the Direction, Se. of the Currents of the Ocean. 3383 
estimate correctly the continually varying effects of the winds 
and sea, on a body that is also continually varying the mea- 
sure of her exposure to their influence. It may be in the 
power of an individual in a vessel, to obtain, by his own ex- 
ertions alone, that portion of the materials towards the evidence 
of currents, which depends on her real change of position; 
but the completion of the evidence by a sufficiently correct 
reckoning must be the result of an interest participated in by 
all the executive officers of a ship; or by the establishment of 
such habits of accuracy, under the authority of her com- 
mander, as are not of usual practice, because they are not ne- 
cessary for the general purposes of navigation; the employ- 
ment of chronometers, by which the position of a ship is ascer- 
tained and a fresh departure taken on every day that the sun 
shines, has superseded the necessity of that vigilant and scru- 
pulous regard, which the older navigators paid to all the de- 
tails of the reckoning, on which alone they had to depend; 
and has tended to substitute general habits of loose and vague 
estimation, for the considerate and well-practised judgement 
with which allowances were formerly made for the incidental 
circumstances of steerage, leeway, making and shortening sail, 
&c. &c., on a due attention to which the accuracy of a reckon- 
ing so materially depends. 
In ships of war especially, the reckoning is further embar- 
rassed by a difficulty less obvious, but not less generally opera- 
tive, by which, if not properly provided against, the know- 
ledge of the true course which the ship has made is necessarily 
_ rendered very uncertain: it arises from the usual practice of di- 
recting the course by the binnacle compasses, which are two in 
number for the convenience of the helmsmen, and being placed 
one on the Jarboard and the other on the starboard side of 
the midship, with a space between them of greater or less ex- 
tent according to the size of the vessel, can scarcely fail, and 
are, in fact, generally influenced differently by the ship’s iron ; 
and being subject to different sys/ems of attraction, the com- 
passes not only disagree, but their disagreement varies ac- 
cording to the direction of ship’s head, the amount of the dip 
of the needle, and the force of terrestrial magnetism. It is 
customary always to steer by the weather compass; and thus 
each is liable to become in its turn the directing compass for 
periods of more or less duration, and the corrections of the 
courses for the disturbing influence of the ship’s iron, becomes 
so various and complicated, as to render the deduction of a 
correct reckoning practically unattainable. [or example, the 
binnacle compasses of the Iphigenia, on her passage from nue 
and 
