In order perfectly to obtain its object, a table of mortality must exhibit the actual mortality at the different ages; and to be of any practical use, we should be able from the results of the past to predict events for the future.
The actual mortality can only be directly determined by knowing the population, as well as the number of deaths, which occur at each age. There are, however, very few countries in Europe in which these two elements are sufficiently ascertained. Even in Belgium, the division of the population by ages was but imperfectly known till the census of 1846, the results of which were only published about the end of 1848. It was therefore necessary, in order to form the tables of mortality, to pass over the important element of the existing population, and frame them from the registers of deaths. In this manner, the tables of mortality which I have given successively since 1827 have been constructed on the hypothesis of a stationary population.