♦ KNOWLEDGE ♦ 



[July 4, 1884. 



relative weight of the birds. The same remark applies to 

 insects ; and we see, in fact, that the bee, the bluebottle, 

 and the common fly — insects not unlike in their proportions 

 — have wings proportioned to their surface dimensions ; 

 the same holding amongst long-bodied insects, like the 

 gnat and the dragon-fly, and the same also among the 

 different orders of flying beetles. 



So that, setting apart difl'erences of muscular capacity 

 and adaptation, a man, in order to fly, would need wings 

 bearing the same proportions to his body as we observe in 

 the wings of the sparrow or the pigeon. In fact, the wings 

 commonly assigned to angels by sculptors and painters 

 would not be so disproportioned to the requirements of 

 flight as has been commonly supposed, if only the muscular 

 power of the human frame were well adapted to act upon 

 wings so placed and shaped, and there were no actual infe- 

 riority in the power of human muscles (cross-section for 

 cross-section) as compared with those of birds. 



So far as the practicability of actual flight on man's 

 part is concerned, these two points are, indeed, among the 

 most important we have to consider. It was to Borelli's 

 remarks on these points, in his famous treatise, De motu 

 Animaiium, that the opinion so long entertained respecting 

 the impracticability of flight must be referred. He com- 

 pared the relative dimensions of the breast muscles of 

 birds with those of the corresponding muscles in man, and 

 thence argued that man's frame is altogether unadapted to 

 the use of wings. He compared also the relative muscular 

 energy of birds and men, that is, the power of muscles of 

 equal size in the bird and the man ; and yet was further 

 confirmed in the opinion that man can never be a flying 

 animal. 



{To ie continued.) 



ELECTRO-PLATING. 



vir. 



By W. Slingo. 



THE copy, on being released from the mould, is gene- 

 rally mDre or less brittle. It is, therefore, made 

 red-hot in a clear fire or by means of a blow-pipe. 

 When cool it is placed in a weak sulphuric acid solution to 

 remove any scale or superficial impurities which may be 

 present. After a few minutes' exposure it is removed into 

 a vessel containing clean water, and subsequently taken out 

 and dried. Any superfluous metal that still remains is 

 now chipped off and the copy cut to form. The sur- 

 face next requires polishing, which is accomplished 

 ■with rotten -stone and oil, applied with a stiff 

 brush. The particles of metal, &c., adhering to the 

 copy are washed off with soap and boiling water, and 

 the surface again polished, using this time moistened 

 rouge and a soft brush. The finger, at this stage, becomes 

 a very useful tool for brightening the smoother portions of 

 the surface. When the copy is required to possess con- 

 siderable substance, that is to say, when it is to be able to 

 withstand rough or hard usage, as in the case of electro- 

 types of wood engravings, &c., a backing of foreign and 

 more easily fusible metal is and must be provided. It 

 would be false economy, and, indeed, almost a practical 

 impossibility, to take copies sufficiently stout to answer 

 such purposes as above indicated. The process of " back- 

 ing " should present no serious difliculties to the amateur. 

 The copy should be laid face downwards and the back or 

 inner surface moistened with killed spirits of salts, that is to 

 say, hydrochloric acid which has been neutralised or killed 



by dropping in it small pieces of zinc. A small piece 

 of pewter solder is then placed on the back of the copy and 

 made to cover that surface. This is easily accomplished 

 with a soldering iron or (holding the copy in the jaws of a 

 pair of pliers, so as to be able to move it about) a V)lowpipe 

 flame.* For want of better means, a good flame may be 

 obtained fiom a gas-jet, and sometimes from a spirit-lamp, 

 with the aid of a piece of non-combustible tubing, such as 

 the stem of a clay tobacco-pipe. Care must be taken that 

 the surface of the copper is fairly coated with solder. This 

 is ensured by an eflScient application of the chloride of zinc 

 solution. The coating of solder being obtained, lead is 

 next poured in tDl the required thickness is obtained. 

 The lead combines or adheres to the solder readily, but 

 would not so adhere to the bare copper, hence the necessity 

 for using the solder. Apart from the scientific or experi- 

 mental interest pertaining to the various operations de- 

 scribed, there are a vast number of practical applications 

 fraught with the greatest importance. Not the least of 

 these is the adaptation of the electrotyping process to 

 printing on a large scale. Where a large number of im- 

 pressions are required, more particularly of diagrams, pic- 

 torial illustrations, newspaper headings, ic, the process is 

 almost invariably resorted to. The Illustrated London 

 News and such-like papers not only treat their illustrations 

 in this way, but apply the process to their type as well 

 The process is a simple one. A ball of gutta-percha is 

 place on the centre of the type and worked outwards gra- 

 dually so as to exclude every possible particle of air, and 

 obtain a perfect negative copy of the type. A weight is 

 placed on the percha, and left there until the latter has 

 cooled down, When cool, the percha is well coated with 

 plumbago, and immersed in the electrolytic bath for a 

 few hours, when, a sufliciently thick deposit having been 

 obtained, it is taken from the bath, the percha se- 

 parated from the copper, and a substantial backing 

 supplied. The backing being made level, it is screwed 

 on to a wooden block, of a thickness suflicient to 

 raise the electrotype to the same level or thickness as 

 ordinary lead type. The advantage of such a process is 

 evident. More especially is this so in the case of an en- 

 graving, which would manifestly suffer very considerably 

 were it used extensively. The durability of a wood en- 

 gi'aving is not extraordinarily great, and once damaged the 

 cost of replacing it is almost, if not quite, as great as the 

 original cost ; whereas, by the adoption of the electro- 

 typing process, additional equally good copies may be pro- 

 cured at onlj-^ a trifling expense. The advantages offered 

 by the process for the preservation of valuable and artistic 

 engravings are too apparent to be further discussed. Some 

 papers, it may be furthermore remarked, are very hard and 

 unyielding, and in consequence they considerably injure 

 the type or block, speedily rendering the impressions 

 blurred and more or less trying to decipher. It is recorded 

 that as many as 20,000,000 impressions have been taken 

 from a single mould. Some typers use plaster of Paris 

 instead of gutta-percha ; but although it is cheaper and 

 sets more rapidly, it does not yield so perfect a copy, there 

 being a want of clearness. There is, however, one great 

 drawback to the adoption of the electrotyping process for 

 books, viz., that in second or subsequent editions, errors, 

 small, it may be, in the amount of type involved, but great 

 in importance, cannot be rectified without sacrificing a 

 page or so of the electrotype. On the other hand, there is 



* A soldering-iron in the bands of the amateur is often a 

 dangerous weapon. He is apt to use it too heavily, and there ia a 

 possibility of his pushing it through the electrotype. A spirit 

 flame is, therefore, safer and equally effectual. 



