HAYES— U. S. NAVY MV TYPE OF HYDROPHONE. 401 



hydrophone in the usual manner. The depth was also noted as 

 accurately as possible from the chart. These data are given below. 

 In order to show more clearly the variation between these dif- 

 ferent methods of sounding, the data have been plotted in Fig. ii 

 where the abscissae represent distance travelled along the course and 

 the ordinates the depth of water. Each of the four curves is a 

 "sounding curve" taken by one of the four different methods as 

 indicated in the key. These curves illustrate several interesting 

 facts. In the first place, in spite of the discrepancies between the 

 various values evident at certain places, it will be seen that, when 

 two or more of the other methods do agree we find that the hydro 

 phone sounding coincides with this point also. It should further be 

 noted that the discrepancy between the hand-lead and the automatic 

 sounding machine is, in general, quite as great as that between one 

 or the other of these instruments and the hydrophone. Again, 

 it is important to note that over the whole course the hydrophone 

 soundng curve agrees most closely with the sounding curve taken 

 from the chart. This fact shows that the hydrophone has a tend- 

 ency to smooth out the local irregularities of an uneven bottom and 

 to give an integrated mean curve which checks well with that ob- 

 tained from the hydrographic charts. The depths given on the 

 charts were obtained by taking an average of a number of sound- 

 ings made in a given locality, thus canceling the local variations 

 which are very prominent in this region. An illustration of this is 

 to be seen on the 120-mile ordinate of the plot where the hydrophone 

 and chart soundings agree, while the sounding machine evidently 

 fell into a ravine, the bottom of which never was reached by the 

 hand-lead. The greatest discrepancy (12 fathoms) between hydro- 

 phone and chart which occurred at 50 miles and whereat the sound- 

 ing machine and lead checked the hydrophone evidently indicates a 

 depression which was not noted on the chart. 



The U. S. Blakeley, Sounding Data Over Favorable 



Sea-Bottom. 



The curves in Fig. 12, plotted from data taken on board the 

 U. S. S. Blakeley during a run from Block Island out to the 100 



