Example 355. In times of epidemics, sinking of ships in time of war, accidents of all kinds that occur so many a week, events of interest to us that occur about so many times on an average in a certain length of time, it is very convenient to carry formulas in the head that will enable us to ascertain approximately the chance that any number will occur, so as to give us an idea whether things are getting better or worse. In the world war when our ships were being sunk by submarines I was anxious to have such a formula, and tried to make one, and that of Proposition 7 is the result. I was surprised to find it so precise. It was so precise that it must represent the law of errors. But it was difficult to get the proof.