SURVEYS. 



13 



favored, and to examine as large an area as could be passed over by 

 the squadron, which generally embraced half a degree of latitude, and 

 from one to two degrees of longitude. The examinations were always 

 made by daylight, and frequently during the most favorable weather, 

 and all the precautions taken that nothing should escape us, by having 

 many " look-outs" and the lead going. In this way we have been 

 able to erase from the charts many islands and reefs, which have been 

 a constant source of alarm to those passing near their assigned posi- 

 tions, not unfrequently causing great detention, from fear of coming in 

 contact with them during the night ; but in many cases, disasters have 

 been brought about, by drift and current (not previously known) 

 carrying vessels beyond their ports, and oftentimes into real dangers. 



No search should be considered effective, unless a sufficient area is 

 passed over, in which, taking all the probabilities of error, the re- 

 ported shoal or danger is said to exist. It is due to the navigating 

 interests of our country, that these searches should be continued, until 

 the existence or non-existence of these dangers is clearly ascertained. 



SOUND. 



Sound has been referred to as the medium through which was ob- 

 tained, oftentimes, bases for the surveys. In order to test the accu- 

 racy of this mode of measurement, I determined to make the experi- 

 ment on an extended scale, by measuring the distance from Cape Frio 

 to Rio de Janeiro ; the ascertainment of the position of the former 

 having been made a part of my instructions. This was a distance of 

 about 76 miles. The diagram of this measurement, and its accuracy, 

 as compared with the meridian distance taken by chronometers from 

 the observatory, will be found in Appendix XXIII, Vol. I, although 

 it more properly belongs to this treatise on Hydrography. The 

 two methods, though very dissimilar, approximate closely in results ; 

 proving the accuracy of the method by sound, and the reliance that 

 can be placed upon its employment for extended bases. Notwith- 

 standing this apparent confirmation, I was not satisfied that the velo- 

 city of sound had been truly ascertained by experiment. The dif- 

 ference between that deduced by theory and experiment, respectively, 

 equal to one-sixth, was, in my opinion, too great ; and in looking for 

 the reason of the discrepancy, I concluded it might arise from the 



