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POPULAR SCIENCE REVIEW. 
earth become overheated, and these put in motion the whole 
system of atmospheric currents ; these too give to the winds 
the moisture destined to be dispersed as clouds, and to fall again 
on the earth as snow and rain. 
The impulse to all these movements of air and water is given 
by the sun’s rays ; and on this luminous body all the life of our 
planet depends. To the facts, then, that the earth is so uneven 
in its surface configuration, that its land and water are so very 
irregularly distributed, and that it receives an unequal share of 
solar heat varying with the seasons and the latitudes, we owe 
that infinite variety of climate by which it is characterized. 
One country near the polar circle receives more warmth than 
does another situated at a less distance from the tropics ; one 
region of the temperate zone is hot in comparison with certain 
spaces in the equatorial zone. And in each place the tempera- 
ture continually varies and oscillates under the action of winds, 
currents, and all the other agencies which affect climate ; and 
when indicated by lines on the surface of the earth, an inex- 
tricable network is formed, of which we can only recognize the 
principal traits. 
Fifty years ago Humboldt first conceived the idea of uniting 
by lines all those parts of the earth’s surface having the same 
annual average temperature. 
These imaginary lines traced on the circumference of the 
globe are called isothermal lines ; they give the thermal latitude, 
which differs widely from the geometrical latitude. 
While the lines of degrees traced every 69^ miles apart are 
parallel to the equator and perfectly regular, the isothermals are 
contorted into numerous and often sharp curves over all parts 
of the earth. 
The thermal equator (or the curve of the greatest average 
heat, on each side of which the temperature gradually decreases 
towards the poles), lies almost entirely in the northern hemi- 
sphere, which is warmer than the southern. 
The district of greatest heat lies between 10° and 60° E. 
longitude, and between 1 5° and 30° H. latitude ; that is to say, 
it is the area to the east and west of the Red Sea, and embracing 
the greater part of that narrow gulf, and also that of Persia. 
The district of greatest cold lies between 120° and 140° E. 
longitude, and between 60° and 80° N. latitude, along the course 
of the river Lena, the principal river of Siberia, covered during 
many month§ of the year by snow and ice ; within the frozen 
mud and ice-cliffs at whose mouth have been found the entire 
carcasses of the mammoth and the woolly rhinoceros. 
All these sinuosities of the isothermal lines over the earth’s 
surface are caused by similar isolated areas of a higher or lower 
temperature, which deflect them in a greater or less degree from 
a straight course. 
