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As this adjustable type of grid has proved to be extremely useful and 
has not been reported elsewhere by Division 2, it will be described in 
greater detail. It was first developed at the Woods Hole Oceanographic 
Institution by Ewing and Vine for use in the analysis of bathythermograph 
records. On two opposite sides of a parallelogram are placed strips with 
holes equally spaced, Thread or wire is strung through these holes to form 
the grid. The most satisfactory material found for use with this particular 
instrument has been black nylon filament. As the parallelogram is closed 
or opened, the spacing between the grid strings becomes smaller or greater. 
This was adapted to the Harp by adding a fixed grid mounted immediately 
below the strings of the adjustable grid. ‘Small angular adjustments are 
allowed for slight deviations from 90°, correcting for any non-orthogonality 
of the cathode-ray tube plates, The whole assembly may be rotated, or moved 
in a vertical or horizontal direction to allow the base line and origin of 
the pressure and time axes to be set to correspond with that on the image 
of the pressure-time curve. These adjustments are 211 made by means of 
screws whose controls are conveniently located for the operator. After ad- 
justing the Harp so that the pressure and time grids read in absolute units, 
and the origin and base lines are properly aligned, the front of the assembly 
is raised to allow the photographic paper to be placed below the grid. The 
back of the assermbly is rigidly hinged so that it cannot get out of adjust- 
ment during this process. The assembly is returned to its rest position, 
a mask is placed around the edges of the paper to identify the axes and 
place a form in the upper right hand corner of the projection for recording 
pertinent data, The timing calibration shows directly on this projection, 
allowing a check of the units of the time axis at any later time. A projec- 
tion of the calibration step is also made which allows the units of the 
pressure axis to be checked later if necessary. 
The Harp reproduces photographically an exact graph of the original 
trace of the pressure-time curve, This obviously has many advantages: (1) 
the tedious readings on the micrometer microscope have been eliminated; (2) 
time and pressure calibrations may be checked dircetly on the prints; (3) 
the process is photographic which means that the original piezoelectric 
record may be inspected in detail in its enlarged form, and fine structure 
which may be present may be immediately detected. This is not the case with 
micrometer microscope readings since the fine structure is often too detailed 
to record point for point, A very important advantage of Harp projection 
is that the projected records themselves indicate the uncertainties in 
values due to poor traces, width of trace, and so forth, This was not 
possible when the records were read on the micrometer microscope by tech— 
nicians. 
Peak pressures may be read directly from the records and the impulse 
and energy integrals may be calculated by the trapezoidal rule as they were 
from the micrometer microscope readings. With regard to the Q=stcp there 
is a small overshoot at the beginning which amounts to from 1 to 3 percent 
(compared to the step height at 1 msec.) due to the frequency response of the 
system. Since the major portion of the pressure-time curve is used for 
measuring impulse and energy, the Harp is set on the Q-step 1 msec after the 
