A defect in the carbon method is found in the manner of making corrections. It is not uncommon for a man to make a slight correction, or add a note, in his own handwriting; such corrections and additions cannot appear on the carbon copy. When corrections are made on the typewriter, it is very difficult to obtain a clear copy of the correction on the second, or carbon, sheet--the correction on the copy is very likely to be blurred. If the correction is written directly on the copy, without using carbon paper, the essential feature of the copy is destroyed; it is no longer a facsimile, and its value as evidence is very materially lessened. =Mechanical Copiers.= There are now on the market several mechanical copiers, without the many disadvantages of the copying press, or the defects of the carbon method. One of the most satisfactory and best known is the roller copier. The fundamental principle of the roller copier is that of the clothes wringer; the original letter, with a wet tissue sheet placed against the written side, is passed between rubber rollers, to which just the right amount of pressure is applied. Working from the clothes-wringer idea as a basis, the inventor of the roller copier set about to remedy the defects of the copying press. It was found that clear copies could be obtained only under certain conditions; the pressure must be absolutely uniform, and the tissue sheet must be moistened evenly. [Illustration: Fig. 4. Mechanism of the Yawman & Erbe Roller Copier] As to the pressure, it was found that too much or too little resulted in unsatisfactory copies--some faint, others blurred--the very defect that had made the use of the copying press objectionable. This was overcome by using an adjusting screw by which the exact pressure required could be obtained. But, if the pressure were left on all of the time, the rubber rollers would soon be flat. So, an adjustment lever was added, by which the pressure could be thrown on before copying, and thrown off when the machine is not in use. To adjust a single sheet of tissue paper so that it would lie smooth on the letter, was out of the question. This difficulty was overcome by copying on a continuous roll, and the problem of moistening was solved by passing the paper through water, insuring equal moisture over the entire surface.