large diameter, are now in general use, and with the excellent
machines, and under special mechanics, are built up of five separate
pieces in such a rigid manner that they possess all the solidity
necessary for a crank shaft. The forgings of iron and steel being much
smaller are capable of more careful treatment in the process of
manufacture. These shafts, for large mail steamers, when coupled up,
are 35 feet long, and weigh 45 tons. They require to be carefully
coupled, some makers finishing the bearings in the lathe, others
depend on the excellence of their work in each piece, and finish each
complete. To insure the correct centering of these large shafts, I
have had 6 in. dia. recesses ¾ inch deep turned out of each coupling
to one gauge and made to fit one disk. Duplicate disks are then fitted
in each coupling, and the centering is preserved, and should a spare
piece be ever required, there is no trouble to couple correctly on
board the steamer.

The propeller shaft is generally made of iron, and if made _not less_
than the Board of Trade rules as regards diameter, of the best iron,
and the gun metal liners carefully fitted, they have given little
trouble; the principal trouble has arisen from defective fitting of
the propeller boss. This shaft working in sea water, though running in
lignum vitæ bearings, has a considerable wear down at the outer
bearings in four or five years, and the shaft gets out of line. This
wear has been lessened considerably by fitting the wood so that the
grain is endway to the shaft, and with sufficient bearing surface
these bearings have not required lining up for nine years. It is,
however, a shaft that cannot be inspected except when in dry dock, and
has to be disconnected from the propeller, and drawn inside for
examination at periods suggested by experience. Serious accidents have
occurred through want of attention to the examination of this shaft;