492 Mr. \V. R. Birt on Atmospheric Waves. 



Pursuing this idea further, there can be no question that Ireland 

 and Scotland become points, or unitedly constitute a great point of 

 rarefaction, forming as they do the nearest land to the nortliern part 

 of the Atlantic, the land becoming hotter than the neighbouring 

 water, and in consequence a N.W. current Avith its compensating 

 current from the S.E. is induced. Not only will the rapidity of the 

 currents reduce the pressure, but the ascending column from the 

 land will transfer some of the air into the general current of the 

 atmosphere, and there will be a real difference in the distribution of 

 air as well as pressure ; a section transverse to the line of greatest 

 velocity will exhibit a hollow or trough, and the same phajnomena 

 will result from this arrangement of the aeVial currents as we no- 

 ticed arising from the N.E. and S.W. currents, the only difference 

 being in direction. 



In thus considering these rectangularly posited systems of parallel 

 and opposite currents, many complex anemonal and barometric 

 phaenomena receive an easy explanation, particularly the revolution 

 of the vane in one uniform direction, and the barometric wind- 

 rose. When the conterminous edges of any two currents pass a 

 station, the barometer is either at a maximum or minimum with 

 respect to that particular system of currents ; the wind also changes 

 at this time. If the barometer has previously been rising with a 

 north-easterly wind, it now begins to fall with a south-westerly : the 

 cross currents are however passing at this time with a lateral motion 

 towards the N.E. ; in this set of cross currents the barometer will 

 rise with a north-westerly wind and fall with a south-easterly. Sup- 

 pose while the posterior slope of a N.W. wave transits, wind S.W., 

 and before its trough passes, the trough of the cross wave from the 

 S.W. also transits, and is immediately succeeded by the following 

 anterior slope with its N.W. current, the wind will pass from S.W. 

 to W. Now while this slope continues, upon the trough of the N.W. 

 sj'stem passing, the wind changes to N.E., and the resultant of the 

 two currents is N. It is easy to pursue this reasoning, and thus 

 trace the changes of the wind arising from these two cross systems 

 completely round the compass. 



The two systems of cross currents naturally divide themselves 

 into four beds of opposite currents, namely, N.E. S.W., N.W^. S.E. ; 

 with the first of each system, N.E. N.W., the barometer rises, and 

 with the last of each, S.W. S.E., it falls, so that in the barometric 

 wind-rose the maximum is found about the N.E., the prevailing 

 system, and the minimum near the S.W., the opposite current of 

 this system. 



The extent of arc which the wind-vane frequently describes, espe- 

 cially in stormy weather, also receives an explanation from these 

 systems of cross currents. A contemporaneous S.W\ with a N.W. 

 wind will occasion large arcs to be described between these points ; 



continent as inducing the N.W. current, but of course, the entire surface, as far as 

 the extreme north of Europe, including Great Britain and Ireland, will act as a 

 rarefying surface. 



