The T‘u Shu adds 哉. The final 之 may refer either to the circle or, more probably, to the 奇正之變 understood. Capt. Calthrop is wrong with: “They are a mystery that none can penetrate.”
12. 激水之疾至於漂石者埶也
The onset of troops is like the rush of a torrent which will even roll stones along in its course.
13. 鷙鳥之疾至於毁折者節也
The quality of decision is like the well-timed swoop of a falcon which enables it to strike and destroy its victim.
For 疾 the Yü Lan reads 擊, which is also supported by a quotation in the 呂氏春秋 [3rd cent. B.C.]. 節 in this context is a word which really defies the best efforts of the translator. Tu Mu says that it is equivalent to 節量遠近 “the measurement or estimation of distance.” But this meaning does not quite fit the illustrative simile in [§ 15]. As applied to the falcon, it seems to me to denote that instinct of self-restraint which keeps the bird from swooping on its quarry until the right moment, together with the power of judging when the right moment has arrived. The analogous quality in soldiers is the highly important one of being able to reserve their fire until the very instant at which it will be most effective. When the “Victory” went into action at Trafalgar at hardly more than drifting pace, she was for several minutes exposed to a storm of shot and shell before replying with a single gun. Nelson coolly waited until he was within close range, when the broadside he brought to bear worked fearful havoc on the enemy’s nearest ships. That was a case of 節.
14. 是故善戰者其埶險其節短
Therefore the good fighter will be terrible in his onset, and prompt in his decision.
Tu Yu defines 節 here by the word 斷, which is very like “decision” in English. 短 is certainly used in a very unusual sense, even if, as the commentators say, it = 近. This would have reference to the measurement of distance mentioned above, letting the enemy get near before striking. But I cannot help thinking that Sun Tzŭ meant to use the word in a figurative sense comparable to our own idiom “short and sharp.” Cf. Wang Hsi’s note, which after describing the falcon’s mode of attack, proceeds: 兵之乘機當如是耳 “This is just how the ‘psychological moment’ should be seized in war.” I do not care for Capt. Calthrop’s rendering: “The spirit of the good fighter is terrifying, his occasions sudden.”
15. 埶如彍弩節如發機