Q. Do not carriage wheels sometimes catch fire?
A. Yes; if the wheels be dry,—or fit too tightly,—or revolve very rapidly,—they often catch fire.
Q. Why do wheels catch fire in such cases?
A. The friction of the wheels against the axle-tree is so great, that their latent heat is disturbed, and produces ignition.
Q. What is the use of greasing cart wheels?
A. The grease lessens the friction; and (by diminishing the friction) the latent heat is less disturbed.
Q. Why is the top of a mountain colder than the valley beneath, although it be two or three miles nearer to the sun?
A. 1st—Because the air on a mountain is less compressed, than the air in a valley.
2ndly—It is more rarefied: and