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If the motion of the carriage is now changed into a non-uniform

motion, as for instance by a powerful application of the brakes,

then the occupant of the carriage experiences a correspondingly

powerful jerk forwards. The retarded motion is manifested in the

mechanical behaviour of bodies relative to the person in the

railway carriage. The mechanical behaviour is different from that

of the case previously considered, and for this reason it would

appear to be impossible that the same mechanical laws hold

relatively to the non-uniformly moving carriage, as hold with

reference to the carriage when at rest or in uniform motion. At

all events it is clear that the Galileian law does not hold with

respect to the non-uniformly moving carriage. Because of this, we

feel compelled at the present juncture to grant a kind of

absolute physical reality to non-uniform motion, in opposition to

the general principle of relativity.