-2 26 



NATURE 



[July 7, 1904 



circumstances we find the motibn taking place in the 

 direction of the forces, we find a condition of things 

 which ought not to be expected, and one which requires 

 explanation. 



The question arises as to what one ought to expect 

 the steady motion to have become in course of time. 

 To afford some idea of the answer to this question, let 

 me refer to the four diagrams here 

 reproduced. The first gives the 

 average isobars for January at the 

 earth's surface, and discloses no 

 simple representation of steady 

 conditions. There are the well 

 known high-pressure areas about 

 the tropics, and isolated regions of 

 low pressure over the North 

 Atlantic and Pacific; but when we 

 look at Fig. 2, the isobars com- 

 puted by Teisserenc de Bort for 

 the 4000-mctre level, there is an 

 indication of comparatively simple 

 steady motion, namely, a motion 

 round the polar axis from west to 

 east, somewhat deviated, however, 

 to south or north by land or sea 

 areas. Now if we assume that 

 the motion is along the isobars 

 thus represented, so that the lines 

 of the diagram practically repre- 

 sent lines of flow of air, we must 

 remember that the motion on a 

 rotating earth implies a certain 

 normal acceleration of the air to 

 keep it in its path, just as 

 the bob of a conical pendulum 

 eicceleration towards its equilibrium 

 maintain its motion in a circular 

 ■effective horizontal acceleration of 

 2<aVsin 



values thus computed are shown in miles per hour 

 by figures between the isobars on the diagram. They 

 must not be confused with the pressures, which are 

 given in millimetres. The average wind velocities 

 thus computed are not at all unreasonable, and it 

 follows that motion along Teisserenc de Bort's isobars 

 at about 50 miles per hour is not at all an unreason- 



requires an 



position to 



path. The 



the air is 



where a is the angular velocity of the 



€arth, V the velocity of the wind, and A the latitude. 



E BELOW 4000METKE5. 



■Of the velocity at the 4000-metre level we can only 

 form an idea from the observed motion of clouds, and, 

 so far as we know, the only forces available to give 

 the necessary acceleration are those due to the pressure 

 distribution which Teisserenc de Bort has plotted. By 

 equating the pressure gradient to the product of the 

 ■density and acceleration we can determine V, and the 



NO. I 8 10, VOL. 70] 



able representation of the average steady motion of 

 the atmosphere at that level in the month of January. 

 That the directions of motion are appropriate is con- 

 firmed by Hildebrandsson's report on cloud motion 

 to the International Meteorological Committee. 



So much for the upper air; the motion is compara- 

 tively simple. Then it might be 

 supposed that the complexity of the 

 surface motion is due to extreme 

 complexity of pressure in the 

 lower stratum. The pressure due 

 to the weight of the lower stratum 

 is shown in Fig. 3, which gives the 

 pressure differences between Figs. 

 I and 2. There is, strange to say, 

 no more complexity about this dis- 

 tribution than there is about the 

 pressure of the upper layer; in fact, 

 the lines of the two are extra- 

 ordinarily similar, only the pressure 

 gradients run in opposite direc- 

 tions. Writing " high " for 

 " low," the one diagram would 

 not be an unsatisfactory duplicate 

 of the other, except that the lower 

 stratum has a dislocation of the 

 pole of high pressure from the 

 geographical pole to north-eastern 

 Siberia. Applying the same prin- 

 ciple of motion to this diagram as 

 to Fig. 2, it would represent, with 

 suitable velocities calculated in a 

 similar manner, a circulation from 

 east to west in each hemisphere round the pole of 

 cold. 



Compare both these diagrams with Fig. 4, repre- 

 senting the surface isothermal lines — the similarity is 

 again conspicuous. The intervals are for every 8° C. 

 of temperature instead of 8 mm. of pressure, and 

 speaking broadly of the temperate latitudes, starting 



