372 Royal Society : — ■ 



atmosphere fi'om lateral sources ; the statical pressure at the base 

 of the column being increased by the augmentation of the super- 

 incumbent mass of air arising- from an influx in the ui)per portion. 

 Such lateral sources may well be supposed to be due to excessive 

 ascensional currents caused bj^ eacessive summer heats in certain 

 places of the globe (as, for example, in Central Asia). Now the 

 lateral overflow from such sources, traversing in the shape of cur- 

 rents the higher regions of the atmosphere, and encountering the 

 well-known general current flowing from the equator towards the 

 pole, has l)een recently assigned with considerable probability (de- 

 rived from its correspondence with many otherwise anomalous 

 phenomena already known, and which all receive an explanation 

 from such supposition) to be the original source or primary cause 

 of the rotating storms or cyclones, so well known in the West Indies 

 and in China under the names of hurricanes and typhoons. A 

 single illustration may be desirable. Let it be supposed that such 

 an excessive ascensional current exists over the greatly heated parts 

 of Asia and Africa in the northern tropical zone, — giving rise, in 

 the continuation of the same zone over the Atlantic Ocean, to a 

 lateral current in the upper regions ; this would then be a current 

 prevailing in those regions from east to west : and it would encounter 

 over the Atlantic Ocean the well-known up])er current proceeding 

 from the equator towards tlie pole, which is a current from the 

 south-M'Cst. An easterly current impinging on a south-west current 

 may give rise, by well-known laws, to a rotatory motion in the atmo- 

 sphere, of which the direction may be the same as that which cha- 

 racterises the cyclones of the northern hemisphere. To test the 

 accuracy of this explanation, we desire to be acquainted with the 

 variations which the mean pressure of the dry air undergoes in the 

 different seasons in the part of tiie globe, where, according to this 

 explanation, considerable variations having particular characters 

 ought to be found. 



2". We have named one of the explanations which have been 

 recently ofl^ered of the primary cause of the northern cyclones. 

 Another mode of explanation has been proposed, by assuming the 

 condensation of large quantities of vapour, and the consequent 

 influx of air to supply the place. In such case the phenomena are 

 to be tested in considerable measure by the variations which the 

 other constituent of the barometric jjressure, namely, the aqueous 

 vapour, undergoes. 



3". The surface of sea in the southern hemisphere mtich exceeds 

 that in the northern hemisphere. It is therefore probable that at 

 the season when the sun is over the southern hemisphere, evapora- 

 tion over the whole surface of tlie globe is more considerable than 

 in the opposite season when the sun is over the northern hemi- 

 sphere. Supposing the pressure of the dry air to be a constant, 

 the difference of evaporation in the two seasons may thus produce 

 for the whole globe an annual barometric variation, the aggre- 

 gate barometric pressure over the ivhole surface being highest 

 during the northern winter. The separation of the barometric 



