384 APPENDIX. 



XX. 



U. S. Ship Vincennes, 



Rio de Janeiro, December 15th, 1838. 

 Sir, — 



You will proceed and make a survey of a shoal said to exist off 

 Cape St. Thomas, about sixty miles north of Cape Frio, with the 

 Peacock, Porpoise,* Sea-Gull, and Flying-Fish, which are placed 

 under your command for the duty. 



The shoal is supposed to be about twenty miles east-half-north from 

 Cape St. Thomas. 



In surveying it, as far as I am able to judge of its locality, I would 

 recommend the following mode to be pursued, viz. : 



On your arrival at or near its supposed locality, anchor your four 

 vessels at convenient distances from each other, within a suitable 

 distance for admeasurement by sound. Here ascertain your latitude 

 and longitude accurately, measure your distance between all the vessels 

 by sound, firing guns in succession, noting the elapse of time between 

 the flash and report ; then, or before, measure the azimuth between 

 each vessel and the sun, and proceed with your boats to sound, 

 radiating from each vessel on the several points of bearings : the 

 position of your boats may be accurately ascertained by the angles on 

 any three of the vessels, and the soundings obtained can at once be 

 inserted on the skeleton chart prepared for the occasion. 



You will, while at anchor, heave the current log every hour, and 

 notice the direction by the head of your ship. After you have satis- 

 factorily explored the ground that your vessel may have anchored on, 

 you will then, in all probability, know the direction in which the 

 shoalest water lies from you, and by shifting the anchorage of each 

 vessel in succession toward that direction, you will occupy new 

 ground, when the same operation of measuring bases by sound, and 

 taking azimuths, will be gone through with, and then you may 

 approach the position without any danger, as your chart will be 

 constructed as you proceed. 



Lieutenant Johnson has been ordered to the Porpoise to superintend 

 her movements in regard to this survey, and Lieutenant Alden to your 

 ship, in whose information, as respects the above mode of proceeding, 

 you may rely. 



Mr. Knox of the Flying-Fish, is also apt at this work. I have 



* The Porpoise was not on this duty ; these orders were countermanded, as she could not 

 be prepared for sea in season. 



