THE AERIAL OCEAN IN WHICH WE LIVE. 69 
ground up into higher parts of the atmosphere. And 
as it rises it leaves only thin air behind it, and this 
cannot resist the strong cold air whose atoms are 
struggling and trying to get free, and they rush in 
and fill the space. 
One of the simplest examples of wind is to b.e 
found at the seaside. There in the daytime the land 
gets hot under the sunshine, and heats the air, making 
it grow light and rise. Meanwhile the sunshine on 
the water goes down deeper,, and so does not send 
back so many heat-waves into the air ; consequently 
the air on the top of the water is cooler and heavier, 
and it rushes in from over the sea to fill up the space 
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 towards 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 
