Development of Crossbows
Crossbows are first found as historical artifacts among the figures in the Chinese Terracotta Army in the tomb of the Emperor Qin Shi Huang, which were buried in the 3rd century BC. The crossbows found here were well-developed and clearly an entrenched part of military armament, implying that they originated much earlier. The classic Art of War by Sun Tsu also outlines the use of crossbows in war, bringing the date of crossbows back at least two more centuries. Some historians think they were developed as early as 2000 BC, an earlier date than bronze weaponry.
Crossbows were developed independently in the West by the Greeks, though they may have been based on observations of Eastern crossbows. They are called gastraphetes by Greek writers in the 5th century BC, and use a different form of action from the Eastern crossbows.
While crossbows have had innovations added to them over the years, they remain essentially the same: a standard small bow with a short draw affixed to a stock which is used to hold the string taut and ready to fire. While they are not as powerful as some compound and even recurve standard bows, they have a variety of advantages:
Crossbows are also not as fast to fire and reload as a bow. While a skilled bowman can nock, aim, and fire an arrow in about five seconds or less, it takes about thirty seconds to recock a crossbow.
The similarities between crossbows and bows do not extend to ammunition. Because of the way bows are fired, a light, aerodynamic design that carries well in the air makes an ideal arrow. Arrows tend to be long, with fletching at the back end for stability, and have a heavy point at the head; their sharpness and slenderness is what carries them into their target, not so much their force.
Crossbows, on the other hand, use bolts. Because the draw of a crossbow is shorter, bolts must be much shorter than arrows. They are fletched, like arrows, but they can be much heavier. A crossbow bolt depends more on its force than on its sharpness for penetration. Bolts tend to have a shorter useful distance than arrows, but they also can do more damage.
Draw weight and draw length are the critical factors separating crossbows from one another. Draw length, on bows or crossbows, is the distance you can pull the string back from its resting position, and is much shorter on a crossbow than on a bow. Draw weight is the amount of force required to pull the string to its full draw length, and can be much higher in a crossbow than in a bow due to the difference in how the crossbow is cocked. Draw weight times draw length divided by two is a measure of a crossbow's strength, and is used by manufacturers as a comparison measure.