304 THE VOYAGE OF H.M.S. CHALLENGER. 



often see a dirty background of cirro-stratus, over which black patches of cirro-cumulus 

 are travelling. Such combinations may be noted in the column for " Remarks." 



Current. — The direction and rate of the current, given in these tables, is derived 

 from the differences in the position of the ship at noon, determined by astronomical 

 observation, and that determined by courses steered and distances run (by patent log) 

 from the astronomical position of the preceding noon. Specific observations on the 

 direction and rate of the surface currents were frequently made during the voyage, 

 either from a boat fast to the trawl rope, or when the ship was kept stationary for 

 sounding purposes ; occasionally by comparing the distances shown by patent log with 

 the distances made good over the ground as ascertained by objects on shore. These 

 several results, independent of the ship's reckoning, are given in the text of the voyage. 



It has been contended that the difference in a day's run between the position of the 

 ship by dead reckoning and by astronomical observation cannot be considered as giving 

 with sufficient accuracy the movements of the surface waters. The long experience of 

 navigators prove, however, that with due care this result furnishes a fair approximation 

 to the truth, and especially so if the astronomical observations are taken at frecjuent 

 intervals during the twenty -four hours. Much care was taken in the " Challenger " by 

 due attention to the steerage and estimation of the leeway to keep an accurate reckoning, 

 and also to ascertain frecpiently the latitude and longitude by astronomical observation. 

 The surface set deduced by these means, was found in the majority of cases to be con- 

 tinuous during the day, and to agree in a striking manner with the surface current as 

 found by anchoring a boat, or other direct method. 



Specific Gravity of Sea Water. — The specific gravity of the sea water during 

 the voyage was ascertained by Mr Buchanan, the analytical chemist of the expedition, 

 with a very delicate hydrometer designed by him for this purpose. (A description of this 

 instrument is given in Vol. I.) The results are reduced to the temperature of 60° Fahr. 

 by the tables and curves given in Maury's "Sailing Directions," 8th edition, p. 239 

 et seq, and Appendix, Plate XVIII. 



