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mosphere is the same as the law of the 
sea, that is, that cold contracts it and 
increases its weight, while the heat ex- 
pands it and lightens its weight, so 
that, because it is heavier the coldest air 
tends to settle into the lowest places on 
the uneven surface of the earth just as 
the coldest water tends to settle into the 
lowest places on the uneven bed of the 
ocean. This law may be demonstrated 
in the heating of our homes. Those whose 
homes are heated with hot water know 
that the pipes which carry the hot water 
from the furnace to the upper rooms are 
called the hot water pipes and that the 
hot water rises from the boiler in the 
furnace room to the upper rooms of the 
building, and that after the same water 
has been cooled by contact with the air 
of the upper rooms, it is carried back by 
return pipes into the furnace room to be 
reheated and rise again. Thus is kept up 
the general round of circulation during 
the winter. Another illustration is in the 
circulation of the air in our homes from 
hot air furnaces. The heated air rises 
from the furnace, pouring from the regis- 
ters and driving the cold air of the rooms 
into the lower story where by means of 
cold air ducts it is carried into the fur- 
nace room either to be reheated or to be 
carried away from the house. Every travel- 
er has noted how, after the sun has set and 
the air is cooling, the colder air tends to 
settle in the valleys, while the warmer 
air tends to rise to the higher altitudes; 
he knows that in ascending the hillsides 
he will sometimes feel very sensibly a 
change in temperature in a distance of a 
few feet. During the day, especially in 
the summer time, the temperature of 
the valleys is warmer than that of the 
higher levels, because the radiation and 
reflection are greater. This can be illus- 
trated. If we stand beside a large build- 
ing on a hot day and get the direct rays 
of the sun at the same time that we 
get the reflected rays from the building, 
we will find that it will be much 
warmer near the side of the build- 
ing that reflects the sun’s’ heat 
than it will some distance away from the 
building. During the day the south slope 
of a hillside receives the direct rays of 
ENCYCLOPEDIA OF PRACTICAL HORTICULTURE 
the sun’s heat, during the night these 
hills radiate that heat and send it into 
the atmosphere to warm the colder air 
coming down from the hills. An illus- 
tration of this heat radiation at night is 
geen in the heated stone buildings and 
pavements of a great city where long 
after midnight the walls and walks are 
hot, especially during the hottest weeks 
of summer. We have seen persons trying 
to sleep on the beaches or in the parks 
at night because the radiated heat from 
the buildings in which their rooms were 
located was unendurable. These facts, 
together with the uneven surface of the 
earth, cause a constant circulation of the 
atmosphere of our globe. 
Why the Highest Mountains Are Covered 
With Snow 
If heated air rises and cold air settles 
it is pertinent to ask why the highest 
mountains are covered with snow and 
why the air is colder as we ascend. 
There are two principal answers to this 
question. 
The first is that or near the sur- 
face of the earth there are innumer- 
able particles of dust and layers of 
clouds and vapor that act as a blanket or 
covering to hold the reflected and radi- 
ated heat from the earth’s surface. It is 
the same principle as when we sleep in 
a cold room, the covering on the bed 
which keeps us warm does not warm the 
atmosphere of the room, but it holds the 
heat radiated from our bodies. So, if 
we rise above a certain altitude, we rise 
above that blanket of dense atmosphere 
which we call the earth’s covering. 
Another reason is that in the highest 
altitudes of the mountains there is less 
friction of air currents, less generation 
of heat through friction, and therefore 
after ascending above the vapor, dust and 
clouds into a rarer atmosphere it be- 
comes colder. We say “we rise above the 
clouds,” but we speak in comparative 
terms, because we have not, in ascend- 
ing the highest mountains, gone beyond 
all clouds, but beyond the general alti- 
tude of clouds. An illustration of this is 
seen on the Pacific coast where the pre- 
vailing winds are from the west. These 
