History for instance. How useful a knowledge of history would be to me now. To lighten an article like this with a reference to what Garibaldi said to Cavour in '53; to round off a sentence with the casual remark, "As was the custom in Alexander's day"; to trace back a religious tendency, or a fair complexion, or the price of boots to some barbarian invasion of a thousand years ago--how delightfully easy it would be, I tell myself, to write with such knowledge at one's disposal. One would never be at a loss for a subject, and plots for stories, plays, and historical novels would be piled up in one's brain for the choosing. But what can one do with mathematics--save count the words of an article (when written) with rather more quickness and accuracy than one's fellow writer? Did I spend ten years at mathematics for this? The waste of it! But perhaps those years were not so wasted as they seem to have been. Not only Functions of a Quaternion, but other of these books, chatty books about hydro-mechanics and dynamics of a particle (no, not an article--that might have been helpful--a particle), gossipy books about optics and differential equations, many of these have a comforting air of cleanness; as if, having bought them at the instigation of my instructor, I had felt that this was enough, and that their mere presence in my bookcase was a sufficient talisman; a talisman the more effective because my instructor had marked some of the chapters "R"--meaning, no doubt, _"Read carefully"_--and other chapters "RR" or _"Read twice as carefully."_ For these seem to be the only marks in some of the books, and there are no traces of midnight oil nor of that earnest thumb which one might expect from the perspiring seeker after knowledge. So I feel--indeed, I seem to remember--that the years were not so wasted after all. When I should have been looking after my quaternions, I was doing something else, something not so useful to one who would be a mathematician, but perhaps more useful to a writer who had already learnt enough to count the words in an article and to estimate the number of guineas due to him. But whether this be so or not, at least I have another reason for gratitude that I treated some of these volumes so reverently. For I have now sold them all to a secondhand bookseller, and he at least was influenced by the clean look of those which I had placed upon the top.