THE AERIAL OCEAN IN WHICH WE LIVE. 73 
on the shore left by the warm air as it rises. This is 
why the seaside is so pleasant in hot weather. During 
the daytime a light sea-breeze nearly always sets in 
from the sea to the land. 
When night comes, however, then the land loses its 
heat very quickly, because it has not stored it up 
and the land-air grows cold; but the sea, which has 
been hoarding the sun-waves down in its depths, now 
gives them up to the atmosphere above it, and the 
sea-air becomes warm and rises. For this reason it 
is now the turn of the cold air from the land to spread 
over the sea, and you have a land-breeze blowing off 
the shore. 
Again, the reason why there are such steady winds, 
called the trade winds, blowing toward the equator, 
is that the sun is very hot at the equator, and hot air 
is always rising there and making room for colder air 
to rush in. We have not time to travel farther with 
the moving air, though its journeys are extremely 
interesting ; but if, when you read about the trade and 
other winds, you will always picture to yourselves 
warm air made light by heat rising up into space and 
cold air expanding and rushing in to fill its place, I 
can promise you that you will not find the study of 
aerial currents so dry as many people imagine it 
to be. 
We are now able to form some picture of our aerial 
ocean. We can imagine the active atoms of oxygen 
floating in the sluggish nitrogen, and being used lip in 
every candle-flame, gas-jet and fire, and in the breath 
of all living beings ; and coming out again tied fast to 
