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A system of co-ordinates of which the state of motion is such

that the law of inertia holds relative to it is called a

"Galileian system of co-ordinates." The laws of the mechanics of

Galilei-Newton can be regarded as valid only for a Galileian

system of co-ordinates.

V.

THE PRINCIPLE OF RELATIVITY (IN THE RESTRICTED SENSE)

In order to attain the greatest possible clearness, let us return

to our example of the railway carriage supposed to be travelling

uniformly. We call its motion a uniform translation ("uniform"

because it is of constant velocity and direction, "translation"

because although the carriage changes its position relative to

the embankment yet it does not rotate in so doing). Let us

imagine a raven flying through the air in such a manner that its

motion, as observed from the embankment, is uniform and in a

straight line. If we were to observe the flying raven from the

moving railway carriage. we should find that the motion of the

raven would be one of different velocity and direction, but that

it would still be uniform and in a straight line.