Perhaps the equivalence is most easily seen when we produce in this manner an artificial field which just neutralises the earth's field of gravitation. Jules Verne's book Round the Moon tells the story of three men in a projectile shot from a cannon into space. The author enlarges on their amusing experiences when their weight vanished altogether at the neutral point, where the attraction of the earth and moon balance one another. As a matter of fact they would not have had any feeling of weight at any time during their journey after they left the earth's atmosphere. The projectile was responding freely to the pull of gravity, and so were its occupants. When an occupant let go of a plate, the plate could not "fall" any more than it was doing already, and so it must remain poised.
It will be seen that the sensation of weight is not felt when we are free to respond to the force of gravitation; it is only felt when something interferes to prevent our falling. It is primarily the floor or the chair which causes the sensation of weight by checking the fall. It seems literally true to say that we never feel the force of the earth's gravitation; what we do feel is the bombardment of the soles of our boots by the molecules of the ground, and the consequent impulses spreading upwards through the body. This point is of some importance, since the idea of the force of gravitation as something which can be felt, predisposes us to a materialistic view of its nature.
Another example of an artificial field of force is the centrifugal force of the earth s rotation. In most books of Physical Constants will be found a table of the values of "g," the acceleration due to gravity, at different latitudes. But the numbers given do not relate to gravity alone; they are the resultant of gravity and the centrifugal force of the earth's rotation. These are so much alike in their effects that for practical purposes physicists have not thought it worth while to distinguish them.
Similar artificial fields are produced when an aeroplane changes its course or speed; and one of the difficulties of navigation is the impossibility of discriminating between these and the true gravitation of the earth with which they combine. One usually finds that the practical aviator requires little persuasion of the relativity of force.
To find a unifying idea as to the origin of these artificial
