Page 537 echo sounding 5615 



Proceed in like manner for the second layer, except that the intercept is added graphically to the 

 distance already laid oflf on the strip of paper for the first layer, always being careful to observe the 

 sign of the correction. Transfer the resultant total distance to Form J-100-5 at the appropriate 

 depth for the second layer. 



Follow the above procedure for other layers until the entire depth range has been covered. 



Instead of a strip of paper a pair of dividers can be used, successive intercepts being added or sub- 

 tracted mechanically by increasing pr decreasing the span of the dividers. 



It is to be noted, as mentioned above, that no correction for pressure is included for depths less 

 than 200 fathoms. 



b. For depths greater than 200 fathoms. — The use of the velocity correction graphs for depths 

 greater than 200 fathoms is exactly the same as described above, except for an additional correction 

 for pressure. Near the right-hand edge of the velocity correction graph is a diagonal line labeled 

 "Pressure Correction Curve for 200-Fathom Laj'ers." This curve is based on corrections computed 

 for 200-fathom layers at the standard velocity of the graph, each correction being based on the mean 

 depth of the layer; thus, the pressure correction of 4.8 fathoms (fig. 115) read on the curve opposite 

 1,200 fathoms is the correction for the 200-fathom layer between 1,000 and 1,200 fathoms, computed 

 for a mean depth of 1,100 fathoms. 



For depths greater than 2,000 fathoms, a pressure curve can be readily constructed by transferring 

 the intercept for 2,000 fathoms (at the bottom of the graph) to the top of the graph and drawing a 

 line parallel to the printed pressure curve. 



For the first point on the deep-water correction curve at 200 fathoms, scale the correction for 200 

 fathoms from the shoal-water correction curve already prepared and divide it by 10 to adapt it to the 

 deep-water scale. Lay this distance off on the edge of a strip of paper, taking care to mark the correct 

 end zero and the value of the correction with the correct sign (plus to the left and minus to the right). 

 To this value apply graphically the correction at 200 fathoms from the pressure correction curve (it 

 will be noted that all corrections for pressure are plus and should be applied to the left), transferring 

 the resultant to Form J-100-5 at 200 fathoms (deep-water scale). 



Enter the velocity correction graph with the temperature and salinity for the mid-depth (300 

 fathoms) of the next layer, and apply in the proper direction the resulting intercept graphically to 

 the previous distance on the paper strip. Then set the paper strip on the pressure curve at 400 

 fathoms and from the last mark on the strip apply the pressure intercept graphically. The resultant 

 distance from the zero on the strip will be the distance to be laid off at 400 fathoms on Form J-100-5. 



Proceed in like manner for other layers to the deepest depth required. 



The construction of a deep-water curve is simplified where it is possible to transfer the pressure 

 correction curve from the velocity correction graph to Form J-100-5, In such cases the corrections 

 for temperature and salinity are laid off from the pressure curve as an initial, instead of from the zero 

 of the velocity correction curve. As each point on the curve is plotted, its distance from the zero line 

 of the curve must be marked on the strip of loaper before the next intercept for temperature and salinity 

 is added graphically to it, the total distance being laid off from the transferred pressure curve as an 

 initial. An analysis of this graphic method of summation will make it clear why this must be done. 

 Each horizontal distance plotted on the velocity correction curve in this manner must consist of the 

 summation of all the corrections for temperature and salinity, and all the pressure corrections except 

 the final one, the latter being accounted for by the transferred pressure correction curve. This possi- 

 bility of utilizing the pressure correction curve should be kept in mind when fixing the zero of the 

 velocity correction curve on Form J-100-5. 



c. For shoal-water depths. — ^ Where corrections in feet are desired, it is necessary to change all units 

 from fathoms to feet on both the velocity correction graph and on Form J-100-5. The corrections for 

 5-fathom layers printed at the bottom of the correction graph will then be in feet, and for 5-foot layers. 

 Likewise, the horizontal scale of Form J-100-5 will be 1 inch equals 0.4 foot, but the horizontal 

 scale of the velocity correction graph will be 1 incli equals 0.1 foot. Therefore, each distance scaled 

 from the velocity correction graph must be divided by four before being plotted on Form J-100-5. 

 This may be done conveniently by using proportional dividers to scale the intercepts. 



When determining velocity corrections in feet, it will also be found convenient to prepare the 

 temperature and salinity curves in feet instead of fathoms. 



After the velocity correction curves have been determined, the procedure is the 

 same as for the numerical method (see 5614). 



465382 —44 36 



