Sunday, February 7, 2010

Off Topic: Parthian Shot

(On occasion there will be additional posts with will deviate from the weekly physics consideration.)

Most are at least vaguely familiar with the exploits of the slave leader Spartacus during the Roman Third Servile War. Without giving the due time to a fascinating piece of history it can be summarized that the revolt of Spartacus was squelched by the Roman General Marcus Licinius Crassus. Crassus one of the wealthiest of all historical figures gained the pinnacle of historical significance for his backing and financial support of the fledgling Julius Caesar, ultimately allowing for his stunning political career. But I digress. The point of the mention of Crassus I am getting to, although somewhat circuitously. Crassus longing for greater military victories and recognition from such victories found himself in a pitched battle in 53BC against the Parthian Empire near the town known in antiquity at Carrhae, often believed to be the Haran of the biblical narrative and auspicious home to Abraham. From the battle, ignominious defeat and death of Crassus came the phrase “Parthian shot” used metaphorically by the likes of Arthur Conan Doyal. The notoriety of this historical nugget begs the questions, where and why was this “shot” notable and what where the mechanics and engineering that made it possible?

The Parthian shot was a tactic used by the skillful Parthian archers. While mounted on light horses the Parthian would feign a hasty retreat inclining the opposing forces to open ranks or drop guard. Then while in a full gallop away from the enemy formation, the Parthian archers would turn their bodies back and shoot with an incredible show of equestrian skill at the unsuspecting troops. This great feat was owed in part to the ability of the Parthians on horseback but was more notably facilitated by the often unappreciated and fascinating engineering of the ancient composite bow.

To appreciate the composite bow it is helpful to step back and look at the notorious long bow of the English empire. The battles of Crecy in 1346 and Henry V’s Battle of Agincourt in 1415 of Shakespearean fame brought the military and historical significance of the long bow and firepower into focus. Technically the bow is one of the most effective ways of storing the energy of human muscles for application toward the projection of a missile weapon. The English long bow of historical fame although initially constructed of English yew, was quickly replaced by the Spanish yew since it manifests high compressive strength and elasticity. In fact the European Yew (Taxus baccata) was found in abundance throughout Spain and most of the Mediterranean. Even to this day it can be found growing amidst the detritus of ancient Pompeii. What proves interesting about this fact is that curious absence of the use of yew for bows in Spain or the Mediterranean in antiquity, including with the ancient Parthians. Why is this so?

The yew has a unique composition that mitigated its use in the southern Mediterranean. Typically the wood of a yew bow tends to break down and lose its mechanical properties in warm weather. In fact at a temperature above 95 degrees Fahrenheit yew bows lose their efficacy as a weapon. This property confined the usefulness of the yew for bow construction to cool climates, making it an ideal weapon of the forces of Henry V but absent in the infamous battles of the Romans or Greeks that preceded.

This brings us to the composite bow of the Parthians. From an engineering standpoint the maximum energy a human can put into a bow is limited by the mechanics of the human body. Typically one can draw an arrow back about 24 inches, and even a strong individual can not pull back with a force exceeding 80lbs, or about 210 Joules (0.05 Calories). This said, if one was to take a bow that starts with a slacked string that is essentially unstressed and draw back the arrow for the total 24 inches, the bow reaches greatest tension at the end of the pull. In this situation the total available energy stored in the bow is one half the available energy, the remaining half retained in the mechanics of the archer. Taking that into consideration the available energy to be transferred to the arrow is approximately 105 Joules (.025 Calories). Generally the measured energy available in an English long bow under strain is just slightly less than this figure. What though of the Parthinian archers?

If we look historically at references to composite bows of the southern Mediterranean from antiquity up to as recently napoleonic times we can find that the Greek word palintonos, or “back-stretched” is used, particularly in the Homeric Classic of the Odyssey. In this arrangement considerable force was necessary to string the bow, bringing it from a bent-backward or negative bend to the strung position. But once done as long in good condition the bow was then in a constant state of pre-stressed prior to being further drawn by the archer. This process leaves the composite bow with distinctive recurve or “cupids bow” like look. The pre-stress stores a portion of energy that was then added to by the application of muscle energy by the archer upon drawing back the bow. This added pre-stress energy allowed for up to 80 percent increase of available energy for application upon the arrow, while still only requiring the equivalent 210 Joules of pull back force by the archer it results in 170 Joules stored energy as opposed to the long bows potential 105 Joules.

In addition to this fact the composite nature of the bow facilitated the greater energy storage potential described above. Where the long bow was typically a yew construction, a composite bow as that made by the Parthians was variably made of a wood core with a tension surface of dried tendon and a compression face of horn. These materials having come mostly from animals where use to operating at body temperature or roughly 100 degrees Fahrenheit, and where well suited for the Mediterranean climate while maintaining their mechanical properties.

So there we have the distinction. The shorter lighter composite bow is set as distinct from the larger heavier wooden yew bow or “long bow”. It was this compact size and mobility facilitated by the construction technique and ideal materials that allowed for the Parthianian archers to shoot backwards from horseback while feigning retreat, forever founding the phrase “Parthian shot”.

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