stances which only a practical printer can understand. For this reason the most beautiful books are printed by the common press; but very good work is done by the platen machine.

The printing machines in general use may be divided into two classes, viz. — Singlecylinder machines, which print one side of the sheet; and double-cylinder machines which print both sides of the sheet before it leaves the machine. A double cylinder machine may be compared to a couple of single-cylinder machines thrown into one.

The following is a skeleton or linear representation of a single-cylinder machine, with the omission of minor details for the sake of clearness:

It is necessary first to describe how the different parts of the machine are set in motion at the same time, and at the same speed, though some of them move in different directions, and even change their directions. If the machine is to be worked by men, a handle

is inserted in the axis of the large wheel A: if it is to be worked by steam, then the wheel is replaced by another, connected by a band, as shown in page 110, with the shaft of a steamengine. The wheel A being turned round, the cord, shown by the dotted lines, turns the cogged wheel B; this wheel works in the cogs of a larger wheel C, and C turns the printing cylinder D behind, the two latter working on the same axis X. The little cogged wheel also turns a universal wheel, the place of which is indicated by the shading E, a part of the machinery on which it works, and the universal wheel sets in motion the table of the machine on which the form of type is placed. This table moves backward and forward within the framework F, F, on the little wheels beneath it. The printing is a very simple process. The sheets to be printed are placed on the board G; a man standing on a platform, as shown in Bensley's machine (p. 110), moves sheet after sheet down to the top of the printing cylinder D, when it is caught by a cleverly contrived apparatus acting like the human hand, and is drawn within the tapes which go round the cylinder, as shown by the dotted line. When the printing cylinder begins to turn, the table also starts from the end at which the large wheel A is placed; the form inks itself by passing under the rollers indicated by a, and then slides towards the printing cylinder D: the form reaches the

printing cylinder exactly when the latter has brought the edge of the sheet to the same point; and the sheet is printed by being pressed between the surface of the cylinder and the type, as they move towards the end of the machine H: The tapes, which have kept the sheet close to the surface of the printing cylinder, it will be seen, are not continued round the cylinder, but end under it. Thus, the printed sheet is not carried upwards as the cylinder moves upwards, but is thrown off on the board I, and the form passes under the board. While that part of the cylinder on which the grippers are placed is rising to roll round another sheet, the table has shot back to the opposite end of the machine, the form obtains another supply of ink, and shoots back again, reaching the printing cylinder the very instant that it has brought down another sheet to be printed, as already described. This ingenious motion, forward and backward, is caused by the working of the universal wheel under the table.

The following is a representation of a double-cylinder machine. (See next page.)

The moving-wheels are at the back instead of the front, as in the representation of a single-cylinder machine; and, as this is a perspective drawing, it will help to make clear what may be obscure in the linear drawing.

AA are the inking-rollers, which supply the forms with ink; a set at each end of the

machine, outside the large printing cylinders. The type is inked by the rollers a a. B is the form of type for printing one side of the sheet: the second form, for printing the second side, cannot be shown, as when one form is drawn out in the position of B, the other is drawn under the cylinder C. The forms glide backward and forward, B under cylinder E, and the other form under cylinder C, on a table similar to the table of the common press, the table being set in motion by a wheel F. The moving machinery is indicated by the cog-wheels a a. A man stands on a platform with the sheets to be printed lying on a board G; he moves sheet after sheet downwards, until its edge meets the roller H, the end of which is just seen, and the sheet is caught within a series of endless tapes, which are shown by dotted lines extending throughout the machine, but they are too complicated to be clearly explained in words. These tapes, in moving in common with the machinery, carry the sheet in the direction of the downward arrow round the first printing cylinder C, and by the time that the sheet is half round, the bed of the machine has moved sufficiently to place the type under the cylinder C, and the cylinder and type move together in the direction of O, so that the first side of the paper is printed as it passes between them. The paper is now carried by the tapes upwards, over the cylinder I, and passing under