586 TRANSACTIONS OF SECTION A. 



representing tbe empirical results of a very elaborate investigation. The em- 

 pirical determinations upon which it is based are certainly not of the highest order 

 of accuracy ; they rely upon two separate investigations besides the statistical 

 comparison, viz., tbo constant of an anemometer and the relation of wind- 

 velocity to wind-pressure, but no subsequent adjustment of these determinations 

 is at all likely to be outside the limits of an error of an estimate of wind-force ; 

 and the equation can be used, quite reasonably, as a substitute for the original 

 specilication of the Beaufort scale, a specification that has vanished with the 

 passing of ships of the type by which it was defined. This result, combined with 

 the equation ' ^; = -003 F", which has been in use in the Office for many years, and 

 has recently been confirmed as suflSciently accurate for all practical purposes by 

 Dr. Stanton at the National Physical Laboratory and Monsieur Eiffel at the 

 Eiffel Tower, places us upon a new plane with regard to the whole subject of 

 wind-measurement and wind-estimation. 



llesults equally remarkable appear in other lines of investigation. Let me 

 take the relation of observed wind velocity to barometric grsidient. You may be 

 aware that in actual experience the observed direction of the wind is more or 

 less along the isobars, with the low pressure on the left of the moving air in the 

 northern hemisphere ; and that crowded isobars mean strong winds. Investigations 

 upon this matter go bade to the earliest days of the Office. 



There can be no doubt that the relation, vague as it sometimes appears to be upon 

 a weather chart, is attributable to the effect of the earth's rotation. In order to 

 bring the observed wind velocity into numerical relation with the pressure- 

 gradient Guldberg and Mohn assumed a coefficient of surface 'friction,' inter- 

 fering with the steady motion. The introduction of this new quantity, not 

 otherwise determinable, left us in doubt as to how far the relation between wind 

 and pressure distribution, deducible from the assumption of steady motion, could 

 be regarded as a really effective hypothesis for meteorological purposes. 



Recent investigations in the OflSce of the kinematics of the air in travelling 

 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 velocity of the wind in all ordinary circumstances 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 

 coui'se of considering the observations of wind velocity in the upper air, obtained 

 in recent investigations with kites. The first conclusion is that the actual 

 velocitj' of wind in tbe upper air agrees Avith the velocitj^ calculated from the 

 pressure distribution to a degree of accuracy which is remarkable, considering the 

 uncertainties of both measurements ; and the second conclusion affords a simple, 

 and I believe practically new, explanation upon a dynamical basis of the marked 

 diff'ereuce between the observed winds in the central portions of cyclones and 

 anti-cyclones respectively, by showing that, on the hypothesis of steady motion, 

 the dift'ei'ence of sign of the effective 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 centres. 



This conclusion is so obviously borne out by the facts that we are now practi- 

 cally in a position to go forward with the considerable simplification which 

 results from regarding the steady state of motion in which pressure gi'adient is 

 balanced by the effective acceleration due to the rotation of the earth and the 

 curvature of the path, as the normal or ordinary state of the atmosphere. 



I cannot forbear to add one more instance of an argosy which has richly come 

 to harbour 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 meteorological phenomena, as it has, 

 in fact, dealt witla those of tides. In this view Sir Richard Strachey and the 

 Meteorological Council concurred, and an harmonic analyser was installed in tlie 



' In this equation^ is expressed, as before, in pounds per .square foot, and T' in 

 miles per hour. 



