' AND DIRECTION OF THE WIND AT SEA. 461 



what will be the direction and the velocity of the wind which a person in the 

 ship will feel ? 



Now, the motion of the ship from A to B being equivalent to a wind moving 

 with equal velocity in a contrary direction, or from B to A, then any particle 

 of air at rest at A will be driven, if acted on by the impact of this adventitious 

 wind only, to a distance, equal to A B, beyond A, and in the same direction, or 

 to E ; but if acted on only by the natural wind, the particle will receive a velo- 

 city and a direction equal to that, and be driven from A to F ; but as the particle 

 is acted on instantaneously by both forces, it will neither go to E nor to F, but in 

 an intermediate direction, and closer to one or other according to the relative 

 strength to the two forces, or in the diagonal of the parallelogram of which A F 

 and A E form two sides. A G, therefore, in the parallelogram A E F G, repre- 

 sents, by its length and position, the distance to which, and the direction in 

 which, a particle at A will be driven under the united influence of the natural 

 wind blowing over the sea, and the artificial wind caused by the motion of the 

 ship ; I. e., A G represents the apparent wind, or that which a sailor would 

 observe. 



For convenience of illustration we may complete the parallelogram A B D C, 

 which is similar to the parallelogram A E F G, and where A D is consequently 

 equal to A G ; and we may now, for the purposes of calculation, omit all but the 

 triangle A B D, in which the sides A B, AD, being given by observation, and the 

 included angle BAD being obtained from the difference of the observed directions 

 of the ship and of the apparent wind, we have merely to compute by the usual 

 rules for plane triangles, the length of the side B D, or the velocity of the true 

 wind ; and the angle A B D, which, being added on to the course of the ship, 

 gives the direction of the true wind. 



Considering, however, the necessary length of time which such a computation 

 must occupy in the hands even of the most expert (the mere preparation of the 

 angles for computation would of itself be no small matter, on account of the 

 rough character of the subtraction of the course of the ship from the direction of 

 the wind — both observed by compass points — the reduction of this to degrees, 

 then the re-reduction of the resulting angle into points, and the addition of them 

 to the course of the ship) : considering, also, the almost infinite number of times 

 the calculations would have to be gone through in any voyage, and the extreme 

 improbability of any amateur undertaking so large a quantity of an unimproving 

 sort of labour, I have had a set of scales made for solving the problem by 

 inspection, and the entry being made with the velocities and directions observed, 

 the velocity and direction of the true wind are immediately given. 



These scales, which are represented in Plate XI., are formed of two ordinary 

 jointed rulers, having moveable circles marked with the points of the compass on 

 the joints, and three sets of divisions on the legs, to include all possible velocities 



