Tracing cloth or linen is especially useful for large work that will require considerable reduction. (See p. 18.) Its advantages are that a tracing that has been carefully made on it over any kind of copy for direct reproduction by a photo-engraving process can be used for making a paper negative for contact printing or blue printing. On the other hand, it is susceptible to atmospheric changes that affect scale, and the lines traced on it are not reproduced as sharply as those made on paper. It can be obtained in rolls 30 to 54 inches wide. Erasures should be made on tracing linen with a hard rubber eraser, not with a sand rubber or a steel eraser. INKS. The best drawing inks are in liquid form, ready for use. They should be waterproof and equal to the grade known as Higgins's waterproof ink. When a suitable waterproof blue ink can not be obtained, a good blue for features of drainage can be made by dissolving a half pan of Winsor & Newton's prussian blue in water. No good waterproof burnt sienna ink seems to be obtainable, but a good substitute can be made by dissolving Winsor & Newton's water color of that name. Ink lines should be drawn in full strength of color--lines that should be black must not appear grayish, for example--and pens should be kept clean. The same pen should not be used for applying two inks, as the mixture thus produced is likely to thicken or coagulate on the pen. A little black should be added to colored inks that are used in making drawings to be reproduced in colors in order to strengthen the lines for photographic reproduction. DRAWING PENS. The pens made by Keuffel & Esser, especially their No. 3202, and Gillott's Nos. 291, 290, 170, and 303 give complete satisfaction. The Gillott numbers are given in the order of fineness of the points. No. 291 being the finest. The best cleaner for a drawing pen is a piece of chamois skin. PENCILS. Pencils used for drawing should have leads of a quality equal to those of the Koh-i-noor brand, in which the grades of hardness are indicated by 3B, 2B, B, HB, F, H, 2H, 3H, 4H, 5H, 6H, 7H, 8H, and 9H; the softest grade is 3B and the hardest 9H. The grades most generally used are B, HB, F, 4H, and 6H. RUBBER ERASERS AND CLEANERS. Two kinds of rubber erasers are usually employed in making erasures on drawings--a hard, dense rubber like the "Ruby," and a soft, pliable rubber like the "Venus" or "H" (Hardtmuth). The soft rubber is also useful for cleaning large surfaces. Art gum is also recommended for this purpose and has the advantage of not disturbing the surface of the paper. COLORED PENCILS AND CRAYONS.