PRINTING 



629 



1665 



1666 



the cup is turned downwards, the greater the pressure of water upwards, the more 

 forcibly are the edges of the leather valve pressed against the insides of the cylinder, 

 and the tighter does the joint become. This was Bramah's beautiful invention. 



Upon the top of the ram, the press-plate, or table, strengthened with projecting 

 ridges, rests, which is commonly called the ' follower,' because it follows the ram closely 

 in its descent. This plate has a half-round 

 hole at each of its four corners, corresponding 

 to the shape of the four iron columns along 

 which it glides in its up-and-down motions of 

 compression and relaxation. 



k, k,figs. 1663 and 1664, is the framing of a 

 force-"pump with a narrow barrel ; i is the well 

 for containing water to supply the pump. To 

 spare room in the engraving, the pump is set 

 close to the press, but it may be removed to 

 any convenient distance, by lengthening the 

 water-pipe u, which connects the discharge of 

 the force-pump with the inside of the cylinder 

 of the press. Fig. 1665 is a section of the 

 pump and its valves. The pump m, is of 

 bronze ; the suction-pipe n, has a conical valve 

 with a long tail ; the solid piston or plunger 

 p, is smaller than the barrel in which it plays, 

 and passes at its top through a stuffing-box q ; 

 r is the pressure-valve, s is the safety-valve, which in Jig. 1664 is seen to be loaded 

 with a weighted lever ; t is the discharge-valve, for letting the water escape from 

 the cylinder, beneath the ram, back into the well. See the winding passages in fig. 

 1666. u is the tube which conveys the water from the pump into the press-cylinder. 

 In fig. 1664, two centres of motion for the pump-lever are shown. By shifting the 

 bolt into the centre nearest the pump-rod, the mechanical advantage of the workman 

 may be doubled. Two pumps are generally mounted in one frame for one hydraulic 

 press : the larger to give a rapid motion to the ram at the beginning, when the resist- 

 ance is small ; the smaller to give a slower but more powerful impulsion, when the 

 resistance is much increased. A pressure of 500 tons may be obtained from a well- 

 made hydraulic press with a ten-inch ram, and a two and a one inch set of pumps. 

 See WATER PRESSURE MACHINE. 



PRINCE'S METAL, or Prince Kupert's metal, is a brass containing about 25 

 per cent, of zinc. 



PRINTING. (Imprimerie, typographic, Fr. ; Buchdruckerkunst, Ger.) The art 

 of taking impressions from types and engravings in relief. 



HISTORY. The art itself is of comparatively modern origin, only 400 years having 

 elapsed since the first book, properly so called, issued from the press ; but we cannot 

 doubt that its principle was known to the ancients. It has certainly been practised in 

 the East from a very early period, and in a manner similar to our own first attempts. 

 That a rude kind of printing was known to the Babylonians is evident from the 

 undecayed bricks of that city which have been found stamped with various cuneiform 

 and hieroglyphic characters ; but as the stamp itself was in one piece or block, it 

 was inapplicable to the propagation of knowledge, from its cost and tediousness of 

 production. 



The Chinese are the only people who have continued this primitive mode of print- 

 ing to the present time. Their earliest attempts are stated in their chronicles to have 

 been made about 50 years before the Christian era ; but it was not till the reign of 

 the Emperor Ming-tsong (927-934 A.D.) that any great advance was made in print- 

 ing large numbers of comparatively cheap books. The name of the Chinese Caxton 

 was Tong-tao. He obtained permission of the Emperor in 932 to print and circu- 

 late copies of the ' Classical Works,' as they are called, by taking impressions 

 from stone plates, the letters cut into them, so that the impression on the paper was 

 black and the letters themselves left white. This is still the case in all Chinese 

 lithographic printing. Tong-tao, however, subsequently obtained the Emperor's 

 sanction to cut in wood and print an edition of the nine ' King,' or classical books, 

 for the use of the Imperial College in Peking. This was completed in 952 ; and, 

 although intended only for the pupils of the college, it was made purchasable by 

 any person in the empire. The process pursued in the printing of this work is pre- 

 cisely the same as at the present day, the following being the 'modus operandi : 



The work intended to be printed is handed to a caligraphist, who writes the sepa- 

 rate pages on fine tracing-paper ; these are given to the engraver, who glues them 

 face downwards upon a thin plate of hard wood, called li, resembling that of the pear- 



