owing to the contraction of the _mass_ of metal forming the coupling;
the outside cooling first tears the center open, and when there is not
much metal to turn off the face of the coupling, it is sometimes
undiscovered. Having observed several of these cavities, some only
when the _last cut_ was being taken off, I have considered it
advisable to have holes bored in the end and center of each coupling,
as far through as the thickness of the flange; when the shafts are of
large size, this is sure to find these flaws out. Another flaw, which
has in many cases proved serious when allowed to extend, is situated
immediately abaft the gun metal liner, in front of the propeller.

This may be induced by corrosion, caused by the presence of sea water,
gun metal, and iron, assisted by the rotation of the shaft. It may
also be caused under heavy strain, owing to the over-finishing of the
shaft at this part under the steam hammer.

The forgemen, in these days of competition and low prices, are
instructed to so finish that there won't be much weight to turn off
when completing the shaft in the lathe. This is effected by the use of
half-round blocks under the hammer, at a lower temperature than the
rest of the forging is done, along with the use of a little water
flung on from time to time; and it is remarkable how near a forging is
in truth when centered in the lathe, and how little there is to come
off. The effect of this manipulation is to form a hard ring of close
grain about one inch thick from the circumference of the shaft inward.
The metal in this ring is much harder than that in the rest of the
shaft, and takes all the strain the inner section gives; consequently,
when strain is brought on, either in heavy weather or should the
propeller strike any object at sea or in the Suez canal, a fracture is
caused at the circumference. This, assisted by slight corrosion, has