5133 HYDROGRAPHIC MANUAL PaGE 442 



rotating neon tube which flashes adjacent to the depth scale at the instant of arrival of 

 the echo, thus acting as an index to indicate the depth. The 312 Fathometer is still 

 used on some ships of the Coast and Geodetic Survey. (See 521.) 



5133. 412 Fathometer 



A 412 Fathometer, a striker type of instrument, was installed on the motor vessel 

 Natoma in 1928, and subsequently other models, similar in principle to the 412, were 

 installed on several ships. Finally all the striker types of Fathometers were discarded 

 because they did not give results sufficiently reliable and accurate for survey purposes. 

 (See 5161D.) 



5134. Dorsey Fathometer No. 1 



Echo sounding proved to be a much more accurate, faster, and easier method of 

 hydrographic surveymg than any method previously used. However, most of the 

 instruments available in the United States prior to 1935 were designed for use on 

 commercial vessels and, while sufficiently accurate for navigation, they did not fully 

 meet the requirements of the hydrographer. 



A precision instrument was needed with which soundings between a few feet and 

 20 fathoms could be obtained with far greater accuracy than with any existing instru- 

 ment. The design of such an instrument was started in 1933 in the Washington labo- 

 ratory of the Coast and Geodetic Survey. This first instrument of its kmd was installed 

 on the Lydonia in 1934. It Avas named later the Dorsey Fathometer No. 1, a detailed 

 description of which is given in 524. The Dorsey Fathometer No. 1 was so successful 

 that similar instruments were installed in subsequent years on all ships of the Coast 

 and Geodetic Survey operating on the East and Gulf Coasts. Although designed 

 primarily for use in depths of 20 fathoms or less, soundings have been obtained in 

 150 fathoms, although 40 fathoms is usually the maximum depth in which it is used. 

 The Dorsey Fathometer No. 1 uses a frequency of sound very close to supersonic; 

 this was the first attempt by the Coast and Geodetic Survey to use the higher-frequency 

 sound waves in echo sounding. 



5135. Dorsey Fathometer No. 2 



In 1937 the Dorsey Fathometer No. 2 was designed to supplement the Dorsey 

 Fathometer No. 1. It incorporated many of the performance and accuracy features 

 of the No. 1 model but, whereas the latter was designed especially for use in depths 

 less than 20 fathoms, the No. 2 model was designed for depths greater than 20 fathoms. 

 The first two instruments of this type were installed on the Oceanographer and the 

 Hydrographer. (See 525.) 



5136. Dorsey Fathometer No. 3 



Performance of these two models of Dorsey Fathometers proved so satisfactory 

 that their features were combined into a single instrument, known as the Dorsey Fatho- 

 meter No. 3, for use in all depths. The first of these instruments was constructed in the 

 laboratory of the Coast and Geodetic Survey and was installed on the motor vessel 

 Westdahl in 1938. This first instrument gave satisfactory results and Fathometers of 

 this type were later installed on most of the ships of the Coast and Geodetic Survey, 

 and were still in use in 1941. (See 526.) 



