ble; and as the lime or cement is usually by far the
most expensive item, it is desirable to use as little of it as is
consistent with strength. When natural flint gravel containing both
stones and sand is used, it is usual to mix so much gravel with so much
lime or cement. The proportions in practice generally run from 3 to 1
for very strong work, down to 12 to 1 for unimportant work. Some
engineers have the sand separated from the stones by screens or sieves
and then remixed in definite proportions. When stones and sand are
obtained from different sources, their relative proportions have to be
decided upon. A common way of doing this is first to choose a proportion
of sand to cement, which will probably vary from 1 to 1 up to 4 to 1. It
then remains to determine what proportion of stones should be added. For
this purpose a large can, whose volume is known, is filled loosely with
stones, and the volume of the voids between them is determined by
measuring how much water the can will hold in addition to the stones. It
is then assumed that the quantity of sand and cement should be equal to
the voids. Moreover, the volume of sand and cement together is generally
assumed to be equal to that of the sand alone, as the cement to a large
extent fills up voids in the sand. For example, suppose it is resolved
to use 2 parts of sand to 1 of cement, and suppose that experiment shows
that in a pailful of stones two-fifths of the volume consists of voids,
then 2 parts of sand (or sand with cement) will fill voids in 5 parts of
stones, and the proportion of cement, sand, stones becomes 1:2:5. There
are several weak points in this reasoning, and a more accurate way of
determining the best proportions is to try different mixtures of cement,
stones and sand, filling them into different pails of the same size, and
then ascertaining, by weighing the pails, which mixture is the densest.

In determining the amount of water to be added, several things must be
considered. The amount required to combine chemically with the cement is
about 16% by weight, but in practice much more than this is used,
because of loss by evaporation, and the difficulty of ensuring that the
water shall be uniformly distributed. If the situation is cool, the
stone hard, and the concrete carefully rammed directly it is laid down
and kept moist with damp cloths, only just sufficient to moisten the
whole mass is required. On the other hand, water should be given
generousl