( 290 ) 

 7. lln- (iiiiniiil (111(1 scciihir ca i'Kdions of' nfmosiiJicric jircsstirc. 



Wlieii wc coiiiparo (he system of isoljjii-s oWlaiiUMl for eacli separate 

 iiioiilli. il heeonies al onee a|(|»areii( llial the annual xariation ofllie 

 Itaronielrie pressnre is very differenl in the varions regions of ihe 

 Earth. In tlie tropics the llnctnatioiis are generally insignilieant ; in the 

 eeiitral pails of the continents of the temperate zones the atmospheric 

 pivssnre is low in summer aiul high in winter; in mid-occan this is 

 llie reverse; Iteyoml 45'' lat. south the pressure is uniformly low; 

 on the other parts of the gloUe the greatest diversity exists in its 

 annual conrse. 



Xevertheless most of the aninud curx'es (especially those of the 

 temjierate zones) display, next to local peculiarities, a common charac- 

 teristic. They exhibit, more or less distijictly, two minima at the times 

 of the equinoxes and maxima in winter and snmmei'. The regions 

 near the N(>rtli Pole seem to make an exce[»tioii to this rule (|terliaps 

 also those at the South Pole); there the maxima occiii- in spring and 

 autunm and the minima in winter and sununer. 



Some important statistics in comiexion with the atmos[)liei'ic ])res- 

 sure in K. America have been jjublislicd in the Jleport of the Chief 

 of the Weather Bureau 1900—1901. \'ol. II. Cha[)ter X treats of 

 tiie anjiual and secular variatioiis ; there we lind the monthly 

 deviations from the mean barometric pressure over a certain number 

 of 3xai's (1873 — 1899) arranged by Prof. Bioki.ow into 10 groups, 

 according to the geogTaj)hical [)Osition of the (^bserxation stations, 

 and the mean annnal course of these deviations charted for each gi-oup. 



The ten curves thus obtained sho\v great diHerences, due to the 

 more continental or more maritime character of the region to which 

 they i-efer, but all reveal a cosmic inlluence iji showing minima at 

 the times of the equinoxes and maxima dnring the solstices. 



From our point of view this cosniic iidlueiice may be thus detined. 



The greater \ariability in the Sun's irradiation during the spring 

 and autumn increases the atmoNplieric circulation, and this augments 

 the average horizontal velocity conq»onent of the aii- as well as the 

 evaporation, and both })rocesses cause the atmospheric pressure to 

 decrease. In the polar regions the solar radiation exercises a lesser 

 inlluence; in those parts compensation can take place and conse- 

 (picntly the maxima occui- in spring and autumn. 



Bkjklow has also tabulate.! the same data in another manner. 

 He has calculated for each station the successive yearly means and 

 subtracted from them the check mean of the whole period (1873 — 



