THE BEASON WHY. G7 



'How much better is it to get wisdom than gold? and to get understanding 

 rather to be chosen than silver." PROVERBS xvi. 



Because, as the warm air flies away, cold air rushes in to occupy 

 its place. 



242. What does this example of the motion of the air in 

 our rooms explain ? 



It explains the movement of volumes of air by convection, and 

 illustrates the origin of breezes and winds. 



243. What is the chief effect of this law of convection ? 

 Under its influence air and water are the great equalisers of solar 



heat, rendering the earth agreeable to living things, and suited to 

 the laws of their existence. 



Owing, also, to this law of convection, the constituents of the air 

 are equalised. The breath of life, supplied by the purer oxygen of 

 the " sunny south," is diffused in salubrious gales over the wintry 

 climes of the north. And the waters, evaporated from the bosom 

 of the central Atlantic Ocean and the Pacific, are borne across vast 

 continents, and poured down in fertilising showers upon distant 

 lands. 



To the educated mind, nothing is too simple to merit attention. To the 

 ignorant, few things are sufficiently attractive to excite curiosity. Knowledge 

 enables us to estimate the varied phenomena that are hourly arising around 

 us, and to see, even in the most trifling effects, illustrations of those great 

 causes and consequences that govern with mighty power the material world. 

 Man, sitting by his firo-side, is enabled to witness the operation of some of 

 nature's grandest laws : light and heat are around him ; conduction, radiation, 

 reflection, absorption, and convection of heat are all going on before him ; little 

 winds are sweeping by his footstool, and warm currents, with miniature clouds 

 folded in their arms, are passing upward before his view. Chemical changes 

 are going on; the solid rock of coal disappears, flying away as an invisible 

 gas. The little " hills are melted," and hard stones have been converted into 

 ' fervent heat." Although some of these changes are imperceptible to the eye, 

 they are manifest to the educated mind; and the pleasures of philosophical 

 observation are as sweet as a poet's dreams. 



CHAPTER XIIL 



244. Why will a piece of paper, held three or four inches 

 over the flame of a candle, become scorched? 



