The strict analogy which exists between these two classes of annuities has doubtless received the consideration of the majority of students, but I am not aware that it has ever in print had that attention which it deserves; and at any rate our usual text books hardly even refer to it. Although it is shown, in the short but suggestive article of the late Prof. De Morgan (vol. iv, p. 277), in the most general way, that years, or periods of time, may be interchanged with any other kind of status whatsoever, whether certain or contingent; yet, as successive statuses are almost exclusively spoken of, the less complex cases are liable to escape notice. Under these circumstances, the following brief notes may not be thought superfluous.