14 Mr. K. M. Deeley on the 



during recent years has failed to reveal the currents postulated 

 by Thomson, and great doubts are now entertained concerning 

 the correctness of the theory. We are consequently faced 

 with the fact that the directions in which the winds blow 

 are not such, in many cases, as the temperature gradients 

 on the earth's surface seem to require. 



A better knowledge of the temperature conditions of the 

 atmosphere, and of the directions of flow of the upper currents, 

 which have been gained during recent years, instead of 

 bringing theory into accord with fact, has to some extent 

 widened the breach ; for in cyclones the temperature gra- 

 dients are in opposition to the circulation of the winds *. 



Halley called attention to the fact that the temperature of 

 the air over different parts of the earth's surface varies greatly 

 owing to the varying amounts of heat received from the sun 

 in different latitudes. These variations of temperature with 

 latitude persist the whole year round, but vary in amount 

 with the seasons. In many cases the temperature along the 

 same line of latitude varies greatly owing to the varying 

 heating of the air over land and water respectively; and such 

 temperature gradients are often reversed with change of 

 season. The atmosphere is thus always in a state of unstable 

 equilibrium, with the result that there are always temperature 

 gradients to cause warm air to flow over the neighbouring 

 colder air, and cold air to flow beneath the warm air from 

 cold to warmer regions. Gradients thus persist throughout 

 the year; but these gradients vary in intensity and often in 

 direction with change of season. 



The manner in which air currents result from differences 

 of temperature is shown in fig. 1. Here the vertical column 



Fig. 1. 



n A „* 



J} B D 



of air AB, being warmer than the columns of air CD and 

 CD' on either side, is the thicker. But the weight of each 

 column is the same. The top of the column AB, being higher 

 than the columns on each side of it, flows in the directions 

 shown by the upper arrows. The columns CD and CD' then 



* The Free Atmosphere in the Region of the British Isles. Dines and 

 Shaw. Meteorological Office Memoir 2106. 1912. 



