494 SCIENTIFIC RECORD FOR 1833. 



Fluids, &c.) is subject to au annual period. It is most intense in the 

 hemisphere in which winter prevails, for in this hemisphere the differ- 

 ence of temperature between tbe tropics and the higher latitudes is 

 greatest, and therefore also the temperature gradient in the upper 

 strata, which sets the circulatory current in action, and which latter, 

 in its turn, brings into play the centrifugal forces. In the hemisphere 

 that is enjoying summer the upper temperature gradient is slightest 

 and the interchange of air between the tropics and the higher latitudes 

 is feeblest. In general an accumulation of air exists always over the 

 hemisphere that is enjoying winter. Since the northern hemisphere in 

 its winter cools much lower than the southern in its, therefore also the 

 upper currents of air are much stronger in the winter of the northern 

 hemisphere than in the winter of the southern. The difference between 

 the warmest and coldest latitudes in the northern hemisphere during 

 its winter is about 60° C, while on the other hand for the winter of 

 the southern hemisphere it is only about 40° C. Since, also, during 

 the winter of the northern hemisphere the local temperature differ- 

 ences between the oceans and the land are very great, reaching, indeed, 

 nearly the same amount as the general difference between the tropics 

 and the pole, therefore the local circulation attains its greatest inten- 

 sity and is able to materially modify the general circulation. There- 

 fore there develop over the warmer northern oceans, the permanent 

 cyclones of the Atlantic and Pacific Oceans and the anti-cyclones over 

 the continents of Asia and North America. Parallel with this there 

 is a more frequent and intense occurrence of smaller whirls — the ordi- 

 nary cyclones — which, in the temperate zones, advance with the gen- 

 eral movement of the atmosphere from west to east. This is therefore 

 in general the season of the development of greatest activity in the 

 movements of the earth's atmosphere, the greatest differences of press- 

 ure and greatest variations of pressure occur, and with these the great- 

 est variability of temperature. In the winter of the southern hemi- 

 sphere no similar increase in the motions of the atmosphere is to be 

 expected, since its cooling is not, by far, so great as that of the north- 

 ern hemisphere. Therefore, also, the upper temperature gradient is 

 not so marked, and consequently the intensity of the upper air cur- 

 rents toward the pole. At the same time also the temperature differ- 

 ences of the meridians and the local currents fail because the conti- 

 nents in high latitudes are missing. Consequently the currents of the 

 general circulation can develop themselves much more regularly and 

 intensely, as we have already seen is indeed the fact. The difference 

 in the movements of the atmosphere between winter and summer is 

 much slighter. Therefore, also, the change in the variations of baio- 

 metric pressure from one season to the other and its consequences. 

 Equally must the variability of the temperature be less. In short, the 

 whole meteorological regime assumes a more constant type and shows a 

 smaller annual periodicity. In the equatorial region of the Atlantic 



