MEMOIR OF C. F. BEAUTEMPS-BEAUPRE. 121 



on board, could not be taken witb too much rapidity, for it was necessary that 

 the ship should not materially change its place during the time of the opera- 

 tion. The principal operations which serve as a foundation for the charts con- 

 structed by M. Beaupre are such as were executed either at midday, or simul- 

 taneously with the observations of horary angles ; that is to say, at such times 

 of each day as the position of the vessel was determined by astronomical ob- 

 servations and the chronometer. On these occasions he assembled around him 

 the greatest possible number of observers, and he had found or formed a great 

 many among the officers of the frigate. Just one minute before taking the 

 observations he made a sketch of the coast under view, beginning with those 

 parts of it which, being most remote, would undergo least change of outline by 

 reason of the movement of the ship ; then, precisely at the moment when the 

 astronomical observations were taken, he measured the angular distance be- 

 tween the object which he had designated to his assistants as the point of de- 

 parture and one of the remarkable places of the coast, while each of the as- 

 sistants measured the angular distance of the same point of departure from one 

 of the other objects embraced in the survey. The results of these simultaneous 

 observations were afterwards transferred to the sketch which had been made 

 of the outline of the land. All the angular measures were taken with Bordd's 

 repeating circle. 



When the suu was not too high above the horizon, one of the observers 

 measured the distance of that body from one of the remarkable points of the 

 coast ; by means of the heights of the suu observed at the same moment by 

 M. de Rossel, and from the distance measured, M. Beaupre obtained the astro- 

 nomical bearing of that point, whence he deduced the bearing of all the points 

 between which angles had been taken. 



Two compasses were always directed, during the observations, on the place 

 chosen as a point of departure for the angles, and the mean of the bearings 

 given by those instruments was transcribed in the collection of notes, and this 

 whether an astronomical bearing had been obtained or not. In the first case the 

 magnetic indication served to show the variation of the needle, and in the second 

 to supply, though imperfectly, the absence of an astronomical observation. If 

 circumstances, which, however, occurred but rarely, prevented the co-operation 

 of a sufficient number of observers to take simultaneously the angles of all the 

 remarkable points necessary to be determined, M. Beaupre arranged several 

 circles of retiection, so that each observer might promptly take two or three 

 angles, without being obliged to write them on the spot; and these observations, 

 made with a rapidity proportionate to the expertness of the observer, were 

 found to agree almost as exactly as those made simultaneously. 



M. Beaupre, who drew the chart with as much facility as exactness, found a 

 marked advantage in embodying the results observed as promptly as possible, 

 for he had then all the circumstances of the observations present to his mind. 

 It was not seldom that he was enabled in this way to detest and remedy inad- 

 vertencies committed in Avriting the angles measured. The precision of his 

 graphic constructions ever rendered it practicable for him to verify, and some- 

 times to correct, with great probability, the positions of the ship, determined 

 * several times a day by astronomical observations, combined with the indica- 

 tions of chronometers and the estimate of courses. 



The means of verification resulted, in part, from the fact that the observa- 

 tions of each station gave him a series of visual lines, springing essentially from 

 the same point, and forming known angles, whether Avith one another or with 

 the astronomic meridian, or at least with the magnetic meridian, itself deter- 

 mined by an observation made at nearly the same time. They resulted, more- 

 over, from the circumstance that all the visual lines directed from diifcrent stations 

 on the same object, such as a cape or a mountain, must, on the draught, intersect 

 one another at the representation of that object. When, at the first trial, these did 



