New-York tribune. [volume] (New York [N.Y.]) 1866-1924, 1897-04-11
AIR VIBRATION TOLD THE TALE.

LOCATING HIDDEN OBSTRUCTION IN AN UN-
DERGRoUND TUBE IN PHILADELPHIA
An exceedingly clever feat recently per-
formed In Philadelphia In that city. perhaps more
extensively than In New-York at present, under
ground tubes used for the conveyance Of tele-
grams letters and small parcels. A few days ago
one Of the cylindrical rubber boxes. or carriers
employed In this service stopped somewhere on a
certain route. Additional air pressure behind It
failed to drive II to It's destination and extra
ton could not recall 1t to the starting point Like
the Greek troops In Crete, It seemed to have re-
solved to stay OF course such situation brought
business along that particular route TO a complete
standstill and II was found necessary to dig down
to the pipe open it and remove the box by hand.
But the great problem was to determine just where
TO dig Where WAS the box7 Ten feet or two thou
sand feet away from the starting point? For many
reasons 1t undesirable to dig up half mile OF
pipe in order to remove this obstruction. Other
means Of locating the seat OF the trouble having
failed the following plan OF operations was adopted
It was proposed to fire off pistol near the open
end OF the tube. and then. If the hidden box gave
back an echo, measure the time required for the
sound to 20 and return. and thus estimate the dis
tance of the object Sound, II Is well known. trav-
els at the of about 100 feet second. Hence
it became desirable to measure the time to within
a thousandth part of n second. Moreover. ms the
velocity would be slightly affected by temperature,
It was necessary TO make a suitable allowance for
the latter element in the problem
The chronograph mprovised for registering tho
time consisted in part OF metal cylinder or drum
ten inches in diameter, which could be revolved by
a hand crank and which would move endwsc very
slowly when in rotation. The polished metal sur
tonless pin point, in contact with the drum. would
trace a fine spiral line thereon. The point was not
motionless though 1t was attached by horse
hair and seang-wax TO one prong OF tuning
fork giving the musical note and th.Tsfore e
waves per second were impaired to the trace; and
these large enough TO admit OF division into
quarters Another disturbance OF the point was
ing half-scconds. Each beat made short, sharp
projection sideways the wavy line. Hence the
marks while the time interval remaining after the
last pendulum could be asceraincd from the tuning
fork waves
Finally, provision was mnds for automatically
inaI shot and also Of the arrival Of the echo. A
vbra ting diaphragm close 10 the drum bore another
STYLUS or sharp point. and this diaphragm WAS 80
Sensitive that when struck by sound II would
move enough 10 make scratch on the sooy sur
face. go
the mean OF the observations gave q 793 seconds
the time required FOR the sound to travel both
WAYS A velocity Of 1903 feet was assumed for
temperature OF % degrees. and correction OF LIZ
feet Der second for each degree above that stand
in the ground was 80 degrees The computed veloc-
ify was J. i0i feet and tho estimated distance.
counting both ways. was therefore a06 feet. Divid
The re M, mSds tho a., HSfS a, S
This was more than quarter OF mile on. When
tyorkmen dug down at the designated spot. several
blocks away they found the box within a foot or
two OF the place A break had occurred IN the
nine about twenty feet further away. but the OBJ
structon was found exactly whore calculation
located IL