WIXD-POWEK SHIPS' SAILS.] 



APPLIED MECHANICS. 



821 



right angles to the direction of the wind intercepted be- 

 tween parallels to it from the extremities of the surface, 

 then the pressure is only that on three feet of 

 surface. 



But besides the loss of surface from obliquity, 

 there is also a loss in intensity of pressure. A 

 column of air AD (Fig. 102) striking a surface 

 exposed obliquely to it, would be reflected in 

 DB, and press upon the surface in the line 

 C D, which is perpendicular to the surface, and 

 bisects the angle contained between A D, the line 



Fig. 102. 



about 14 feet ; therefore the effective surface of the sail 

 is 14 width X 20 height = 280 square feet. Tho velo- 

 Fig. 104. 



of impact, and D B, the line of reflection. The quantity of 

 pressure compared with what it would be if the surface 

 were not oblique, is found by taking A D any length, to 

 represent the direct pressure, and drawing A C and D C 

 respectively parallel and perpendicular to the oblique 

 surface. Then, while A D measures the direct impulse, 

 C D measures iU effective impulse to move the surface 

 in the direction D E, and A C measures the loss from 

 obliquity, or the amount of force expended parallel to 

 the oblique surface, and therefore not effective upon it. 



Fig. 103. 



But farther, it is seldom the case that the effective 

 pressure of the wind on the surface of a sail set obliquely 

 to it, is employed directly. If A B (Fig. 103) represent 

 the surface of the sail of a vessel, C D the direction of 

 the wind, D E the direction of the vessel's course, we 

 may take C D any length, representing the force of the 

 wind ; then C F, perpendicular to the sail, measures its 

 effective force on the sail, and the length of C G, parallel 

 to D E, and intercepted by F G, which is perpendicular 

 to D E, measures the effective force in the direction of 

 the ship's course. The line F G measures the effort of 

 the wind to propel the vessel sideways, or to make what 

 is technically called leeway. 



\\:: may take a practical example as an illustration of 

 how the effective impulse of the wind may be computed : 

 A sail 20 feet wide and 20 feet high is set with its 

 edges directed to N.W. and S.E. ; the wind blows at 

 30 miles an hour from the south, and the vessel sails due 

 east with a speed of 10 miles per hour. Having drawn 

 a plan of the vessel (Fig. 104), showing D C the direction 

 of the wind, and A B that of the sail, in the first place 

 from A and B we draw parallels to D C, cutting off F G, 

 i-ffective width of sail to receive the wind. A B 

 measures 20 feet, and F G would be found to measure 



city of the wind 30 miles per hour being represented by 

 D C, its velocity, resolved perpendicularly to the sail, is 

 measured by D H, about 21 miles per hour. This velo- 

 city, resolved into D K, in the direction of the vessel's 

 course, becomes 15 miles per hour, which is to the actual 

 speed of the vessel, 10 miles, as 3 to 2. The pressure 

 on the surface of the sail is not that due to 21 miles per 

 hour, the measurement of D H, because the vessel and 

 sail are continually escaping from the wind ; but to the 

 excess of D H over the speed of escape, or the same part 

 of 21 as 5 (the excess of the wind's velocity in 1) K over 10, 

 the velocity of the vessel) is of 15, the wind's velocity in 

 D K. As 6 is Jrd of 15, and 7 is jrd of 21, the effective 

 pressure on the sail is that due to 7 miles per hour 

 about 0'245 Ibs. per square foot. This multiplied by 280 

 square feet, the effective surface of the sail, gives 68 Ibs. 

 for the pressure tending to blow off the sail from the 

 mast while the vessel is moving at 10 miles per hour. 

 Were the motion of the vessel entirely obstructed, the 

 pressure on the sail would be that due to 21 miles per 

 hour, about 2^ Ibs. per square foot, or 280 X 2J = 

 C30 Ibs. The tendency of the wind to produce leeway, 

 or to force the vessel sideways from its course, happens 

 in this case to be the same as its direct force, for K H is 

 equal to D K. If the vessel be six times as long as it 

 is broad, the resistance to its movement sideways should 

 be 6 times that to its direct movement, without taking 

 into consideration the form of the part immersed. The 

 shape, however, is made so as to offer as much resistance 

 as possible sideways ; so that, under equal impulses, the 

 resistance to side-movement or leeway may probably be 

 at least 10 times that to direct movement. It might be 

 possible to compute the angle at which a sail should be 

 set when the direction of the wind and the course of the 

 vessel are given, so as to obtain the greatest possible 

 effect. But as, in practice, the sails of a vessel are nume- 

 rous and various in position and direction of action, it 

 would be difficult to apply such a computation. A sailor, 

 after a few trials of his vessel under sail, is able to esti- 

 mate very correctly the angles of his sail and course 

 with the direction of the wind, so as to get the best 

 effect. If the wind blow almost directly against the 

 ship, it is necessary to tack. If, for instance, in sailing 

 from A to B (Fig. 105), the wind blew from B towards 

 A, or nearly so, the vessel would be steered in various 

 tacks or directions oblique to that of the wind, making 

 progress on the whole. The choice of those tacks, so as 

 to get over the given distance with as little deviation as 

 possible, or, at all events, to do it with the least possible 

 loss of time, is an important part of the sailor's art. 



The force of wind has occasionally been applied to 

 moving bodies on land, but not in a manner that can be 

 generally used. Some of the Arctic voyagers having to 



