462 ON THE STRENGTH AND DIRECTION OF THE WIND AT SEA. 
of the wind and ship, from 1 mile to 100 per hour, each mile being capable of 
subdivision into tenths. 
A leg of each scale being made to slide one upon another, with the joint of 
each turned outwards, the centres of these joints are to be placed at such a 
distance apart as corresponds to the velocity of the ship in terms of any one of 
the three sets of divisions on the legs. Both circles are then turned, so as to 
shew on one edge of that double leg the observed direction of the ship’s course; 
the single projecting leg of one scale being then placed in the observed direction 
of the apparent wind, and the projecting leg of the other moved until one edge of 
it cuts on the first, the divisions indicating the velocity of the apparent wind ; 
then the length of the intercepted portion of that second leg shews the velocity of 
the true wind, and its reading on its own circle gives the direction. 
The circles are graduated to degrees as well as to points of the compass, so 
as to be capable of solving all ordinary cases of plane triangles, as well as those 
immediately contemplated, and to suit the instances sometimes found of the 
more exact determination of the direction of the wind in degrees instead of 
points. 
As the scales stand at present, they require, when the significant numbers 
of the anemometer shall have superseded the present unmeaning ones entered in 
a ship’s log, no new data from observation; but by merely running down, scale 
in hand, the columns of “Directions and Velocities of the Ship, and Apparent 
Wind,” as entered in the book for the ordinary purposes of navigation, the direc- 
tion and velocity of the true wind may be rapidly entered in a couple of adjacent 
columns; and the importance of making this correction before beginning to reason, 
from the data at present afforded by ships, on the general character of the winds 
on the surface of the ocean, may be seen in the accompanying supposed extract 
from the journal of a man-of-war or a steamer; where, though the true wind was 
the same during the whole interval, yet, on account of the different courses 
pursued, and velocities attained, by the ship at the various hours, the wind did alter 
to those on board at times, almost of 90° in direction, and from 9 knots to 23 knots 
in velocity. 





Date. Ship’s Apparent Wind. True Wind. 
Tote silk: cas JRA. CRWRLLEOS ri 
Course : Direction ‘ Direction ae 
1848. Hour. (to) Velocity. (from) Velocity. (from) Velocity. 
Knots. | Knots. | Knots. 
N 10°5 WSW 12-1 SW. by S 19:3 
April 17 

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