5. HYDROGRAPHY 



185 



(f) Add the values in column (F) alge- 

 braically and enter the progressive sums 

 with their correct signs in column (G). 



(g) For convenience enter in column (H) 

 the depth of the bottom of each layer. This 

 is the depth for which the entry on the same 

 line in column (G) is the correction. 



(h) The values in column (G) are then 

 plotted on Form 117 with reference to the 

 depths in column (H) to which they apply 

 and a smooth curve is drawn through them. 

 A table of corrections is compiled from this 

 curve as shown in Figure 71. The units of 

 the corrections depend on the depth and 

 character of the area (see Table 2 and Sec- 

 tion 5-101). 



5-118 Graphic determination of velocity 

 corrections. — Specially prepared velocity cor- 

 rection graphs are used in determining ve- 

 locity corrections by the graphic method. 

 Three graphs are available from the Wash- 

 ington Office for calibration velocities of 

 800, 810, or 820 fathoms per second and 

 are identified by form numbers 117A, 117B, 

 and 117C respectively. Form 117C is re- 

 produced at a reduced scale in Figure 72. 



The velocity correction graphs are de- 

 signed for use directly with Form 117. They 

 are constructed so that horizontal scales for 

 the 20-fathom and 200-fathom layers corre- 

 spond to the scales used on the above form. 

 Where 5-fathom layer-intervals are used, the 

 values taken graphically from the graphs 

 must be divided by four to reduce them to 

 the 20-fathom layer scale for use on Form 

 117. This can be done by using proportional 

 dividers. 



By use of a graph the velocity correction 

 curve can be constructed directly on Form 

 117 from the temperatures and salinities in 

 columns (B) and (C) of Table 11. The 

 procedure is as follows : 



(a) For depths of 200 fathoms or less. 

 — Using the mid-depth of the first layer and 

 corresponding temperature and salinity 

 values, as in columns (A), (B), and (C) in 

 Table 11, enter the appropriate correction 

 graph with the temperature as an ordinate 

 and find where it intersects the salinity curvt 

 for the corresponding salinity. With a pro- 



portional divider set to a ratio of 1 to 4, 

 measure the distance from the intercept to 

 index line of the graph, and transfer one- 

 fourth of the distance to Form 117 in the 

 proper direction at the bottom of the first 

 layer. This gives one point on the desired 

 velocity correction curve. 



Proceed in like manner for the second 

 layer, except that the intercept is added 

 graphically to the distance already laid off 

 for the first layer, always being careful to 

 observe the sign of the correction. Transfer 

 the resultant total distance to Form 117 at 

 the bottom of the second layer. 



Follow the above procedure for other lay- 

 ers until the entire depth range has been 

 covered. Note that the one-fourth reduction 

 is not made for 20-fathom layers. It is to be 

 noted that no correction for pressure is in- 

 cluded for depths less than 200 fathoms. 

 Consequently there will be a slight, but neg- 

 ligible difference between curves derived by 

 the two systems. 



(b) For depths greater than 200 fathoms. 

 — The use of the velocity correction graphs 

 for depths greater than 200 fathoms is ex- 

 actly the same as described above, except for 

 an additional correction for pressure. Near 

 the right-hand edge of the velocity correc- 

 tion graph is a diagonal line labeled "Pres- 

 sure Correction Curve for 200-Fathom Lay- 

 ers." This curve is based on corrections 

 computed for 200-fathom layers at the stand- 

 ard velocity of the graph, each correction 

 being based on the mean depth of the layer ; 

 thus, the pressure correction of 4.8 fathoms 

 (Fig. 72) read on the curve opposite 1,200 

 fathoms is the correction for the 200-fathom 

 layer between 1,000 and 1,200 fathoms, com- 

 puted 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 cor- 

 rection curve at 200 fathoms, scale the cor- 

 rection for 200 fathoms from the shoal-water 

 correction curve already prepared and divide 



