PRN Ee LIES: OM S Oe RE G OTT E p ES 
1878. | Physiography. 675 
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 vapor 
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 bet- 
ter, 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 burn 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 sup- 
ply 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 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 un- 
der 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 
VOL. XII.—NO X. 46 
