The Washington times., 1901-11-10

great, and other places where they are
hardly noticeable; and naval architects
have had no hesitation in explaining the
difference by pointing out that a ship,
too. has it's nodes and loops. and vibrates,
to all intents and purposes, just like the
string of a violin or the steel bar between
the saw horses. Later it was found that
the shaking at some points prevented the
reading of fine type, so that the page Of
a book might be used to test the compare
tive vibrations Of a steamer. Very soon,
however, testing instruments were de-
vised so that an accurate record Of the
number of vibrations a minute could be
taken, and from the definite data thus
afforded a mathematical formula has
been worked out by means Of which the
principal nodes and loops Of any ship
can approximately be obtained from cal
culation.
The practical purpose Of all this work,
of course, has been to determine such an
arrangement of a ships engincS-which
are the cause of most Of the difiiculty-as
should reduce the vibrations to a mini
mum, partly because they result in 80
much discomfort to passengers, and
mean loss Of business, and partly because
they constitute II wear and tear on the
with a single vertical piston rod, or, in
other words. those which had an up and
down motion, exerting free force" so
called, like that OF the man eXerting his
weight on the experimental steel bar,
should be kept away from the ships loops
and placed as near as possible TO the
ships nodes. Other machinery which
exerted a rocking couple'-an alternat-
ing movement comparable to that of the
man astride of the support Of the steel
bar-should be kept away from the sup
ports or nodes.
Certain practical difficulties, of course,
prevent the perfect application of these
rules. as for example, the impossibility of
placing TIE engines very near the bow or
the stern Where more complicated en-
gines are employed, moreover-engines
which exert an up-and-down and back
and-forth motion simultaneously-the
best that can be done is to select a COM
promise Sjtuation.
When the great express steamers We're
put on the Atlantic service It was noticed
that from time 10 time tho ships vibra-
tions would Increase without apparent
cause The ship might be steaming
through comparatively quiet seas but the
shaking and qualing Of the deck would
manifest llseIi at INltINdls ullll IL might
send a score Of Taasser gers below with
all the distressing symptoms OF seasick-
ness. Careful observation, however,
showed that such excessive vibrations
took place when the twin engines got to
running in unison. so that when one en-
gine raced ahead Of the other the vibra-
tion was again decreased. When fast
torpedo boats were built there was a
similar trouble, and Yarrow. the famous
English shipbuilder. learned by expert
ment that just as q violin string has a
certain number Of vibrations a minute
which determines its note. so also ship
vibrates IN accordance with its own natu-
raj time and is most seriously affected
when the revolutions OF its engines coin
eide with that time or any Of its multiples
oT sub b.multlDies.
For example. If the greatest vibration
Is noticed when the engines are making
COO revolutions q minute, the vibration
viii be less at 500 or 700 than at 400 or 500