LIFE, duration qf. The uncertainty of • the continuance of human life, has been a fruitful source of serious reflections not only to divines and moralists of all ages, but occasionally to every individual of the human race. Independent of the host of fatal diseases which are continually aug menting the list of their victims, the fre quently occurring instances of persons apparently in full possession of all the requisites to the continuance of life, be ing unexpectedly consigned to the grave, would cause men to think life more un certain than they generally appear to consider it, (lid not the experience ofliv ing from one day to another, confirmed by the whole of their past lives, impress them with the expectation of continuing so to do, while they do not feel any known impediment to it; and it is neces. sary to the well being of society that this idea should in general preponderate. But as the property or income from which many persons derive their subsist ence depends on the continuance of their life, or that of others, cases will fre quently occur in the adjustment of pecu niary concerns, in which it is desirable to be able to form an estimate of the dura tion of life, and as it is evidently a subject on which certainty cannot be attained, we must be content with that species of knowledge which rests on probability, This degree of knowledge, which is the limit of our acquaintance, with many other important facts, is, in a comprehen sive view of this subject, infinitely more useful and proper than more positive knowledge would be.
At whatever period the world was first inhabited, there is undoubted evidence that for at least 3000 years past the ge neral duration of human life has been much the same as it now is ; nor has any great difference been observed between the inhabitants of different climates, the negro of Africa (in some instances at least) attaining to as great age as the Eu ropean. The human frame appears to adapt itself with little difficulty to the at mosphere and local peculiarities of the country in which it is born, or even into which it is afterwards removed. Thus not only the children of persons who have removed from Great Britain to different parts of the continent of North America, but also the emigrants themselves, have been found to live as long as in the for mer country. Men can live equally well under very different circumstances ; it is sudden changes that are injurious to the human frame ; and temperate climates being less liable to such changes are found to be most favourable to the dura tion of life. There are however, in almost every country, particular districts in which the inhabitants are found to live longer than in other situations, which proceeds chiefly from a free circulation of air, uncontaminated by the noxious va pours and exhalations which destroy its purity in other parts ; thus hilly districts are almost universally found to furnish more instances of long life, than low and marshy situations.
The knowledge of the duration of hu man life in general, and of its probable continuance at all 'ages, has been ascer tained with sufficient correctness for all practical purposes, from the observations which have been made on the bills of mortality of different places. Dr. Halley formed a table of the probabilities of life from the registers of the births and bu rials of the inhabitants of the city of Bres low, the capital of the duchy of Silesia in Germany, from the year 1687 to 1691. A similar table was formed by Mr. Thomas Simpson from the London bills of morta lity, from 1728 to 1737; and other tables of the same kind have been since pub lished by M. Dupre de St. Maur, M. Kerseboom, Al. de Parcieux, Dr. Price, and others, from which the following are selected.
The probability that a life of any pre sent age shall continue a certain number of years, or shall attain to any other given age, is the fraction whose numerator is the number of the living in the table opposite to the given age, and the denominator the number opposite to the present age of the given life. Thus the probability that a life of 25 shall attain to the age of 45, 3248 or live 20 years, is The difference between this fraction and unity gives the probability-that the event will not happen; the probability that a life of 25 will not live 20 years, is therefore 1512 476J, conse quently the odds of living to dying in this period are more than 2 to 1. The pro bability that a person of 32 years of age shall attain to 59 years, appears by the ta 2120 ' ble to be — or nearly an even chance. 4235 In order to find the expectation of life at any age, from a table, like the above, which shows the number that die annually at all ages, divide the sum of all the living in the table, at the age whose expectation is required and at all greater ages, by the sum of all that die annually at that age and above it ; or, which is the same, by the number of the living at that age ; and half unity subtracted from the quotient will give the expectation required. Thus, at the age of 65, the sum of all the living at that and all greater ages, is 18,580 ; the number living at that age is 1,632; and the former number divided by the latter, and half unity subtracted from the quotient, gives 10.88 for the expectation of the age of 65. In this manner the fol lowing table is firmed.