358 



Introduction to the Study of Science 



have but to apply the facts developed in the example of the 

 wagon, and illustrated in towing a ship in a canal. 



Exercise : Relation of forces in sailing. In diagram A of Fig. 

 126, is shown the relation of the forces when only the sail and the 

 wind are considered. The wind, let us say, is from the north and the 

 boat is headed westward. The force of the wind, AF, is equivalent 

 to 300 pounds. This is resolved into two forces, the one, A P, which 

 is perpendicular to the plane of the sail, and represents the pressure 

 on the sail, and the other, A W, which is parallel with the sail and repre- 

 sents the waste force called the " spill " of the sail. The figure is 



RELATION OF FORCES WHEN RELATION OF FORCES WHEN 



SAIL AND WIND ARE CONSIDERED HULL AND WATER ARE CONSIDERED 



FIG. 126. Boat sailing with the wind abeam at 45 angle. Scale T y to 



30 pounds. 



drawn to the scale of one eighth of an inch to 30 pounds. What is 

 the length of the diagonal? Of the perpendicular and parallel lines 

 respectively? What is the effective force? What is the waste force 

 shown by the spill of the sail? 



The resistance of the hull and centerboard of the boat together 

 with that of the water must be considered. This requires that the 

 sail pressure (AP) or 240 pounds must be further resolved. The 

 step is illustrated in diagram B of Fig. 126. The pressure on the sail 

 (AP) exerts an effective force, AE, parallel to the centerboard, and 

 also a drift force, A D, perpendicular to the centerboard. How much 

 force is exerted against the hull and centerboard and wasted in drift to 

 leeward ? How much force is effective in moving the boat forward ? 



