548 LECTURE LVI. 



than a few feet above it ; but that in cloudy weather there is less differ- 

 ence : and it is possible that this circumstance may be derived from 

 the difference of the quantity of evaporation from the earth's surface, 

 which occasions a different degree of cold in different states of the atmo- 

 sphere. 



The motions of the air, which constitute winds, are in general dependent, 

 in the first instance, on variations of temperature. They are so accidental 

 and uncertain, as to be subjected to no universal laws ; as far however as 

 any regularity can be observed in their recurrence, it may in most cases 

 be sufficiently explained.* 



The principal phenomena of the periodical winds may be reduced to six 

 distinct heads : first the general tendency from north east and south east 

 towards the equator, in latitudes below 30 ; secondly, the deviation of this 

 . tendency from the precise situation of the equator ; thirdly, the prevalence 

 of westerly winds between 30 and 40 or more, especially in the southern 

 hemisphere ; fourthly, the local modifications to which these general effects 

 are subjected ; fifthly, the monsoons, which vary every half year ; and 

 lastly, the diurnal changes of land and sea breezes. 



With respect to the general tendency of the trade winds to the west, it 

 may be sufficiently explained by Hadley's theory t of the difference of the 

 rotatory motion of different parts of the atmosphere, combined with the 

 currents occasioned by the greater heat at the equator. For the sun's rays, 

 expanding the air in the neighbourhood of the equator, and causing it to 

 ascend, produce a current in the lower parts of the atmosphere, which rush 

 southwards and northwards towards the equator, in order to occupy the 

 place of the heated air as it rises : and since the rotatory motion of the 

 earth is greatest at the equator, and is directed eastwards, the air coming 

 from the poles has of course a relative motion westwards ; and hence the 

 joint motion of the current is directed, in the northern hemisphere, from 

 north east to south west, and in the southern, from south east to north 

 west. [As the winds on both sides approach the equator, the friction of 

 the earth's surface is constantly tending to give them an easterly direction ; 

 and since the lengths of the diurnal circles increase very slowly in the 

 immediate vicinity of the equator, this friction is even more effective than 

 the change of latitude ; and the westerly direction of the winds is gra- 

 dually lessened. Moreover, the northerly and southerly currents, coining 

 here into opposition, mutually annihilate each other's effects. At the equa- 

 tor, therefore, the trade winds lose their distinctive character, and consti- 

 tute only currents which depend on the preponderancy of local causes, and 

 thus vary in different places.^] Dr. Halley supposed that the air was 

 made in some measure to follow the sun round the earth, simply by means 

 of the expansion of the atmosphere, which takes place immediately under 

 him, and accompanies him round the globe ; but it does not seem evident 

 that the air could have any greater tendency to follow the sun that to meet 



* See Dove, Meteorologische Untersuchungen, Berlin, 1837. Fechner's Reper- 

 torium, vol. Hi. f Hadley, Ph. Tr. 1735, xxxix. $8. 



See Hall's Fragments of Voyages and Travels, 2nd Series, i. 162. 

 Ph. Tr. xvi. 152. 



