C. Lloyd Morgan — Tliymgraphy. 249 



at night. The cause of these land and sea-breezes, with which every 

 yachtsman is acquainted, is simple. In the morning the sun shines 

 alike on land and sea : the land, however, most readily takes up the 

 undulations of heat. The air above the land thus warmed expands, 

 and forms an upward current, while a refreshing breeze comes along 

 the surface from the sea, just as a cold current passed along the 

 floor from the hall. 



At nightfall the reverse is the case. The sun withdraws his rays 

 from land and sea : but the land, which was the first to be heated in 

 the morning, is the first to cool in the evening. Soon it is as cool as 

 the sea. Ere long it has become colder than the sea. And the current 

 now sets outwards from the land. We have changed the conditions. 

 We have brought a refrigerator into the dining-room, and the lower 

 cold current now sets outwards into the hall. It is, of course, under 

 ordinary conditions, only the under current which we on the earth 

 feel. The upper current is far above our heads. A French balloonist 

 (Tissondier) rose from Calais into the upper current, and was 

 carried -far out to sea ; on descending he entered the under current, 

 which bore him safely back to Calais. 



The same laws are seen in operation in the Indian Ocean. There 

 for half the year the North-east Monsoon which blows from the 

 continent of Asia is the prevalent wind. During the summer, how- 

 ever, it is forced back by a South-west wind, caused by the great 

 upward draught over the glowing plains of Central Asia. 



Far away on the broad Atlantic and Pacific Oceans, we may see 

 the same thing on a scale so magnificent as to form a healthy and 

 vigorous circulation for the whole world. In the great system of 

 winds, of which the trade winds are the most constant, we have 

 mighty currents of air which sweep from pole to pole, and are the 

 very life of the earth over which they pursue their ceaseless course. 



Thus the existence of the winds is due to sun-heat.^ 



Let us pause here for a moment to see what we have learnt. We 

 have seen that the waves which beat on our shores, and denude our 

 coast-lines, are due to the winds ; that the rivers which cut down 

 trenches into the earth are due to rain, which is itself brought to us 

 as vapour of water by the winds ; and we have seen that both the 

 formation of water vapour, and the existence of the winds, are due 

 to sun-heat. This sun-heat is therefore the highest link we have yet 

 reached in the chain of causation. We have also seen incidentally that 

 the sand and clay at the top of the cliff were built up of mud and 

 sand grains, carried down mechanically by rivers to the sea : and that 

 the chalk has been separated by living creatures from the sea-water 

 to which the lime had been carried down in solution by rivers. The 

 question — how came this life upon the earth ? — now arises. It will 

 not however be discussed here. It is enough to state that it is 

 almost universally believed by those competent to give an opinion, 

 that all life forms have come into being by a process of evolution 

 from primitive organic germs. It may be noticed, however, that all 

 life, whether vegetal or animal, is made possible only by solar 



^ Their direction is modified by the rotation of the Earth. 



