The Worcester spy. [volume] (Worcester, Mass.) 1898-1904, 1901-11-24
SEASICKNESS, MUSIC.
SHIPS, AND VIOLINS

tons Of Violin Strings-The
Principle of Nodes and Loops
Applies to Both
Boston, Nov. 29,-Not all seasickness
is due to the rolling of the ocean
much Of it is caused by the ship itself
if it be steam ship, that Island
comes from the vibrations of the en-
gines. Some interesting studies have
recently been made of these vibrations,
among others by Professor Cecil H.
Peabody of the department of naval
architecture at the Massachusetts In
/ stitute of Technology. In these stud
les, queerly enough, the phenomena of
seasickness are found to be most east
~ explained by reference to the phe-
nomena of music with the result that
an entirely scientific parallel has been
/ established between the steel hull of
thirty knot torpedo boat vibrating
under the strain of her vertical, In
4 Verted, triple-expansion" engines, and
I o violin or piano string, vibrating to
/ the touch of bow or key
| The comparison between the violin
string and the steamship will be easier
to understand by introducing still a
| third factor, long bar of steel such
| as may be found in any blacksmith's
SHOP, which may represent either the
string or the steel hull If this steel
bar Is laid on two saw-horses or some
other convenient support one near
either end, and struck with hammer,
1t gives out musical note comparable
to that of the violin string when it is
touched by the resined bow If a man
will contrive to stand on a bar of
sufficient length he can produce such
vibrations by jumping up and down, !
if he times his jumping so as to allow !
his full weight to coincide with the |
downward rather than with the up |
ward bending of the bar. Should the /
man stand astride Of one of the sup |
ports an then move the bar after the !
manner of a see-saw, by alternating
his weight from one foot to the other. !
he will also give it a trembling motion
which may be compared to the vibra-
tons of violin string under the pres
sure of the players finger. Under these
stresses both the string and the bar
VIII have what are called in the lam
guage of physics loops and nodes"
the loops being represented by
curved departure from the straight
line OF the bar, first one way and then
another, while the nodes are the points
where there is no movement or vibra-
ton.
These loops and nodes can be located,
roughly, by putting bits Of paper upon
/ the steel bar at short intervals The
DTS that are placed at loops will be
thown off by the vibrations, while
those placed at nodes win be undis-
turbed. In the case of the steel steam
shlp-which the steel bar has for the
moment been representing-exac the
same principle applies; except that the
test may be made with human beings
! instead of with bits of paper Both
passengers and crew have learned by
| experience that there are certain places
| on ship where the vibrations are
very 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 ayioln
! 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 book might be used to test the
comparative vibrations of steamer
Very soon, however, testing instru-
ments We're devised, so that an ac-
curate record of the number of vibra-
tons 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
1 calculation
Inc practical purpose of all this
work, of course, has been to determine
such an arrangement of a ships en-
glnes-which are the cause of most of
! the difflculty-as should reduce the vi
| brations to a minimum, partly be-
cause they result in so much discomfort
| to passengers, and mean loss of BUSH
ness, and partly because they consti-
tute wear and tear on the ship itself
1t was apparent that engines with
single virtical 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 pos
sible TO the ships nodes. Other ma
chinery which exerted a rocking
couple'-an alternating movement
comparable to that of the man astride
of the support of the steel bar-should
be kept away from supports or nodes
Certain practical difficulties. of course.
prevent the perfect application of
these rules, as for example, the impos-.
sibility of placing the engines very
near the bow, or the stern. Where
more complicated engines are en-
ployed. moreover,-engines which ex-
ert an up-and-down and back-and-
forth motion simultaneously-th best
that can be done is to select a COM
promise situation.
When the great express steamers
were put on the Atlantic service 1T
was noticed that from time to time the
ships vibrations would increase with
out apparent cause. The shin might
be steaming through comparatively
quiet seas, but the shaking and quak-
ing Of the deck would manifest itself
at intervals until it might send a
score of passengers below with all the
distressing symptoms of seasickness. /
Careful observation. however, showed
that such excessive vibrations took
place when the twin engines got to
running in unison, so that when one
engine raced ahead of the other the
vibration was again decreased When
fast torpedo boats were built there was
a similar trouble, and Yarrow. the ia
nous English shipbuilder, learned by
experiment that just as violin string
has a certain lumber of vibrations a
minute which determined its note, so
also a ship VibTates in accordance
with its own natural time and is most
seriously affected when the revolutions
Of its engines coincide with that time
or any Of its multiples or sub-multi-
pies. For example. II the greatest vi
bration is noticed when the engines are
making 900 revolutions a minute. the
vibration will be less at 500 or 700 than
at 400 or 800 revolutions In other
words, Just as the open string Of a
piano may be made to vibrate by the
sound of the corresponding note, or as
bridge of building may be Jared