Because the fat reduces the specific gravity of the body, so that it floats more easily.
Why can a person float more easily in salt water than in fresh water?
Because salt increases the weight of the water, and, therefore, it buoys up with more force.
What familiar example have we of this?
An egg will sink when placed in fresh water; but if salt be dissolved in the water, the egg will float near the surface.
CHAPTER IX.
Motion.
When is a body said to be in motion?