It is certain that all the friction exercised on the earth--by the tides, for instance--must slowly lengthen the duration of the day, and must influence the movement of the earth round the sun.

Such influence is certainly very slight, but it nevertheless gives an unfortunately arbitrary character to the unit adopted.
We might have taken as the standard of time the duration of another natural phenomenon, which appears to be always reproduced under identical conditions; the duration, for instance, of a given luminous vibration.

But the experimental difficulties of evaluation with such a unit of the times which ordinarily have to be considered, would be so great that such a reform in practice cannot be hoped for.

It should, moreover, be remarked that the duration of a vibration may itself be influenced by external circumstances, among which are the variations of the magnetic field in which its source is placed.

It could not, therefore, be strictly considered as independent of the earth; and the theoretical advantage which might be expected from this alteration would be somewhat illusory.
Perhaps in the future recourse may be had to very different phenomena.
Thus Curie pointed out that if the air inside a glass tube has been rendered radioactive by a solution of radium, the tube may be sealed up, and it will then be noted that the radiation of its walls diminishes with time, in accordance with an exponential law.