10 



HYDROGRAPHY. 



many corresponding observations were obtained of moon-culminating 

 stars with each of these places. The duties of these gentlemen con- 

 tinued through the period of four years, or during the absence of the 

 Expedition, and from their untiring assiduity, perseverance, and atten- 

 tion, many comparative results were also obtained in magnetism and 

 meteorology. 



At some points they have been more numerous than at others, in 

 consequence of the weather being more favorable. These points 

 have been connected by meridian distances measured by chronometers, 

 and a net of triangles, if I may so express it, has thus been spread 

 over both the North and South Pacific, which include positions whose 

 longitudes have been ascertained by others as well as by ourselves. In 

 many cases the meridian distances have been measured several times. 

 In this way I have not only been able to satisfy myself of the perform- 

 ance of our chronometers, but of the accuracy of the positions assigned 

 these points, which in turn aided to prove those of the islands, reefs, 

 and shoals, which were encountered on the route from one point to 

 another. The correctness of the determinations of the meridian dis- 

 tances we have had frequent opportunities of proving, by different 

 chronometers and observations, entirely independent of each other, 

 increasing the dependence that may be placed on them. 



Although we were well provided with instruments, but few oppor- 

 tunities offered for the observation of occultations, or the eclipses of 

 Jupiter's satellites. A single eclipse of the sun was observed, and only 

 the end, owing to the first contact being lost by obscuration. This 

 gave the position of Peacock or Ahii Island (south side), within a few 

 minutes of the chronometric measurement. As respects chronometric 

 distances, I have preferred to place greater reliance upon the determi- 

 nations of short intervals, confining each to its own limit, and not to 

 allow, by an extension of the series, the accumulation of error by any 

 change that the comparison of chronometers may have shown in longer 

 periods. I have invariably preferred adopting the determinations by 

 a chronometer whose rate had proved most uniform during our passage 

 from port to port, instead of taking the mean of the whole. The reduc- 

 tions of meridian distances have been computed for all intermediate 

 points, by the supposition of a gradual increase or decrease of rate ; 

 and for any intermediate point, where a rate was obtained, the correc- 

 tion has been applied by the usual formula, according to the time that 

 had elapsed when the observations were made. The weekly rates of 



