MECHANICS AND USEFUL ARTS. 85 



The average results of Crane's paper, 14 Ibs. to the ream, with sizing, was 

 an average perpendicular strain of 3 '35 Ibs to the square inch, with an aver- 

 age of 3'151 grammes; and an average transverse strain of 4'75 Ibs. to the 

 square inch, with an average of 3'134 grammes weight. Wilcox & Co.'s, with 

 sizing. 14 Ibs. to the ream, stood an average perpendicular strain of 3 '66 Ibs. 

 to the square inch, the average weight being 3 '195 grammes; and a trans- 

 verse strain of 4~81 Ibs., with 2*991 grammes weight. 



MANUFACTURE OF BANK-XOTES. 



At a recent meeting of the Society of Arts, London, Alfred Smee, F.R.S., 

 and chemist of the Bank of England, read a paper and unfolded publicly the 

 improved method employed by that great institution for manufacturing the 

 bank-notes, to prevent forgery by the substitution of surface printing on the 

 notes, for line printing. 



The notes and checks of the Bank of England have, up to the present time, 

 been invariably printed from copper and steel plates, in which the lines were 

 engraved, or cut into the metal, and into these lines was introduced the ink, 

 which in the progress of printing was transferred from the plate to the paper. 

 In surface printing the reverse of this state of things occurs, as the design, in- 

 stead of being cut into the metallic plate, is raised in relief, and the ink being 

 applied to the raised portions by means of rollers, is transferred by the press 

 to the paper in order to produce the impression. A single cut with a graving 

 tool forms the groove which holds the ink in plate printing, while for surface 

 printing- a line must be cut on both sides, and equally finished on both, thus 

 materially increasing the difficulty of engraving. From a steel die electro 

 moulds were obtained, and electro copper produced by this system is found to 

 have hardly any limit to its durability. The limit to the duration of the 

 electro casts, for the purposes of bank-notes, has yet to be ascertained, as 

 almost a million of copies have been already printed from one without any 

 sensible effect The platinized silver voltaic battery is the one used by the 

 bank as a source of power, and its successful operation was exhibited to the 

 meeting. It was devised by Mr. Smee, who received for it a gold medal from 

 the Society, and it has stood for 14 years the test of experience. At the bank 

 large batteries are employed, holding several gallons of the acid charge, the 

 platinized silver plate being of fair thickness, and the thick rolled zinc plates 

 being so arranged that they can be easily changed. For charging the battery 

 dilute sulphuric acid is used, generally mixed in the proportion of - acid to -J 

 of water. In order to secure a strength suitable to the purposes of the battery, 

 it is convenient to adjust the mixture to a specific gravity of 1130, and a 

 battery so charged will continue hi action nearly three weeks before it will be 

 completely exhausted. It is found, however, in practice, that after having 

 done efficient duty for from 7 to 14 days it becomes feeble, and requires a 

 fresh supply to resuscitate its former vigor. A hygrometer is generally used 

 to ascertain the changes, but Mr. Smee described an instrument constructed 

 by himself for the purpose, which he termed a battery meter. The point cor- 

 responding to specific gravity 1.130 is called unity, the interval between that 



