Because there is but little friction between the sleigh and the snow.
Why do wagons run more heavily when the ground is soft?
Because their wheels then sink farther into the earth, and thus the friction is increased.
Why do wagons run more heavily when the ground is covered with snow?
Because their wheels sink into the snow, and thus the friction is increased.
Why will a person slip down upon the ice more easily than upon the earth?
Because there is less friction between his feet and the ice than there is between his feet and the earth.
Why do we put ashes on our walks when they are covered with ice?
We put ashes on our walks to increase the friction between the ice and our feet, so that we may not slip down when walking on them.
Why is a lock or a brake used on a wagon?