APPENDIX. 429 



U. S. Ship Vincennes, 



At Sea, August 8th, 1839. 



In order more effectually to observe the periodical meteors which 

 may be repeated, on the nights of the 8th, 9th, and 10th of August, 

 the following officers will keep watch on those nights, and assist the 

 officers of the watch in making observations. 



With the 1st watch, Mr. Couthouy and Mr. Elliott. 



With the 2d watch, Dr. Pickering and Mr. Howison. 



With the 3d watch, Mr. Drayton and Dr. Fox. 



With the 4th watch, Dr. Gilchrist and Dr. Whittle. 



The officer of the watch will assign a quarter of the heavens to each 

 person in his watch, who will note at once on paper, any meteors that 

 may be visible, viz., their size ; with or without train ; their height ; 

 the points of appearance and disappearance ; and the time of flight. 



Although it may seem difficult to accomplish all this on a meteor, 

 yet it is possible, and even easy, viz., by noting the star of the constel- 

 lation near which it appears and disappears ; counting until its dis- 

 appearance ; the arc may be accurately measured by a sextant ; the 

 bearing of the two stars gives the direction ; and the counting com- 

 pared with a chronometer gives the true time ; and yet one has time 

 to examine its length of train, brilliancy, &c. 



Charles Wilkes, 

 Commanding Exploring Expedition. 



MODE OF SURVEYING THE CORAL ISLANDS. 



As the surveys of islands are frequently spoken of in the body of 

 the work, I have deemed it proper to give in this place a concise 

 explanation of the method employed for this purpose, and applied 

 particularly to the low coral islands, together with an example to 

 serve as an illustration of the mode in which it was carried into effect. 



In consequence of the time that could be allotted to such examina- 

 tions being limited, it became desirable that such a course should be 

 pursued in our surveys as should unite to the utmost expedition the 

 greatest attainable accuracy. A method, which it is trusted united 

 these requisites, was, therefore, devised and communicated to the 

 officers of the squadron in the shape of a syllabus. In this I pointed 

 out the mode of operating, and it was recommended to their study, in 

 order that all might become familiar with its details. The basis of 

 this method rested upon the measure of distances by sound. For this 

 we had ready means by firing guns alternately from the different 



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