5. HYDROGRAPHY 



187 



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 pres- 

 sure 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 117 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 dis- 

 tance 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 re- 

 sultant distance from the zero on the strip 

 will be the distance to be laid oflf at 400 

 fathoms on Form 117. 



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 veloc- 

 ity correction graph to Form 117. In such 

 cases the corrections for temperature and sa- 

 linity 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 paper before the next intercept 

 for temperature and salinity is added graph- 

 ically 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 ex- 

 cept the final one, the latter being accounted 

 for by the transferred pressure correction 

 curve. This possibility of utilizing the pres- 



sure correction curve should be kept in mind 

 when fixing the zero of the velocity correc- 

 tion curve on Form 117. 



(c) For shoal-water depths. — Where cor- 

 rections in feet are desired, it is necessary 

 to change all units from fathoms to feet on 

 both the velocity correction graph and on 

 Form 117. The corrections for 5-fathom 

 layers printed at the bottom of the correc- 

 tion graph will then be in feet, and for 5-foot 

 layers. Likewise, the horizontal scale of 

 Form 117 will be 1 inch equals 0.4 foot, but 

 the horizontal scale of the velocity correction 

 graph will be 1 inch equals 0.1 foot. There- 

 fore, each distance scaled from the velocity 

 correction must be divided by four before be- 

 ing plotted on Form 117. 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. 



5-119 Velocity corrections by Veloci- 

 meter. — The velocity of sound in sea water 

 can be determined in situ by use of the ve- 

 locimeter described in 3-132. When this in- 

 strument is used, the observations are trans- 

 lated to velocity and a velocity curve related 

 to depth can be drawn on any suitable graph 

 paper. The layer velocities shown in Column 

 (D) in Table 11, can be scaled from the 

 velocity curve at the mid point of the layer. 

 The correction factor is obtained by refer- 

 ence to Table 17 or can be computed. The 

 procedure from this point is the same as for 

 the numerical method of deriving the 

 corrections. 



The present stage of development of the 

 velocimeter limits its use for this purpose 

 to depths of about 500 feet. True depths of 

 the various velocity measurements depend 

 on the verticality of the cable used to sus- 

 pend the instrument. Therefore, it should 

 be used only under favorable conditions of 

 wind, sea, and current. 



