The Deacon's Masterpiece or, the Wonderful "One-hoss Shay": A Logical Story

The term one-hoss shay in John Walker's crisis letter (http://news.ycombinator.com/item?id=349676). I didn't know what it meant, so I looked it up and found that it referred to this poem by Oliver Wendell Holmes. Holmes gives this description:

"There is a practical lesson to be got out of the story. Observation shows us in what point any particular mechanism is most likely to give way. In a wagon, for instance, the weak point is where the axle enters the hub or nave. When the wagon breaks down, three times out of four, I think, it is at this point that the accident occurs. The workman should see to it that this part should never give way; then find the next vulnerable place, and so on, until he arrives logically at the perfect result attained by the deacon."