or cement
will be killed by the excess of water, or at least have its binding power
affected. It is not advisable to carry out work when the temperature is
below freezing point, but in urgent cases bricklaying may be successfully
done by using unslaked lime mortar. The mortar must be prepared in small
quantities immediately before being used, so that binding action takes
place before it cools. When the wall is left at night time the top course
should be covered up to prevent the penetration of rain into the work,
which would then be destroyed by the action of frost. Bricks used during
frosty weather should be quite dry, and those that have been exposed to
rain or frost should never be employed. The question whether there is any
limit to bricklayers' work in frost is still an open one. Among the members
of the Norwegian Society of Engineers and Architects, at whose meetings the
subject has been frequently discussed, that limit is variously estimated at
between -6 deg. to -8 deg. Reaumur (181/2 deg. to 14 deg. Fahr.) and -12 deg. to -15 deg. Reaumur (5 deg.
above to 13/4 deg. below zero Fahr.). It has been proved by hydraulic tests that
good bricklayers' work can be executed at the latter minimum. The
conviction is held that the variations in the opinions held on this subject
are attributable to the degree of care bestowed on the preparation of the
mortar. It is generally agreed, however, that from a practical point of
view, bricklaying should not be carried on at temperatures lower than -8 deg.
to -10 deg. Reaumur (14 deg. to 91/2 deg. Fahr.), for as the thermometer falls the
expense of building is greatly increased, owing to a larger proportion of
lime being required.

For grey lime mortar the usual proportion is one part of lime to two or
three parts of sand; lias lime mortar is mixed in similar proportions,
except for work below ground, when equal quantities of lime and sand should
be used. Portland cement mortar is usually in the proportions of one to
three, or five, of sand; good results are obtained with lime mortar
fortified with cement as follows:--one part slaked lime, one part Portland
cement, and seven parts sand. Roman cement mortar should consist of one or
one and a half parts of cement to one part of sand. Selenitic lime mortar
is usually in the proportions of one to four or five, and must be mixed in
a particular manner, the lime being first ground in water in the mortar
mill, and the sand gradually added. B