556 HYDROGRAPHIC MANUAL PaGE 524 



per second. Instruments of this type are now used simultaneously with, and to 

 supplement, the Dorsey Fathometer No. 3. Since the motor of the latter is tuning-fork 

 controlled, its speed is quite accurate and may be used as a speed standard for the 

 Veslekari instrument. 



The Veslekari may be adjusted to a cahbration velocity of 820 fathoms per second 

 by comparison with the Dorsey No. 3. The two instruments should be warmed up for 

 a half-hour, after which the gain of the echo amplifier of the Veslekari is increased until 

 the transmitted signal of the Dorsey Fathometer records on the fathogram of the 

 Veslekari. The Dorsey Fathometer should have the depth-selector switch on position 

 3 and the starting-frequency switch on position 4. 



Because the Dorsey Fathometer is transmitting more than four signals for each 

 stylus cycle of the Veslekari there will always be one and sometimes two transmitted 

 signals of the Dorsey Fathometer recorded for each of these cycles. These may occur 

 at any position relative to the index of the Veslekari. 



If the speeds of the two motors were synchronized for the same velocity of sound, 

 the Dorsey signals would record in a vertical line on the fathogram. If the two speeds 

 differ, the successive signals of the Dorsey instrument are recorded at advanced 

 or retarded positions for each cycle of the Veslekari, resulting in a line of recorded 

 signals that crosses the fathogram at an angle. 



For perfect instrumental speed, the governor of the Veslekari motor should be 

 adjusted so that the Dorsey signals record in a nearly vertical line, but such an adjust- 

 ment is almost impossible to effect and to maintain. The governor adjustment may 

 be considered satisfactory if the time from the first recorded Dorsey signal at one edge 

 of the fathogram to the last recorded signal (of the same cycle) at the other edge is 

 more than 3^ minutes. 



Where the Veslekari is on a ship where there is no Dorsey Fathometer No. 3 with 

 which to compare its motor speed, the speed may be verified by timing the revolutions 

 of the phasing-switch brush. For a velocity of sound of 820 fathoms per second, the 

 phasing-switch brush of the Veslekari should rotate at 0.3417 r.p.s. The instrument 

 should be run for a half-hour before starting the test. The front of the recorder 

 cabinet should be open while the test is made. A reference mark should be made or 

 noted on the stationary part of the phasing switch, which is directly behind the phasing 

 dial when the front of the cabinet is closed. As the phasing-switch brush passes the 

 reference point, start a stop watch and count 41 revolutions of the brush, and stop the 

 stop watch at the end of the 41st revolution. If the motor speed is correct the measured 



time should be 120 seconds. 



556. Separation Effect 



For an echo-sounding mstrument with separate transmitting and receiving units, 

 the physical separation of the units must be taken into account. This separation has 

 two effects on echo soundings, (a) to make the registered depth greater than the actual 

 depth, and (6) to make the transmitted signal register later than it would if there were 

 no separation between imits, unless the units are electrically coupled. 



The first of these two effects is caused by the inclination of the sound path {AC 

 and CB in fig. 113) due to the separation of the acoustic units. This introduces an 

 error that increases with an increase in the distance between the acoustic imits but 

 decreases with an increase in the depth of water. It is appreciable only in compar- 

 atively shoal water. Where the depth of water is five times the distance of separation 

 between acoustic units, the true depth is 99.5 percent of the registered depth. Where 



