Although a force is something that cannot be pictured it can be represented graphically by means of a line, letting the direction of the line represent the direction of the force and the length of the line the strength of the force. In Figure 58 we may measure off 3 inches from _O_ to _a_ to indicate the 3-pound force and 4 inches from _O_ to _b_ to represent the 4-pound force and 5 inches from _O_ to _c_ to represent the 5-pound force. Now if from _a_ a line is drawn parallel to _O b_ and from _b_ a line is drawn parallel to _O a_ we shall have a parallelogram _a O b d_, and if we extend the line _O c_ it will bisect the parallelogram, running diagonally from _O_ to _d_, and this diagonal will be found to measure exactly 5 inches which represents the 5-pound force. This is what is known as the _parallelogram of forces_. It shows us the resultant of any two forces that are not directly opposite and it gives us the direction as well as the strength or magnitude of this resultant. It is only because we happen to choose the forces 3, 4 and 5 that the angle at _O_ is a right angle. In Figure 59, where the forces are all equal, our parallelogram is lozenge-shaped and the line _O d_ is just as long as the line _O a_ and _O b_, showing that its magnitude is the same as that of the two forces that balanced it. [Illustration: FIG. 57.--BALANCED FORCES--UNEQUAL WEIGHTS] The greater the angle between the two lifting forces the less weight can they lift. If two men are carrying a ten-pound satchel, each will be lifting five pounds, if the pull is directly upward; but this is a rather inconvenient way of carrying the bag and usually they pull at a slight angle from the vertical, and so each must carry more than half the weight. If they move so far apart that the angle between them is more than 120 degrees, each will be carrying more than the full weight of the bag. [Illustration: FIGS. 58 AND 59.--PARALLELOGRAMS OF FORCES] SAILING AGAINST THE WIND