A good many readers make notes as they go. If these are written in books they soon become so numerous, so various of topic, as to demand laborious indexing. It is better to take the notes in a form which will index itself. Slips of good paper can be bought at low cost, and, as in the accompanying illustration, “Astronomy,” “Glass,” “Photography,” or other headings may be adopted. All the slips under a given head are numbered consecutively. Kept on edge in a shallow box, or tray, they are self-indexing, and a new slip takes its proper place at once. From its compactness this kind of note-keeping puts a premium on the abbreviations which suggest themselves in a special study. Unit Systems. A card system employed as a catalogue, or for account keeping, is made up of simple units which may be added to or deducted from with utmost ease. They may be manipulated as readily as the bricks, all alike, with which a child builds a house, a box, or a steeple. This principle a few years ago was extended to book-cases, each about a foot high and about thirty-three inches long; while each formed a unit by itself it could be combined with other such units to furnish forth a library. This plan had been adopted for office furniture of all kinds,--cabinets in which papers may be filed away, or which are divided into pigeon-holes for blanks and the like. In some handsome designs a unit unfolds as a small writing desk, while adjacent units contain drawers of various sizes. Each unit is so moderate in dimensions as to be readily portable; a dozen, a score, a hundred may be joined together to equip a sitting-room or the cashier’s office in a bank. [Illustration: Sectional book-case, desk, and drawers.] Numbering as a Fine Art. When an American visits London for the first time, he may fall into an error which will much provoke him. Suppose that he has to call at 457 Strand. He begins at number 1 in that thoroughfare, and proceeds a goodly distance when, to his dismay he observes that the numbers he is passing on his right are strictly consecutive,--100, 101, 102 and so on. A weary trudge brings him to 457, opposite number 1, whence he started. That odd numbers should be on one side of the street, and even numbers on the other, did not occur to the city fathers of London centuries ago. In this regard a forward step was taken in Philadelphia, where the streets parallel with the Delaware River are First, Second, and so on, while each house on the streets crossing them from the river westward is so numbered as to tell between what streets it stands. Thus, when we walk up Chestnut Street, the first door above Ninth Street, on the right, is 901, although the house next below it, across Ninth Street, is 839; and so on with all parallel streets. If the thoroughfares in Philadelphia, running at right angles to the Delaware River, were labeled avenues, and consecutively numbered, the system would be a troublesaver indeed.