Having thus found the setting and drift of the current, it remains to apply this experiment to the purposes of navigation. If the ship sails along the direction of the current, then the motion of the ship is increased by as much as is the drift or velocity of the current.
If a current sets directly against the ship’s course, then her motion is retarded in proportion to the strength of the current. Hence it is plain,
1. If the velocity of the current be less than that of the ship, then the ship will advance so much as is the difference of these velocities.
2. If the velocity of the current be more than that of the ship, then will the ship fall as much astern as is the difference of these velocities.
3. If the velocity of the current be equal to that of the ship, then will the ship stand still, the one velocity destroying the other.
If the current thwarts the course of a ship, it not only diminishes or increases her velocity, but gives her a new direction, compounded of the course she steers, and the setting of the current, as appears by the following
LEMMA.
If a body at A be impelled by two forces at the same time, the one in the direction A B, carrying it from A to B in a certain space of time, and the other in the direction A D, pushing it from A to D in the same time; complete the parallelogram ABCD, and draw the diagonal A C: then the body at A, (which let us suppose a ship agitated by the wind and current; A B, being the line along which she advances as impressed by the wind, and A D the line upon which she is driven by the current) will move along the diagonal A C, and will be in the point C, at the end of the time in which it would have moved along A D or AB, as impelled by either of those forces (the wind or current) separately.
CUTTER, bateau, a small vessel commonly navigated in the channel of England; it is furnished with one mast, and rigged as a sloop.