ON CLIMATES AND WINDS, 701 



cnt of the sun's immediate action. Thus, it has been observed, that when 

 the weather has been clear, and a cloud passes over the place of observation, the 

 thermometer frequently rises a degree or two almost instantaneously. This 

 has been partly explained by considering the cloud as a vesture, preventing 

 the escape of the heat which is always radiating from the earth, and reflecting 

 it back to the surface : the cloud may also have been lately condensed, and 

 may itself be of ahigher temperature than the earth. Mr, Six has observed that in 

 clear weather, the air is usually some degrees colder at night, and warmer by day, 

 close to the ground, than a few feet above it; but that in cloudy weather there 

 is less difference: 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 atmosphere. 



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 re- 

 gularity can be observed in their recurrence, it may in most cases be suffi- 

 ciently 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 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 



VOL. I. 4s 



