Spread out the mixture of sand and gravel as much as the board will
readily permit, add enough water to wet the gravel and sand thoroughly,
spread the cement evenly in a thin layer over the sand and gravel, and
turn over, as described previously, at least three times, adding the
rest of the water necessary to get the required consistency while the
materials are being turned. It requires some experience to work up a
natural mixture of bank sand and gravel, and if at all doubtful about
the concrete made from it, first screen the sand from the gravel, and
then mix in the regular way.
[Illustration: Fig. 10.—Mixing the Stone (or Gravel) with the Sand and
Cement.]