OCTOBEB 9, 1908] 



SCIENCE 



467 



an inch of mercury— but certainly identi- 

 fiable. 



The reality of this variation of pressure, 

 common to the whole earth, can not be 

 doubted, and, so far as it goes, we may 

 represent it (if indeed we may represent 

 pressure differences as differences in ver- 

 tical heights of atmosphere) as the de- 

 formation of a spherical atmosphere into 

 an ellipsoid, with its longest axis in the 

 equator pointing permanently 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 re- 

 mains fixed in direction with regard to the 

 sun, while the solid earth revolves beneath 

 it. Whatever may be the cause of this 

 effect, obviously cosmieal, and attributable 

 to the sun, at which it indirectly points, 

 its existence has long been recognized, and 

 further investigation only confirms the 

 generalization. It is now accepted as one 

 of the fundamental general facts of mete- 

 orology. 



Professor 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 relation of pressure and 

 wind, in accordance with the dynamical ex- 

 planation of Buys Ballot's law, we should 

 expect the effect of a pressure variation 

 that has its counterpart in that of ter- 

 restrial magnetism to be traceable also in 

 wind observation. 



Mr. J. S. Dines has just given me par- 

 ticulars of the discovery of that effect in 

 the great air-current, the variations of 

 which I have called the pulse of the atmo- 

 spheric circulation — I mean the southeast 

 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 1891 the Meteorological Council set 

 up an anemometer at St. Helena, in the 

 very heart of the current, and we have 

 just got oiit the results of the hourly tabu- 

 lations. When the observations for the 

 hours 1 to 24 are grouped separately for 

 months, so as to give the vector resultants 

 for each hour and for each month, it ap- 

 pears 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 April gives the most stri- 

 king 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 result- 

 ant wind for the whole day is very nearly 

 southeast, and practically remains so for 

 all the months of the year, the monthly 

 variation of resultant wind being confined 

 to a change of velocity from about four- 

 teen miles per hour in May to about 

 twenty-one miles per hour in September. 



If, instead of combining the south and 

 east components to form a vector diagram, 

 we plot their variations separately, the 

 semi-diurnal variation in each is plainly 

 marked; and the calculation of its con- 

 stants shows that its amplitude is about 

 three quarters of a mile per hour both in 

 the south and rather less in the east com- 

 ponent. 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 correspond 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 super- 

 position upon the resultant wind of a 

 northeasterly component of about one mile 



