, since
. We may conclude that the velocity of the ball with respect to the embankment is equal to its velocity in the train plus the velocity of the train with respect to the embankment. This expresses the classical belief that velocities add up like numbers.
[39] In the classical theory, however, aberrational observations of very high refinement should reveal our speed through the ether, but observations of this sort are beyond our present powers; and Einstein’s theory has since proved that however precise our experiments, this velocity could never be revealed.
[40] Although no experimental results could be claimed to have justified any such assumption, Maxwell introduced it unhesitatingly into his theory. His celebrated equations of electromagnetics represented, therefore, the results of experiment, supplemented by this additional hypothetical assumption. The advisability of making this hypothesis was accentuated when it was found to ensure the law of the conservation of electricity.
[41] If two magnetic poles of equal strength, situated in empty space at a distance of one centimetre apart, attract or repel each other with a force of one dyne, either pole is said to represent one unit of magnetic pole strength in the electromagnetic system of units. Owing to the interconnections between magnetism and electricity, we can deduce therefrom the unit of electric charge also in the electromagnetic system. Likewise, if two electric charges of equal strength, also situated in empty space at a distance of one centimetre apart, attract or repel each other with a force of one dyne, either charge is said to represent one unit of electric charge in the electrostatic system of units. From this we may derive the unit of magnetic pole strength in the electrostatic system.
[42] It is to this pressure of light that the repulsion of comets’ tails away from the sun is due.
[43] By a first-order experiment, we mean one that is refined enough to detect magnitudes of the order of
