6343 HYDROGRAPHIC MANUAL PaGE 588 



should be shown on both curves. Only one serial temperature shall be plotted on a 

 sheet. Where a serial temperature extends into depths greater than 1,200 fathoms, 

 the observations at depths below 1,200 fathoms should only be recorded on Form 717, 

 but a notation should be added at the bottom of the graph stating the maximum 

 depth of observations. For the sake of uniformity serial temperatures shall be plotted 

 only on Form B- 1528-5, and only at the specified scales. 



The temperature and salinity curves may be inked any time after a serial tempera- 

 ture has been completed. The two sections of the temperature curve may be inked as 

 heavy black lines, unless they cross and there is probability of confusion, in which case 

 the deep section should be inked in red. Each observed value should be indicated by 

 a small circle. The depth of some one observation on each curve, or on each section 

 if there are two, shall be indicated in numerals. 



The graphs of serial temperatures, with the records of observations, should be 

 arranged in chronological order and bound together and forwarded to the Washington 

 Office. They should not be bound with any other report or records. (See 8331.) 



6343. Computation of Velocities 



As stated in 63, the velocity of sound in sea water depends on the temperature, 

 salinity, and pressure. Therefore, if the temperature and salinity are known at any 

 depth, the velocity of sound at that depth may be computed from tables 32, 33, and 

 34 in 9611. These tables are based on Tables of the Velocity of Sound in Pure Water 

 and Sea Water, H. D. 282, published by the Hydrographic Department of the British 

 Admiralty. Table 32 contains the velocity in meters per second for various tempera- 

 tures under atmospheric pressure and at a salinity of 35.0 %o. The correction to be 

 applied to the velocity in table 32 for the difference between the actual salinity and 

 35.0 %o is given in table 33, the correction being subtractive where the actual salinity is 

 less than 35.0 %o and additive where it is greater. Table 34 gives the correction for 

 pressure, to 4,000 fathoms, to be applied to the velocity in table 32 ; it is always additive. 

 The pressure correction varies not only with hydrostatic pressure but also with gravity 

 and therefore with latitude. Table 34 is computed for latitude 45°, a mean value 

 sufficiently accurate for surveying purposes (see 63). The pressure correction is rela- 

 tively uniform and for depths less than 1,000 fathoms it may be found by dividing the 

 depth in fathoms by 30, which will give the correction in meters per second. 



Velocities are derived on Form 717 from entries made from the above tables. 

 Using the " corrected temperature at depth," the velocity found from table 32 is entered 

 in the column headed "Velocity at temp." The corrections to this value for salinity 

 and pressure at depth of observation are found from tables 33 and 34, respectively, 

 and, with their algebraic signs, are entered in the double column headed "Corrections." 

 The algebraic sum of these three column entries is entered in the column headed 

 "Velocity (theoretical)." This value is the computed velocity of sound in sea water 

 at the depth of the observations. Velocities are usually computed and recorded to 

 tenths of a meter per second. 



Echo-sounding corrections and occasionally velocities for use in R.A.R. are based 

 on theoretical velocities, meaned from the surface to the various depths of water. The 

 temperatures and salinities, for a period of time in which they can be assumed to have 

 remained approximately the same throughout an area, are used (see 5612). 



The velocity for each depth at which observations are made in a serial temperature 

 is usually computed and entered on Form 717. Instead of graphs of temperature and 



