The second class of facts to which we have alluded has reference to the question whether or not the motion of the earth in space can be made perceptible in terrestrial experiments. We have already remarked in Section V that all attempts of this nature led to a negative result. Before the theory of relativity was put forward, it was difficult to become reconciled to this negative result, for reasons now to be discussed. The inherited prejudices about time and space did not allow any doubt to arise as to the prime importance of the Galileian transformation for changing over from one body of reference to another. Now assuming that the Maxwell-Lorentz equations hold for a reference-body _K_, we then find that they do not hold for a reference-body _K′_ moving uniformly with respect to _K_, if we assume that the relations of the Galileian transformation exist between the co-ordinates of _K_ and _K′_. It thus appears that, of all Galileian co-ordinate systems, one (_K_) corresponding to a particular state of motion is physically unique. This result was interpreted physically by regarding _K_ as at rest with respect to a hypothetical æther of space. On the other hand, all coordinate systems _K′_ moving relatively to _K_ were to be regarded as in motion with respect to the æther. To this motion of _K′_ against the æther (“æther-drift” relative to _K′_) were attributed the more complicated laws which were supposed to hold relative to _K′_. Strictly speaking, such an æther-drift ought also to be assumed relative to the earth, and for a long time the efforts of physicists were devoted to attempts to detect the existence of an æther-drift at the earth’s surface.