present, as a result of the reaction of the sulphuric acid of
the salt dissolved by the water upon a part of the lime in the
cement. The ferric oxide of the cement is unaffected by sea-
water.

The neat cement briquette tests showed that those immersed in
sea-water attained a high degree of strength at a much quicker
rate than those immersed in fresh water, but the 1 to 3 cement
and sand briquette tests gave an opposite result. At the end of
twelve months, however, practically all the cements set in
fresh water showed greater strength than those set in sea-
water. When briquettes which have been immersed in fresh water
and have thoroughly hardened are broken, the cores are found to
be quite dry, and if briquettes immersed in sea-water show a
similar dryness there need be no hesitation in using the
cement; but if, on the other hand, the briquette shows that the
sea-water has permeated to the interior, the cement will lose
strength by rotting until it has no cohesion at all. It must be
remembered that it is only necessary for the water to penetrate
to a depth of 1/2 in on each side of a briquette to render it
damp all through, whereas in practical work, if the water only
penetrated to the same depth, very little ill-effect would be
experienced, although by successive removals of a skin 1/2 in
deep the structure might in time be imperilled.

The average strength in pounds per square inch of six different
well-known brands of cement tested by Mr. O'Hara was as
follows:--