A correct knowledge of the rate of mortality among mankind is of such importance for answering many interesting questions, that we might hav expected it would hav been diligently studied from the earliest times. This, however, appears not to hav been the case, the reason probably being that questions as to the expectation of life arise in the first instance with reference to particular persons, and not to men in general. The proverbial uncertainty of human life—so often seen in the occurrence of death when least expected—is unfavourable to the notion of a general law of mortality, but naturally suggests the search after a means of predicting the duration of individual life. As thus understood, the question of the duration of life has at all times attracted great attention. Soothsayers and magicians claimd to hav the power of discerning from signs and omens of many kinds every event of life, its termination not excepted, and astrologers pretended to read in the stars the precise hour of each man's death.