450 APPENDIX. 
the bearing of the two stars gives the direction ; and the counting 
compared 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 
vessels, any three of which being stationary, the distances and 
positions of objects could be determined from them by direct angles, 
giving a double result, or by the angles between them, taken from • 
the shore or from boats, furnishing data for the problem of "the 
three points." When both methods can be applied at the same time, 
it is evident that the utmost accuracy may be obtained. Upon the 
land, by employing many observers, and occupying all the points of 
a trigonometric survey simultaneously, the whole work might evi- 
dently be completed in a very short space of time; and in like 
manner upon the water, creating, by means of vessels and boats, a 
number of artificial stations around an island, measuring angles 
simultaneously at them all, and measuring the base lines by sound, 
the rapidity with which a survey can be performed is equally as 
great. 
