248 C. Lloyd Morgan — Physiography. 



pounds weight of cloud formed in our skies, an amount of heat is 

 set free sufficient to melt 45 lbs. of cast iron. 



A valuable lesson may be learnt from this behaviour of water and 

 water vapour. When the liquid water became gaseous vapour, a 

 certain amount of heat energy disappeared. But it was not de- 

 stroyed. It was converted into another form of energy which we 

 may call the energy of separation. The particles were forcibly 

 separated from one another, and a certain amount of energy was 

 necessary to keep them apart. Presently, however, they clashed 

 together again, and the energy of separation was reconverted into 

 the energy of heat. The amount of heat given out was exactly 

 equal to the amount of sun-heat absorbed. Day by day fresh 

 experiment and observation make clearer this great law of nature : 

 that by no means at our disposal can we either destroy or create 

 energy. We may change it in a number of ways. We may con- 

 vert chemical separation into electricity, this into mechanical motion, 

 and mechanical motion into heat. But we can neither call into 

 existence or put out of existence any portion of the energy of the 

 universe, any more than we can call into existence or put out of 

 existence any portion of the matter of the universe. 



One more fact must be noticed. Though the same amount of 

 heat is given out by the condensation of the aqueous vapour, as was 

 absorbed on the Atlantic during its formation, it is no longer useful 

 in the same way. It does not possess the power of again converting 

 water into water vapour. It has become degraded. It is the same 

 in amount, but different in value. The water which turns a mill is 

 the same in amount, whether it lies above or below the water-wheel ; 

 but it differs vastly in value. That above the mill is useful to the 

 miller : that below the mill is useless. It is the same with energy. 

 Just as water tends to run down from higher to lower levels, so does 

 energy tend to run down from higher to lower forms. All forms of 

 energy tend to be degraded to heat uniformly diffused throughout 

 space. 



To the energy of sun-heat, then, we owe the existence of vapour 

 of water in the wind. And to what do we owe the wind itself? 

 To the same cause. On any winter's evening, the colder the better, 

 we may make the following experiment, first performed by Franklin. 

 When the dining-room is warm, but the hall outside cold, we may 

 throw open the door to its full extent. On holding a lighted candle 

 in the doorway near the top, we shall find that the flame is blown 

 outwards : on holding it near the bottom, we shall find that it is 

 blown inwards : midway between the top and the floor, the flame 

 will bum steadily. The cause of this is obvious when we remember 

 that warm air is lighter than cold air. When the door is opened, 

 warm air rushes out near the top, and, to supply its place, cold air 

 rushes inwards along the floor. The two currents are divided by a 

 calm. 



At the seaside we may watch the same sort of experiment per- 

 formed on a larger scale by nature. In settled summer weather 

 sailors count on a sea-breeze in the morning, and a breeze from the land 



