unanimous, has 

 already contributed a paper to the Royal Society pointing 

 out the possible relations between the diurnal variations of 

 pressure and those of terrestrial magnetic force. Going 

 back again to the ubiquity of the application of the rela- 

 tion of pressure and wind, in accordance with the 

 dynamical explanation of Buys Ballot's law, we should ex- 

 pect the effect of a pressure variation that has its counter- 

 part in that of terrestrial magnetism to be traceable also 

 i[) wind observations. 



Mr. J. S. Dines has just given me particulars of the 

 discovery of that effect in the great air-current, the vari- 

 ations of which I have called the pulse of the atmospheric 

 circulation — I mean the south-east Trade Wind, the most 

 persistent atmospheric current in the world. It is difficult 

 as a rule to get observers to pay much attention to that 

 current, because it is so steady ; but in iSgi the Meteor- 

 ological Council set up an anemometer at St. Helena, in 

 the very heart of the current, and we have just got out the 

 results of the hourly tabulations. When the observations 

 for the hours i to 24 are grouped separately for months, 

 ST as to give the vector resultants for each hour and for 

 each month, it appears that there is a conspicuous semi- 

 diurnal variation in the current, which shows itself as a 

 closed polygon of vector variations from the mean of the 

 day. 



The month of .\pril gives the most striking diagram of 

 the twelve. It displays the superposition of two practically 

 complete dodekagons, one a large one, completing its 

 cycle from 6 a.m. to 6 p.m.. the other a small one, for 

 6 p.m. to 6 a.m. The resultant wind for the whole day 

 is very nearly south-east, and practically remains so for 

 all the months of the year, the monthly variation of re- 

 sultant wind being confined to a change of velocity from 

 about fourteen miles per hour in May to about twenty-one 

 miles per hour in September. 



If, instead of combining the south and east components 



NO. 2027, VOL. 78] 



to form a vector diagram, we plot their variations 

 separately, the semi-diurnal variation in each is plainly 

 marked ; and the calculation of its constants shows that 

 its amplitude is about three-quarters of a mile per hour 

 both in the south and rather less in the east component. 

 The easterly increment has its maxima at 10 a.m. and 

 10 p.m., and at these hours the phase of the variation of 

 the southerly component is nearly opposite. Thus, to cor- 

 respond with the semi-diurnal variation of pressure, there 

 is a semi-diurnal variation in the Trade Wind at St. 

 Helena, which is equivalent to the superposition upon the 

 resultant wind of a north-easterly component of about one 

 mile per hour amplitude, with maxima at to a.m. and 

 10 p.m., the hours when the ellipsoidal deformation of the 

 spherical atmosphere is passing over the locality. 



I have only dealt with one month. I believe that when 

 all the results that flow from this simple statement can be 

 put before you, you will agree with me that the argosy 

 which the Meteorological Council sent out in 1891 has in- 

 deed richly come to harbour. 



Let me digress to say a word in illustration of the prin- 

 ciple I laid down that, if one would avoid waste in 

 n:eteorological work, the observations must be examined 

 forthwith and so far discussed that any ambiguities may 

 be cleared up. 



.After some years of wear at St. Helena the persistent 

 rubbing of the south-east part of the spiral metallic pencil 

 upon the metallic paper wore away the metal and left a 

 flat place. This got so bad that the instrument had to 

 come home for repairs, and when it was set up again, 

 after a vear's absence, the average direction of the Trade 

 Wind differed by two points from the averages of most, 

 but not of all, of the previous years. So far as we know, 

 the orientation has been attended to, as before, and yet 

 it is hardlv possible to resist the suggestion that the ane- 

 mometer has been set slightly differently. We are now 

 making verv careful inquiries from the observer; but, in 

 the mcantirne, it seems to me that there is a great oppor- 

 tunity for a competent mathematical physicist to help us. 

 Dynamical explanations of the Trade Winds have been 

 given from the time of Hafley. Let me offer as a simple 

 question in the mathematical physics of the atmosphere 

 whether a variation of two points in the direction of the 

 south-east Trade Wind between the years 1903 and 1905 

 can be regarded as real, and, if not, which of the two 

 recorded directions is the correct one? 



It would be appropriate for me to add some words about 

 the results of last year's work upon the upper air, in 

 which we have had the valuable cooperation of the Univer- 

 sity of Manchester. These results have disclosed a num- 

 ber of points of unusual interest. But we are to have an 

 opportunity of considering that subject in a discussion be- 

 fore the Section, and I need not deal with it here. 1 must, 

 however, pause to give expression of the thanks of all 

 meteorologists to Prof. Schuster for his support of the- 

 Manchester University station at Glossop Moor. I may 

 remind you that this generous contribution for the ad- 

 vancement of science on the part of Prof. Schuster is in 

 addition to the foundation of a readership in mathematical 

 physics at Manchester and a readership in dynamical' 

 me'teorologv, now held by Mr. Gold at Cambridge. 



I have said enough to show that the speculative ventures 

 of official meteorologists are not all failures, and I will only 

 add that if anv mathematician or physicist would like to 

 take his luck on a meteorological argosy he will be heartily 

 welcomed. Part of the work will be drudgery ; he must 

 be prepared to face that ; but the prospects of reaching port 

 are reasonably good, so much so, indeed, that such a 

 voyage might fairly lead to 'a claim for one of the higher 

 academical degrees. 



Up to now I have been dealing with the adjustment of 

 official scientific work to reduce waste to a minimum, in 

 so far as it lies within the control of those responsible for 

 an office. I turn now to an aspect of the matter in which 

 we require the assistance of others, particularly of the 

 British Association. 



The most serious danger of waste in a busy office is that 

 it should carry on its work without an adequate knowledge 

 of what is being done in advancing science and improving- 

 methods elsewhere. 1 speak myself for the Meteorologicaf 



