AND THE PREVAILING WINDS OVER THE GLOBE. 579 



rapid diminution of the pressure indicated as taking place in the north of the 

 Atlantic and adjoining regions. This is the season of the year when the first 

 great decrease of temperature takes place, which is accompanied by heavy rains 

 and furious storms. The increase of pressure in Sweden in October, taken in con- 

 nection with the simultaneous decrease in Greenland, Iceland, north of Norway, 

 and the British Islands, is interesting, as bearing on the transference of masses of 

 the atmosphere from one region to another. 



In November, pressure rises considerably over the continents of the northern 

 hemisphere, and falls in the south temperate zone; and the belt of low pressure 

 in the equatorial regions may be regarded as now passing completely round the 

 globe. This belt, towards which the trades on each side of the equator blow, 

 does not occur in the summer months in the Indian Ocean ; but, on the contrary, 

 there is a continuous diminution of pressure northwards, from Australia and 

 Mauritius to the interior of Asia. It will be seen that in November, as compared 

 with October, the isobars have advanced a little northwards from the British Isles 

 to Iceland, and eastwards from Baffin's Bay to Iceland, thus indicating a general 

 increase of pressure over the north of the Atlantic and regions adjoining. 

 Coincident with this increase of pressure, there occurs a diminution of pressure 

 to the south-east of it, including Austria, Italy, and countries adjoining the 

 Mediterranean ; and in the Atlantic to the south of it, from about latitude 45° to 

 ] 5° N. Probably these extensive oscillations of the pressure are parts of one 

 general movement of the atmosphere, which in one of its manifestations has been 

 long known to meteorologists under the name of the great November wave, but 

 of which no very satisfactory account has yet been given. 



In addition to these changes in the monthly distribution of the pressure, it is 

 probable that a system of low pressures traverses the continent of Africa, follow- 

 ing the sun's course ; but since the grounds of this supposition have been recently 

 laid before the Society, in a paper on ' k The Determination of Heights, chiefly in 

 the Interior of Continents, by Observations of Atmospheric Pressure,"* it is not 

 necessary to reproduce them here. The probable pressure for the months is 

 shown on the separate charts. 



Prevailing Winds. — It will be seen that every one of the charts shows con- 

 siderable disturbance of the equilibrium of the atmosphere at the surface of the 

 earth. If the pressure was equal in all parts of the globe, we should have the 

 physical conditions of a stagnant atmosphere. But such is not the case. From 

 the different pressures which the charts show in different regions, it might be 

 expected, from the laws of aerial fluids, that movements of the atmosphere 

 would set in, giving rise to the prevailing winds of these regions. 



* Proceedings of the Roy. Soc. Edin. vol. vi. p. 465. 

 VOL. XXY. PART II. 7 L 



