We must next consider, not weight in general, but the weight of water.
We say that a tumbler full of water is heavier than an empty tumbler,
because the full tumbler gives us a greater feeling of effort when we
lift it than the empty tumbler does. The more water there is in the
tumbler the greater is the effort. A pail full of water requires still
more effort, though the empty pail feels quite light; and, when we come
to deal with a large tub full of water, we may be unable to stir it,
though the empty tub could be lifted with ease. Thus it seems that the
greater the bulk of water the more it weighs, and the less the bulk the
less it weighs. But then a single drop of water in the palm of the hand
seems to weigh nothing at all. However, this clearly cannot be, for the
drop falls to the ground readily, and therefore it must have weight.
Moreover, a few thousand drops would fill the tumbler, and if a thousand
drops weigh something, each drop must have a thousandth of that weight.
The fact is that our feeling of effort is a very rough measure of
weight, and does not enable us to compare small weights, or even to
perceive them if they are very small. To know anything accurately about
weight we must have recourse to an instrument which is contrived for the
purpose of measuring weights with precision.