PARALLELOGKAM OP JTOECBS

267

When a ship steams at the rate of 10 knots directly into the teeth of
a 40 miles per hour gale (force 8 on the Beaufort Scale), the velocity of
the wind as registered by an anemometer on deck would be 50 m.p.h.,
and, conversely, if she were going in the opposite direction at 10 knots
she would be running before the wind and the anemometer would register
30 m.p.h., that is 40 m.p h less 10 knots The anemometer registers
the apparent wind, which is the resultant effect of the true wind and the
current of air due to the ship's motion.

Suppose a ship to be stopped, heading North, and the wind to be
blowing from East at the rate of 20 m p h., then the wind would be on

the starboard beam and the steamer's-
smoke would trail to the port beam at
20 rn p.h. If the ship now gets under
way and steams j^orth at 10 knots, the
wind would appear to come from the
starboard bow and the smoke would
trend towards the port quarter. But
how far abaft the beam? The par-
allelogram of forces will help us 'as
illustrated m Figure 8.

Fig 8 —True Wind East;
Apparent Wind N 63° E

From the ship A draw AB, West, and mark ofE 20 parts from* any
convenient scale of equal parts, then draw AC, South, and make it
equal to 10 of the same parts. Complete the parallelogram by drawing
CD parallel to AB and DB parallel to AC. The diagonal AD is the
resultant of the two velocities (wind and ship) and^gives the direction
the steamer's smoke would go.

AD is the direction of the apparent wind as felt on board the ship,
and its length, measured from the scale of equal parts, will be its velocity.
The apparent wind comes from N. 63° E., velocity 22£ m.p.h., and
the smoke will, of course, go S. 63° W.

Another Example.—Given a true wind from N.E , at 10 m.p.h., ship
steaming East, 12 m.p.h. Find the
direction and velocity of the apparent
wind. Draw AB to the S.W. and equal
to 10 units. Draw AC to the East
and equal to 12 units. Join CB.
Then CB is the apparent wind.

Ans.—N. 69° E., 20 m.p.h.

APPARENT

Fig. 9.—True Wind NE.;
Apparent Wind N. 69° E.

It will be noted that half the parallelogram is really all that is
needed and ABC is called the triangle, of forces