444 PHYSICAL GEOGRAPHY. 



considerable distance over-land, and deposited in the most uncongenial situations ;. so that 

 now, during the violence of these tropical storms, fish are found alive on the tops of 

 houses ; nor is this any longer a matter of surprise to the established resident in India, 

 who sees every year a repetition of this singular phenomenon. During the extreme 

 violence of the storm, the heat was occasionally almost beyond endurance, particularly 

 after the first day or two, when the wind would at intervals entirely subside, so that not 

 a breath of air could be felt, and the punka afforded but a partial relief to that distressing 

 sensation which is caused by the oppressive stillness of the air so well known in India/' 

 It is an extraordinay but well-ascertained fact, that as soon as one monsoon ceases, 

 though a month may elapse before the succeeding one appears, the clouds take the direc- 

 tion of the approaching monsoon, and thus from the regions of the atmosphere herald 

 its advent to the dwellers below. 



We naturally inquire concerning the origin of these peculiar movements, but must be 

 content with a very scanty measure of information upon the subject. The laws which 

 nature obeys in these periodical changes are undoubtedly identical with those which give 

 rise to atmospheric currents in general, but their mode of operation is in this case obscure. 

 The north-east and 'south-east monsoons, the former on the north and the latter on the 

 south side of the equator, may be considered as trade winds, explicable upon the same 

 principles, but counteracted for a certain time by causes which produce winds from a 

 different quarter, the south-west and north-west monsoons. It has been observed that 

 the south-west monsoon, which prevails to the north of the equator, is coincident with the 

 sun being vertical to that region, when Hindustan, Siam, and the adjacent countries 

 receive their maximum of heat. Consequently, the incumbent air, being rarefied, ascends, 

 and a rush of colder air to supply its place is produced from the southward, which is then 

 receiving the oblique rays of the sun, and which presenting a surface of water is immensely 

 less heated than the lands to which the luminary js perpendicular. In like manner, the 

 north-west monsoon, which prevails south of the equator, is coincident with the sun being 

 south of it likewise, and vertical to the region, when the sandy plains of New Holland 

 become powerfully heated, and the air over them-rarefied, creating a wind by the rush of 

 the colder northern air towards the point of rarefaction. These are the explanations 

 commonly given, and though in several respects they do not account for all the phenomena, 

 yet the probability is, that they present the correct theory, anomalous circumstances 

 arising from the influence of causes which are local and as yet unknown. The monsoons 

 are more valuable as auxiliaries to commerce than the trade winds, owing to the change 

 in their direction, for a ship may proceed to a distant port with one monsoon and be aided 

 on its return by its successor. 



3. Land and sea breezes. A line in one of our popular songs, 



" How sweetly the breeze blows off the shore," 



refers to the wind which begins at evening to blow from the coasts situated between and 

 near the tropics ; and an equally grateful breeze blows by day from the sea to the shore 

 in those warm climates. The inequality of the solar action on the land and water, 

 together with the tendency of the atmosphere to preserve an uniform density, is the cause 

 of these periodically shifting currents. During the day the land acquires a temperature 

 higher than that of the ocean, and the air over it is therefore rarefied and ascends, and the 

 cooler air from the sea glides in to fill the partial vacuum produced. At night, the land 

 rapidly cools with the atmosphere over it, but the sea and the air in connexion with it 

 retain a nearly equal temperature, in consequence of which, the colder and heavier 

 land-air displaces the less dense or lighter air over the water, and a wind from the shore 

 is created* The smoke of Vesuvius beautifully exemplifies this diurnal change in the 



