THE PRINTING TRESS. 



Above the form and slab are mounted, also in juxtaposition, a 

 large cylinder or drum, which carries upon it the sheet of paper 

 to be printed, and three or four inking-rollers similar to that 

 already described. There are also three or four other rollers in 

 juxtaposition with the latter, one of which supplies ink to ^the 

 others, which severally spread it in a uniform stratum upon the 

 slab. The paper-cylinder and the inking and diffusing rollers are 

 so mounted, that when the horizontal table, carrying the form 

 and inking slab, moves alternately backwards and forwards under 

 them, they roll upon it. 



In this way, when the table is moved towards the rollers, the 

 form, passing under the inking-rollers right and left, receives 

 from them the ink upon the face of the type ; and at the same time 

 the slab, moving backwards and forwards under the diffusing 

 rollers, receives from them, upon its surface, the proper stratum 

 of ink to supply the place of that which was taken from it by the 

 inking-rollers. 



20. Single Printing Machines When the table is moved 

 alternately towards the other side, the form, with the types 

 already inked, passes under the cylinder carrying the paper, the 

 motion of which is so regulated as to correspond exactly with the 

 rectilinear motion of the table carrying the form. The cylinder 

 is urged upon the type with a regulated force, sufficient and not 

 more than sufficient, to impress the type upon the paper. 



The sheets of paper are supplied in succession to the cylinder, 

 and held evenly upon it by bands of tape while they pass in 

 contact with the type. After receiving the impression of the 

 type, the tapes which bound them are separated, and the printed 

 sheets discharged. 



Such is the general principle of single printing-machines. 



21. Double Printing Machines. In these the table which 

 is moved alternately right and left, carries two forms, one corre- 

 sponding to the pages to be printed on one side of the sheet, and 

 the other to those to be printed on the other side. There are also 

 two inking-slabs, one to the left of the left-hand form, and the 

 other to the right of the right-hand form. There are also two 

 paper cylinders, and two sets of inking and diffusing-rollers. 

 Each sheet of paper to be printed, being held between tapes, as 

 already described, is carried successively round the two cylinders, 

 being so conducted, in passing from one to the other, that one 

 side of it passes in contact with, and is printed by, one form, and 

 the opposite side by the other form. The proportion of the 

 motions is so nicely regulated, that the impression of each page or 

 column made on one side of the paper, corresponds exactly with 

 that of the corresponding page or column on the other side. 



12 



