"What is that inequality of temperature which is produced by the shape of the earth?" "The five zones," answered Peter. "This subject is so well understood," said Mr. Wilton, "that I need not spend time in explaining it. Every boy knows the difference between setting his wet slate before the fire to dry so that the heat will fall squarely and perpendicularly upon it and placing it edgewise to the fire. Upon the torrid zone the sun shines perpendicularly, upon the temperate zones obliquely, and upon the frigid zones still more obliquely, and during a part of the year the sun is entirely hidden. In proportion as the rays of heat fall obliquely, any given amount of heat is spread, so to speak, over a larger surface, and the larger the space over which it is spread, the feebler it becomes. What is another cause of inequality of temperature?" No one answered. "Samuel, what is the cause of day and night?" "The turning of the earth upon its axis." "And the rotation of the earth upon its axis," continued Mr. Wilton, "brings not only an alternation of light and darkness, but also of heat and cold. The heat of the sun is withdrawn along with the light. The heat of the sun is not withdrawn from the earth, but one-half of the earth's surface is constantly turned away from its influence. This must produce a daily change of temperature. This diurnal fluctuation of temperature may be very small or it may amount to seventy or eighty degrees. Samuel, what is a third cause of unequal temperature?" "The inclined position of the earth's axis and the revolution of the earth around the sun cause the change of seasons."