It is confidence which causes accidents and worry which prevents them. — J E Gordon

If a ship runs into a large bridge, destroying both the bridge and ship, , it is very difficult to see what either the naval architect or the bridge designer could have been expected to do about it from the structural point of view. The problem is not one for the structural engineer but for the local Pilotage Association. Again, aircraft cannot be designed to be flown into mountains.

In the course of a long professional life spent, or misspent, in the study of the strengths of materials and structures, I have had cause to examine a lot of accidents, many of them fatal. I have been forced to the conclusion that very few accidents just "happen" in a morally neutral way. Nine out of ten accidents are caused, not by more or less abstruse technical effects, but by old-fashioned human sin — often verging on plain wickedness. Of course I do not mean the more gilded and juicy sins like deliberate murder, large-scale fraud, or Sex. It is squalid sins like carelessness, idleness, won't-learn-and-don't-need-to-ask, you-can't-tell-me-anything-about-my-job, pride, jealousy and greed that kill people.

Used with caution, such formulae really are very useful indeed, and indeed they form the professional stock-in-trade of most engineering designers and draughtsmen. There is not the slightest need to be ashamed of using them; in fact we all do. But they must be used with caution.