There is a striking passage in Addison's "Vision of Mirza," in which life is pictured as a passage over a bridge of about a hundred arches. A black cloud hangs over each end of the bridge. At the entrance to it there are hidden pitfalls very thickly set, through which throngs disappear, so soon as they have placed their feet upon the bridge. They grow thinner towards the centre; they gradually disappear; until at length only a few persons reach the further side, and these also having dropped through the pitfalls, the bridge at its further extremity becomes entirely clear. The description of Addison corresponds with the results of the observations made as to the duration of human life. Thus, of a hundred thousand persons born in this country, it has been ascertained that a fourth of them die before they have reached their fifth year; and one-half before they have reached their fiftieth year. One thousand one hundred will reach their ninetieth year. Sixteen will live to a hundred. And only two persons out of the hundred thousand--like the last barks of an innumerable convoy, will reach the advanced and helpless age of a hundred and five years. Two things are very obvious,--the uncertainty as to the hour of death in individuals, but the regularity and constancy of the circumstances which influence the duration of human life in the aggregate. It is a matter of certainty that the _average_ life of all persons born in this country extends to about forty-five years. This has been proved by a very large number of observations of human life and its duration. Equally extensive observations have been made as to the average number of persons of various ages who die yearly. It is always the number of the experiments which gives the law of the probability. It is on such observations that the actuary founds his estimates of the mortality that exists at any given period of life. The actuary tells you that he has been guided by the Laws of Mortality. Now the results must be very regular, to justify the actuary in speaking of Mortality as governed by Laws. And yet it is so. Indeed, there would seem to be no such thing as chance in the world. Man lives and dies in conformity to a law. A sparrow falls to the ground in obedience to a law. Nay, there are matters in the ordinary transactions of life, such as one might suppose were the mere result of chance, which are ascertained to be of remarkable accuracy when taken in the mass. For instance, the number of letters put in the post-office without an address; the number of letters wrongly directed; the number containing money; the number unstamped; continue nearly the same, in relation to the number of letters posted, from one year to another.