572 HYDROGRAPHIC MANUAL PaGE 552 



572. Instrument Tests and Maintenance 



Throughout the text of chapter 5 various tests and the maintenance of the various 

 instruments used by the Coast and Geodetic Survey have been described. For con- 

 venience these have been summarized in table 21 with reference numbers to the text 

 where they are described more fully. The X entries in the table are just as important 

 as the others although they may not have been particularly stressed in the text. Each 

 entry is listed according to time and this is intended as a reminder to the hydrographer 

 when each should be accomplished. It is not the intention of this table to prescribe 

 the exact frequency of these operations. Experience with each instrument under vary- 

 ing conditions of operation may establish this better. However, it must be borne in 

 mind that no departure should be made from the requirements of this schedule which 

 will in any way result in less accm'ate soundings or endanger the instrument because of 

 inadequate maintenance. 



One blank column has been provided for similar entries for any new type of instru- 

 ment acquired. 



58. ECHO-SOUNDING EXPEDIENTS 



When results cannot be obtained with echo-sounding instruments operated conven- 

 tionally, either because the depths are beyond the effective range of the instrument or 

 for other reasons, unconventional expedients can sometimes be used in emergencies to 

 obtain results. The two described here are only typical and the inference should not 

 be drawn that there are not others which will give equally good results. No such 

 method should be utilized, however, except in an emergency. 



581. Deep-Water Sounding With the 312 Fathometer 



Where the depths are beyond the effective range of the 312 Fathometer, the index 

 disk can be disengaged so that a signal of longer duration can be transmitted by making 

 the necessary contact manually, as is done in adjusting the index (see 5543). The 

 echo from this longer transmitted signal can often be heard in earphones, when no results 

 can be obtained by operating the instrument conventionally. The elapsed time between 

 the transmitted signal and the reception of the echo must be accurately measured on a 

 stop watch or chronograph. The transmitted signal will record automatically on the 

 chronograph (see 5553) but because the echo will be weak its reception will probably 

 have to be recorded manually. 



582. Sounding With Bombs 



To sound in deep water, in the absence of a deep-water echo-sounding instrument, 

 or in water whose depth exceeds the range of the available instrument, bombs can be 

 substituted for the oscillator, and the R.A.R. chronograph (673) used as a time-measur- 

 ing device. The time required for the sound of the bomb explosion to travel to the 

 bottom and return is measured on the chronograph tape in seconds, as in standard 

 R.A.R. practice (685), and after having been corrected it is multiplied by the mean 

 velocity of sound from surface to bottom. 



The bombs must be exploded at a known depth. This may be effected by attaching 

 each bomb to an inflated paper bag by a cord of the desired length. Four fathoms is 

 suggested as a proper depth. This involves no hazard since the paper bag soon becomes 

 water-soaked and permits any unexploded bomb to sink. (See also 6846.) 



