(2) It must produce a uniform impression without weak spots or streaks.
The other properties it needs in common with other presses, such as:—
(3) It must be powerful enough to produce the necessary pressure.
(4) It must combine the greatest possible speed with this power.
(5) It must be easily operated, to save the workman.
All these qualities combined are not to be found in any press hitherto applied to lithography.
II
APPLICATION OF BOOK- AND COPPER-PLATE PRESSES TO LITHOGRAPHY
If we consider the peculiarities of book and copper print, we find a decided difference between them that affects printing importantly.
The letters of book-type are raised, the engraving in copper is depressed. It is evident that the former requires no such power for making impressions as the latter. Therefore the presses are so different that copper plates cannot be printed on a book-press and vice versa. Now, as the stone combines both the elevated and the depressed principles, the natural idea would be to combine the fundamental principles of both presses as nearly as possible for stone-printing. In book-print, only the types are exposed to the pressure, and in the average printed sheet these are only one fourth part of the entire surface. The remaining white space is not affected at all by the press. In the stone, however, the elevation of any part of a design is so slight that the entire surface is affected, and consequently a stone plate offers four times as much resistance. A book-press therefore would print a stone only if it were arranged for a pressure four times greater. Now, for a stone of the size of a letter-sheet the power required to print with one vertical pressure would be five or six hundred hundredweight, a pressure that could be supported only by a thick stone laid very exactly on a perfect foundation.
An ordinary copper-plate press increases the pulling of the paper so much in the case of a stone plate that the impression would be worthless. This pulling is not caused, as in the case of the scraper, during the impression itself, as already described, but it is caused before the impression through the endeavor of the cylinder to force the plate along under it. Once the stone is under the cylinder, the paper is not pulled noticeably, because the cylinder glides over the leather much more gently and with much less friction than the scraper.