430 



APPENDIX. 



vessels, any three of which being stationary, the distances and posi- 

 tions 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 evidently 

 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. 



By means of the system of signals prepared by me for the Exploring 

 Expedition, I could direct the vessels to assume any position I might 

 select as most fit for our purpose. When these were reached, general 

 but minute instructions, directed the observations that were to be taken 

 at each, in 'doing which there was no difficulty. These observations 

 were entered upon a deck-board, for which the following form was 

 prescribed : 



FORM OF DECK-BOARD. 



TIME. 



POSITION. 



TIME. 



AZIMUTH 

 OF 0. 



LEFT-HAND 

 OBJECT. 



CENTRE 

 OBJECT. 



RIGHT-HAND 

 OBJECT. 



REMARKS. 



















To illustrate still further the mode in which the whole squadron was 

 made to concur in obtaining unity of action, a detail of a survey and 

 a plot of the work are subjoined. 



On approaching the island to be surveyed, signal was made to 

 prepare for surveying duty ; if boats were to be used, the number of 

 them and the vessels whence they were to be despatched was next 

 indicated; and finally, the position each vessel was to occupy was 

 shown. The vessels having reached their assigned places, hove-to 

 and the boats, having been previously despatched, would about the 

 same time have anchored in their assigned stations, at the points of 

 reefs, and hoisted their appropriate signals. The vessel that is first to 

 fire a gun then hoists an ensign at the foremast-head, which is answered 

 by all, and the flag is in like manner displayed previous to each sue 



