Readers may be divided into four classes: I. Sponges, who absorb all they read, and return it nearly in the same state, only a little dirtied. II. Sand-glasses, who retain nothing, and are content to get through a book for the sake of getting through the time. III. Strain-bags, who retain merely the dregs of what they read. IV. Mogul diamonds, equally rare and valuable, who profit by what they read, and enable others to profit by it also.