446 THE SCIENTIFIC MONTHLY 



first such table actually to be computed in anything like the modern 

 fashion was made by the astronomer, Dr. E. Halley, and was pub- 

 lished in 1693. Since that time a great number of such tables have 

 been calculated. Dawson fills a stout octavo volume with a collection 

 of the more important of such tables computed for different countries 

 and different groups of the population. Now they have become such 

 a commonplace that elementary classes in vital statistics are required to 

 compute them. 



2. Changes in Expectation in Life 

 I wish to pass in graphic review some of these life tables in order 

 to bring to your attention in vivid form a very important fact about 

 the duration of human life. In order to bring out the point with 

 which we are here concerned it will be necessary to make use of an- 

 other function of the mortality table than either the l x or d x lines 

 which you have seen. I wish to discuss expectation of life at each age. 

 The expectation of life at any age is defined in actuarial science as 

 the mean or average number of years of survival of persons alive at the 

 stated age. It is got by dividing the total survivor-years of after life 

 by the number surviving at the stated age. 



In each of the series of diagrams which follow there is plotted the 

 approximate value of the expectation of life for some group of people 

 at some period in the more or less remote past, and for comparison 

 the expectation of life either from Glover's table, for the population 

 of the United States Registration Area in 1910 — the expectation of life 

 of our people now, in short — or equivalent figures for a modern Eng- 

 lish population. 



Because of the considerable interest of the matter, and the fact that 

 the data are not easily available to biologists, Table 1 is inserted giv- 

 ing the expectations of life from which the diagrams have been plotted. 



