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SCIENCE 



[N. S. Vol. XXVIII. No. 719 



motion. The introduction of this new 

 quantity, not otherwise determinable, left 

 us in doubt as to how far the relation be- 

 tween wind and pressure distribution, de- 

 ducible from the assumption of steady 

 motion, could be regarded as a really effect- 

 ive hypothesis for meteorological purposes. 



Recent investigations in the office of the 

 kinematics of the air in traveling storms, 

 carried out with Mr. Lempfert's assistance, 

 have shown that, so far as one can speak of 

 the velocity of wind at all— that is to say, 

 disregarding the transient variations of 

 velocity of short period and dealing 

 with the average hourly velocity, the ve- 

 locity of the wind in all ordinary circum- 

 stances is effectively steady in regard to the 

 accelerating forces to which it is subject. 

 This view is supported by two conclusions 

 which Mr. Gold has formulated in the 

 course of considering the observations of 

 wind velocity in the upper air, obtained in 

 recent investigations with kites. The first 

 conclusion is that the actual velocity of 

 wind in the upper air agrees with the 

 velocity calculated from the pressure dis- 

 tribution to a degree of accuracy which is 

 remarkable, considering the uncertainties 

 of both measurements ; and the second con- 

 clusion affords a simple, and I believe prac- 

 tically new, explanation upon a dynamical 

 basis of the marked difference between the 

 observed winds in the central portions of 

 cyclones and anti-cyclones respectively, by 

 showing that, on the hypothesis of steady 

 motion, the difference of sign of the effect- 

 ive acceleration, due to curvature of path 

 and to the earth's rotation respectively, 

 leads to quite a small velocity and small 

 gradient as the limiting values of those 

 quantities near anti-cyclonic centers. 



This conclusion is so obviously borne out 

 by the facts that we are now practically in 

 a position to go forward with the consider- 

 able simplification which results from re- 

 garding the steady state of motion in which 



pressure gradient is balanced by the effect- 

 ive acceleration due to the rotation of the 

 earth and the curvature of the path, as the 

 normal or ordinary state of the atmosphere. 

 I can not forbear to add one more in- 

 stance of an argosy which has richly come 

 to harbor so lately as this summer. You 

 may be aware that Kelvin was of opinion 

 that the method of harmonic analysis was 

 likely to prove a very powerful engine for 

 dealing with the complexities of meteorolog- 

 ical phenomena, as it has, in fact, dealt 

 with those of tides. In this view Sir 

 Richard Strachey and the Meteorological 

 Council concurred, and an harmonic an- 

 alyzer was installed in the office in 1879, 

 but subsequently numerical calculation was 

 used instead. A considerable amount of 

 labor has been spent over the computation 

 of Fourier coefficients. Not many great 

 generalizations have flowed from this meth- 

 od 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 pres- 

 sure—that is, to the component of twelve- 

 hour period — we get a variation very 

 marked in inter-tropical regions, and grad- 

 ually diminishing poleward in both hemi- 

 spheres, but synchronous in phase through- 

 out 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 



