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 douljle 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 

 tlie 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 Avell 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 of the anemometer shall have superseded the present unmeaning ones entered in 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 extratt 

 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 

 jnirsued, and velocities attained, by the ship at the various hours, the wind did alter 

 to those on board at times, almost of 00° in direction, and from 9 knots to 23 knots 

 in velocity. 



