630 SCIENTIFIC RECREATIONS. 



So it is in nature. The warm air ascends, cooler air rushes in to fill the 

 space, and a storm or a breeze is created. The balance must be restored. 

 The upper current probably moves one way, and the lower the other way. 

 Thus clouds are said to be " coming up against the wind " when they are 

 moving in an upper current, or in a different direction to that the wind is 

 blowing just above the earth's surface. 



The wind moves with varying velocity. We have a gentle breeze 

 when the motion of the air is about five or six miles an hour, a good 

 breeze at twenty-five miles an hour, a high wind at thirty-five, and a gale at 

 fifty. Hurricanes travel at sixty and seventy miles an hour, and do enor- 

 mous damage. Near the Equator we do not find much wind, and this fact 

 has caused the name of the Region of Calms, or "The Doldrums" of sailors, 

 to be bestowed upon that portion of the globe, but this belt of calm has 

 no fixed position. It follows the sun's course, and is the region of greatest 

 heat, and, as it were, the centre of a concentric circle of currents. The hot 

 air rises and goes away ; air rushes in north and south, and causes what are 

 called the North-East and South-East Trades, or Trade Winds, owing to 

 their being so useful in commerce for ships, or to the old meaning of the 

 word trade, a " regular course." The calms of the Tropic of Cancer are 

 called the " Horse Latitudes." 



Readers of the life of Columbus will remember how his crew were 

 affrighted at the persistency of the wind which bore him across, for no sail 

 requires shifting, nor is a sheet altered while the vessel is making way with 

 the "Trades." Were the earth covered with water, we should find the trade- 

 winds blowing equally over the surface, but the varying temperature of the 

 land diverts them. The rarefaction of the air in the Sahara causes a westerly 

 wind to prevail, which blows towards the land, instead of the trade wind we 

 might expect to find. 



The MONSOONS, again, are caused in like manner, for the ordinary 

 "trade" from the south-east is changed by the elevation of the heated air in 

 Central Asia into a south-west wind, and so in the south, in consequence of 

 the heated air from Australia, the north-west trade appears as a north-east 

 monsoon, but is altered to a north-west wind. Nearly all the year round, 

 therefore, we find the two winds, which are modifications of the " trades," 

 blowing in different directions and from different quarters. From November 

 to March there is a north-east wind north of the Equator, and a north-west 

 wind blows south of the Equator. From April to September a south-west 

 wind blows at the north, and a south-east wind at the south of the line. 

 The term monsoon signifies a " season," and the changes of these winds give 

 rise to tremendous storms causing great havoc. 



SEA and LAND breezes are really little monsoons ; they are caused by 

 the heat of the- sun in just the same way t but with miniature results. We 

 all know the sea-breeze which comes in as the land gets hot during the day, 

 for the land warms more quickly than the sea under equally existing circum- 

 stances. So again, in the evening, the land loses its heat more quickly, and 



