A. On a fine day, the sun heats the surface of the earth, and the air (resting on the earth) is heated by contact; as soon as it is heated it ascends, and other air succeeding is heated in a similar way, till all is heated by convection.
Q. If air be a bad conductor, why does hot iron get cold, by being exposed to the air?
A. A piece of hot iron exposed to the air, is made cold—1st—By “convection;” and
2ndly—By “radiation.”
Q. How is hot iron (exposed to the air) made cold by convection?
A. The air around the iron (being intensely heated by contact) rapidly ascends, carrying some of its heat with it: other air succeeds, absorbs more heat, ascends, and gives place to that which is colder; till the hot iron is cooled completely down.
Q. How is hot iron cooled by radiation?
A. While the heat of the iron is being carried off by “convection,” it is throwing off heat (on all sides) by radiation.
Q. What is meant by radiation?
A. Heat emitted (in all directions) from any surface, by innumerable rays.