very
great care has to be taken to prevent the cement from being washed away
from the other constituents when passing through the water. Indeed, this
is bound to happen more or less, but it is guarded against by lowering
the concrete slowly in a special box, the bottom of which is opened as
it reaches the ground on which the concrete is to be laid. This method
can only be carried out in still water, and where strong and tight
framing can be built which will prevent the concrete from escaping. For
small work the box can be replaced by a canvas bag secured by a special
tripping noose which can be loosened when the bag has reached the
ground. The concrete escapes from the bag, which is then drawn up and
refilled.

Strength.

Concrete may be compared with other building materials like masonry or
timber from various points of view, such as strength, durability,
convenience of building, fire-resistance, appearance and cost. Its
strength varies within very wide limits according to the quality and
proportions of the constituents, and the skill shown in mixing and
placing them. To give a rough idea, however, it may be said that its
safe crushing load would be about 1/2 cwt. per sq. in. for lime concrete,
and 1 to 5 cwt. for Portland cement concrete. The safe tensile strength
of Portland cement concrete would be something like one-tenth of its
compressive strength, and might be far less. On this account it is usual
to neglect the tensile strength of concrete in designing structures, and
to arrange the material in such a way that tensile stresses are avoided.
Hence slabs or beams of long span should not be built of plain concrete,
though when reinforced with steel it is admirably adapted for these
purposes.

Durability.

In regard to durability good Portland cement concrete is one of the most
durable materials known. Neither hot, cold, nor wet weather has
practically any effect whatever upon it. Frost will not injure it after
it has once set, though it is essential to guard it from frost during
the operations of mixing and depositing. The same praise cannot,
however, be given to lime concrete. Even though the best hydraulic lime
be used it is wise to confine it to places where it is not exposed to
the air, or to running water, and indeed for important structures the
use of lime should be avoided. Good Portland cement is so much stronger
than any lime that there are few situations where it is not cheaper as
well as b