525 HYDROGRAPHIC MANUAL PaGE 478 



ing for the decreasing beats between the sound made by the rotor teeth and the 2050- 

 cycle sound, synclironization of the motor occurring when the beats are no longer heard. 

 The Fathometer is now ready for sounding. The gain control on the front of the 

 echo amplifier should be adjusted to as high a value as practicable without causing 

 strays and an excessive number of multiple echoes, 



525. DoRSEY Fathometer No. 2 



The Dorsey Fathometer No. 2 is a combined sonic and supersonic visual-type of 

 echo-sounding instrument, designed to supplement the Dorsey No. 1, and for measure- 

 ments in moderately shoal to deep depths. It incorporates some of the accuracy fea- 

 tures of the Dorsey Fathometer No. 1 and the depth range of the 312 Fathometer. 

 The Dorsey Fathometer No. 3 has now superseded the Dorsey No. 2 because it com- 

 bines the features of the Dorsey No. 1 and Dorsey No. 2. For this reason the Dorsey 

 Fathometer No. 2 is described only briefly. 



The transceiver, one of the echo amplifiers, and the power supply of the Dorsey No. 

 2 are very similar to those of the Dorsey No. 1. The echo amplifier of the Dorsey No. 2 

 is provided with an audio outlet, making it possible to listen to the returning echo with 

 headphones. The principal difl^erences between the two systems are in the indicating 

 mechanisms, and the incorporation of the 312 Fathometer system in the Dorsey 

 Fathometer No. 2. The parts of the 312 Fathometer (see 521) which are used in 

 conjunction with the Dorsey No. 2 are the oscillator, the 525-cycle motor generator, 

 hydrophones, and the 1050-cycle echo amplifier. 



There are two concentric depth scales; the outer graduated from to 100 fathoms, 

 and the inner from to 1,000 fathoms. One neon tube serves for both scales, its 

 light being refracted to the inner scale by means of a prism rotated on a radial arm. 

 An annular disk, carrying the index, rotates between the circular neon tube and the 

 100-fathom scale at 4.1 revolutions per second, and the radial arm, carrying the 

 prism, rotates inside the disk, just behind the 1,000-fathom scale, at 0.41 revolutions 

 per second. The arm and disk are geared to the same tuning-fork controlled reluctance- 

 type synchronous motor, the speed of which is 4.1 revolutions per second. 



The selection between the two scales of the dial is made by throwing a toggle 

 switch on the inside of the indicator cabinet. This selects the photoelectric tube keying 

 circuit associated with one of the two depth scales. A selector switch mounted along- 

 side the cabinet makes the necessary circuit changes from the sonic to the supersonic 

 system or vice versa. 



The supersonic system utilizes circuits similar to those in the Dorsey Fathometer 

 No. 1 and is used for the shoaler depths, while the sonic system is operated through 

 circuits similar to those used in the 312 Fathometer and is used for the deeper depths. 



526. Dorsey Fathometer No. 3 



The Dorsey Fathometer No. 3 is a combined sonic and supersonic visual-type 

 soundino- instrument, designed for depth measurements from a few feet to several 

 thousand fathoms. This instrument is an outgrowth of the Dorsey No. 1. It embodies 

 all the principles of the Dorsey No. 1, and is essentially the same instrument with the 

 addition of certain equipment, similar to the Dorsey No. 2, to extend the depth range 

 to deeper water. Certain parts, such as the echo amplifier, power supply, and trans- 



