NARRATIVE OF THE CRUISE. 155 



Stream, when out of sight of land or unable to anchor a boat, observations of tlie 

 heavenly bodies are the only resource, and when the weather is clear, so that these 

 observations can be obtained at short intervals, and especially when stars have been 

 taken at sunrise and sunset, with a well-defined horizon, so that both latitude and 

 longitude are determined at the same instant twice during the day, a very accurate 

 estimate can be formed, by astronomical observations alone, of the rapidity of the stream. 

 If star observations cannot be obtained, the operation is by no means so simple ; and is 

 sometimes impossible, as then the latitude cannot be determined at the same instant as 

 the longitude, 1 double altitudes of the sun not being available owing to the want of 

 knowledge of the alteration in the position of the ship between the times of observation ; 

 the latitude, therefore, can only be got at noon by meridian altitude of the sun. 



Unfortunately, nearly all the great ocean streams, the phenomena of which it is most 

 desirable to investigate, have in their neighbourhood cloudy, thick weather, with short, 

 sharp gales, rendering their exploration at all times difficult. The officers were so 

 far fortunate in the weather in the Challenger, whilst taking temperatures, soundings, 

 &c, in the Gulf Stream from 6 a.m. to 5 P.M. on the 1st May, as to be able to obtain 

 frequent observations of the sun, although the sky w T as too cloudy to permit the 

 determination of the position by star observations at sunrise or sunset. It was there- 

 fore only possible to obtain one latitude at noon by meridian altitude of the sun. The 

 positions of the ship at various times during the day were determined in the follow- 

 ing manner : — A set of observations for longitude was taken at 6.40 a.m., when the 

 bearing of the sun was E. 6° N., or nearly on the prime vertical, and another at 4.27 p.m., 

 when its bearing was W. 1° S., or almost exactly on the prime vertical. Any error, 

 therefore, of the latitude used in working these observations would have but a slight 

 effect on the longitude. Now, the resulting longitude at 6.40 a.m. was 72° 4' W., and 

 at 4.27 p.m. 71° 31' 30" W., it is, therefore, evident that between those hours the drift 

 in longitude experienced by the ship was 32^ minutes. But in keeping the sounding 

 line perpendicular it was found necessary to steam W.S.W. (S. 60° W. true) at the rate 

 of 3 miles per hour, from which it is evident that the direction in which the stream was 

 running was N. 60° E. Having then the direction in which the current w T as going, and 

 the alteration of longitude due to it in a given time, it was possible to calculate the 

 speed at which it was running, which amounted to 3j miles per hour, or very nearly the 

 same rate at which it was necessary to steam in a W.S.W. direction whilst sounding. 



If this conclusion as to the direction and rate of the current be correct, it is evident 

 that by applying the amount due to a given interval of time, the longitude of the ship 

 at that time, as determined by this method, should agree with the longitude obtained by 

 actual observation. By taking a proportion of the whole alteration in the longitude be- 

 tween 6.40 a.m. and 4.30 p.m., it was found that the longitude at noon was 71° 45' 54" W.. 



1 Unless the moon is visible, or Venus or Jupiter passes the meridian during the d;iv. 



