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In contrast to electric and magnetic fields, the gravitational

field exhibits a most remarkable property, which is of

fundamental importance for what follows. Bodies which are moving

under the sole influence of a gravitational field receive an

acceleration, _which does not in the least depend either on the

material or on the physical state of the body._ For instance, a

piece of lead and a piece of wood fall in exactly the same manner

in a gravitational field (in vacuo), when they start off from

rest or with the same initial velocity. This law, which holds

most accurately, can be expressed in a different form in the

light of the following consideration.

According to Newton's law of motion, we have

(Force) = (inertial mass) x (acceleration),

where the "inertial mass" is a characteristic constant of the

accelerated body. If now gravitation is the cause of the

acceleration, we then have

(Force) = (gravitational mass) x (intensity of the gravitational

field),