b. per square inch, it has been
found that not more than 120 lb. per square inch should be exerted to
keep them cool (this varies according to the material of which the
bearing is composed), without having to use sea water and prevent them
being ground down, and thus getting out of line. I have known a
bearing in a new steamer, in spite of many gallons of oil wasted on
it, wear down one-eighth of an inch in a voyage of only 6,000 miles,
from insufficiency of bearing surface.

Several good rules are in use governing the strength of shafts, which
treat of the diameter of the bearings only and angles of the cranks;
and the engine builder, along with the ship owner, has been chary of
increasing the surfaces by lengthening the bearings; for to do this
means increase of space taken up fore and aft the vessel, besides
additional weight of engine. Engine builders all aim in competing to
put their engines in less space than their rivals, giving same power
and sometimes more. I think, however, this inducement is now more
carefully considered, as it has been found more economical to give
larger bearing surfaces than to have steamers lying in port, refitting
a crank shaft, along with the consequences of heavy bills for salvage
and repairs, also the risk of losing the steamer altogether.
Proportioning the bearings to the weights and strains they have to
carry has also been an improvement. The different bearings of marine
engines were usually made alike in surface, irrespective of the work
each had to do, with a view to economy in construction.

In modern practice the after bearings have more surface than the
forward, except in cases where heavy slide-valve gear has to be
supported, so that the wear down in the whole length of the shaft is
equal, thus avoiding those alternate bending strains at the top and
bottom of the stroke every revolution. Another improvement that has
been successfully introduced, adding to the duration of life of crank
shafts, is the use of white bearing metal, such as Parson's white
brass, on which the shafts run smoothly with less friction and
tendency to heat, so that, along with well proportioned surfaces, a
number of crank shafts in the Peninsular and Oriental Co.'s service
have not required lining up for eight years, and I hope with care may
last till new boilers are required. Large and powerful steamers can be
driven full speed from London to Australia and back without having any
water on the bearings,