216 EEPORT OF COMMISSIONER OF FISH AND FISHERIES. [100] 



COS ] COS d versin t, A form and examples are to be found in Bow- 

 ditch i Navigator, page 200. It does not appear in the revised edition. 

 The formula used for deducing the latitude from an altitude of Polaris, 

 observed at any time, is — 



L=1i—p cos t, 

 in which 7i=true altitude, 



j)=polar distance, expressed in minutes of arc, 

 f=hour angle = sidereal time— *'s E. A. 



A form and example are to be found in the revised edition of Bow- 

 ditch's Navigator, page 118. In practice, however, it has been found 

 simpler to disregard the sign of cos f, merely adding or subtracting 

 the correction p cos t, according as t is more than 6 and less than 18 

 hours, or the reverse. 



For determining the circles of equal altitude of any body, the hour 

 angle for each latitude is obtained from single altitudes by the ordi- 

 nary formula — 



sin ^ t= / cos S sin (S-fe) 

 V cos L cos p 

 in which S = J (h-^-Ij+p). 



In addition to the above-mentioned methods, advantage is taken when- 

 ever possible of the simple problem of finding the exact distance from 

 an object by reading the tafi'rail log when it bears exactly four points 

 off the bow, and again when it bears exactly abeam ; the distance from 

 the object at the second bearing being equal to the distance run be- 

 tween the two plus or minus the current. 



The current at each sounding is carefully estimated by noting the di- 

 rection and speed of the ship necessary to keep the sounding wire ver- 

 tical after the shot has passed below the surface current. A fair guide 

 is thus afforded to what allowance should be made in shaping the course 

 to the next position, as well as in connecting the run up to that point. 

 Such help is particularly desirable when clouds by day, or clouds or the 

 absence of the moon at night, prevent taking frequent observations. 



Case I. — On April 28 single altitudes of the sun were observed at 

 6.43 a. m., 8.47 a. m., and 10.23 a. m., a sounding being taken at the 

 time of each sight. The meridian altitude was observed at noon. The 

 three time sights were worked out for latitudes 35° 20' N, and 35° 30' 

 N., placing the vessel respectively on the lines AA, BB, CC (accompa- 

 nying chart, see Plate) ; and the meridian altitude placed her in latitude 

 350 31' 35", BB. From the first sounding, ran 10 miles ESE. (mag.) to 

 the second, where the temperature of the surface water and the current 

 showed that the edge of the Gulf Stream had been reached. From the 

 second to the third the drift in trawling and current were estimated at 

 3 knots jSTE. From the end of the third cast to noon the drift and cur- 

 rent were about 2 miles NISTE. These being plotted place the ship in 

 the positions 1, 2, 3, 4, 



