It is known to all students of physics that when a number of different observations have been made of the same constant quantity, the best result is, in general, obtained by taking the arithmetic mean of the observed values.

If the observed quantity instead of being constant is increasing at a uniform rate, and we require to determine this rate of increase, we should naturally expect that the proper course to follow would be to find the rates of increase of the quantity in successive equal intervals of time and take their arithmetic mean.