§ 3. THE MEASURE OF MASS

On the subject of measures of mass, similar remarks to those on
measures of length might be made. The confusion here was perhaps still
greater, because, to the uncertainty relating to the fixing of the
unit, was added some indecision on the very nature of the magnitude
defined. In law, as in ordinary practice, the notions of weight and of
mass were not, in fact, separated with sufficient clearness.

They represent, however, two essentially different things. Mass is the
characteristic of a quantity of matter; it depends neither on the
geographical position one occupies nor on the altitude to which one
may rise; it remains invariable so long as nothing material is added
or taken away. Weight is the action which gravity has upon the body
under consideration; this action does not depend solely on the body,
but on the earth as well; and when it is changed from one spot to
another, the weight changes, because gravity varies with latitude and
altitude.

These elementary notions, to-day understood even by young beginners,
appear to have been for a long time indistinctly grasped. The
distinction remained confused in many minds, because, for the most
part, masses were comparatively estimated by the intermediary of
weights. The estimations of weight made with the balance utilize the
action of the weight on the beam, but in such conditions that the
influence of the variations of gravity becomes eliminated. The two
weights which are being compared may both of them change if the
weighing is effected in different places, but they are attracted in
the same proportion. If once equal, they remain equal even when in