The general subject of extra risks, and the special method of meeting such risks by making temporary deductions from the sum assured in lieu of charging an extra premium, have been more than once under discussion at this Institute. The latter problem in particular was somewhat exhaustively considered upon the occasion of a paper by the late Mr. Sunderland, “On a Method frequently adopted of treating Under-Average Lives for Assurance purposes, &c.” (J.I.A., xxix, 419).

The method about to be described is based upon a special application of the principle involved in the slide-rule. As this instrument is less familiar now than it was formerly, and as its working presents many points of interest, I propose to begin with a short account of its construction and use.