BRAHMAGUPTA'S problem * appears to be the earliest instance of a kind of joke which has been much used by mathematicians. For the sake of giving a certain picturesqueness to the data of problems, and so to excite that sort of interest which is partly expressed by a smile, mathematicians have got into the habit of talking, for example, of monkeys in the form of geometrical points climbing up massless ropes. Professor P. Stackel2 truly remarked that physiological mechanics-the mechanics of bones, muscles, and so onis wholly different from this. There was once a lecturer on mathematics at Cambridge who used yearly to propound to his pupils a problem in rigid dynamics which related to the motion of a garden roller supposed to be without mass or friction, when a heavy and perfectly rough insect walked round the interior of it in the direction of normal rolling.