• Readers may be divided into four classes: 1) Sponges, who absorb all that they read and return it in nearly the same state, only a little dirtied. 2) Sand-glasses, who retain nothing and are content to get through a book for the sake of getting through the time. 3) Strain-bags, who retain merely the dregs of what they read. 4) Mogul diamonds, equally rare and valuable, who profit by what they read, and enable others to profit by it also