Dec. 1 8, 1879J 



NATURE 



*55 



The "Encyclopaedia Britannica " — The Nile 



The volume of the Nile is made a thousand times greater 

 than the truth in the new " Encyclopaedia," vol. vii. p. 706, 

 art. Egypt, by an error copied from the last edition. The 

 same mistake occurs in Raw linson's " Herodotus," vol. ii. p. 7, 

 Note G\V. ; in the Geographical Society's Journal, vol. xix. ; 

 in Fullarton's Gazetteer, and probably elsewhere, and some 

 fables have been founded on it. 



I observed the error myself some years ago, after being per- 

 plexed by it in some rainfall estimates, and mentioned it to the 

 late Sir Gardner Wilkinson, who intended to have it corrected ; 

 but there has been no fresh edition of the " Herodotus," and it 

 has escaped revision in the " Encyclopedia. " As it illustrates a 

 special danger, easily overlooked, in copying French figures, it 

 deserves perhaps a few lines in Nature to put it right. 



In English notation we mark decimals with a point, and use 

 commas to divide periods ; but the French generally use commas 

 to mark off decimals. The authority for the volume of the Nile 

 is I.inant's measurement, given by Clot Bey in the " Apercu 

 general sur l'Egypte," tome i., pp. 40, 41, and the figures are 

 given as follows, in cubic metres of water discharged into the 

 Mediterranean at full flood in twenty-four hours : — 



English authorities 705,514,667,440 



Linant 705 514 667,440 



The last three figures are decimals, and the quantity is in 

 millions, not in thousand millions. Albert J. Mott 



December 14 



Lunar Rings 



According to your suggestion I have followed up my experi- 

 ments with lunar light on bromo-gelatine plates, and at midnight 

 on November 28, for the third time at full moon period I 



obtained on one plate three well-defined rings round the photo- 

 graphic image of the moon with 1 minute, lj minute, and 2 

 minutes' exposure. 



The I minute exposure is fainter than the above woodcut, the 

 I $ minutel the same in density, and the 2 minutes' exposure is 

 denser and more defined ; while six consecutive nightly observa- 

 tions previous to the 2Sth failed to give any vinculum cr indica- 

 tion of refraction of light. 



One of the six taken on the night of November 24 with two 

 /tours' continuous exposure gave a bright clean well-defined line 

 2\ inches long, gapped here and there by passing clouds, but not 

 the slightest indication of blurr or dispersion was shown on the 

 brightest parts of the line. 



Whether the cause which produces these rings at full moon 

 phase only, depends upon the greater effulgence of lunar reflection 

 at that particular time ; whether it is cosmical or atmospheric in 

 its nature, or optical or chemical, there can be no doubt that 

 there is refraction of the lunar light ; the existence of a dark 

 space between two luminous (or more correctly speaking actinised 

 regions) as manifested by the above annular periodical impressions 

 is a clear indication of the dispersion of light, but how, why, or 

 where the decomposition takes place is not so obvious. 



Sunderland, December 5 Georce Berwick 



Stag's Horns 



It is well known to be the universal belief in the Highlands 

 that stags eat the horns they shed, and every gillie will tell you 

 that no one ever picked up a horn. Can any of your readers 

 inform me w hat really becomes of them ? 



There must be abundant opportunities of observing the whole 

 process in places like Windsor Park, where red deer are kept in 

 a domesticated state. G. W. H. 



ON A NEW COPYING PROCESS 



AVERY elegant process has recently been introduced 

 into this country for copying and multiplying letters 

 and documents. It is known by various names, according 

 to the etymological skill of the makers. One calls it a 



"hektograph," another less pardonably calls it the "cento- 

 graph," while yet another, to bridge the gap between 

 ancient Greek and modern English, styles it the " printo- 

 graph." But whether it is introduced by these names, or 

 the polygraph, the compo-lithogTaph, or the velocograph, 

 the principle is the same ; though the details are slightly 

 varied in each case. A slab of gelatinous material in a 

 shallow tin tray forms the type. The letter is written 

 with a special ink on any kind of paper, and when dry is 

 placed face downwards upon the jelly, and allowed to 

 remain a minute or more. On removal it is found that 

 the greater part of the ink has been left behind on the 

 jelly. It is only necessary to place pieces of paper on the 

 latter, and on their removal they are found to be perfect 

 fac similes of the original copy. The number of copies 

 obtainable varies with the ink, the most potent being 

 aniline violet, such as Poirrier's. With this a hundred 

 copies may be produced. Others, such as Bleu de Lyon, 

 Bismarck brown, or Roseine, 1 yield forty to fifty. It was 

 with a view to determine the principles which govern this 

 beautiful process, that I made an examination of the 

 subject. The slab consists of gelatin and glycerine, with 

 carbolic or salicylic acid to prevent fungoid growth, and 

 in the " chromograph " a quantity of barium sulphate 

 is added, which gives the slab a white, enamel-like 

 appearance. 



If a hot, strong solution of gelatin in water be prepared, 8 

 and then a certain quantity of glycerine stirred in, the 

 whole mass will become solid in cooling. This might at 

 first sight appear to be a solution of gelatin in water and 

 glycerine ; but such is not the case, the gelatin being 

 quite insoluble in glycerine. When the aqueous solution 

 solidifies, the gelatin still retains the water, but the large 

 quantity of glycerine being dispersed through the mass, 

 makes the whole into what is practically a very fine gelatin 

 sponge containing glycerine in its pores. 



The moisture-loving nature of the glycerine prevents 

 the "sponge" from getting dry, while the insolubility of 

 the gelatin in the glycerine prevents its becoming liquid. 

 When the copy is placed on the jelly, the glycerine comes 

 out to meet the ink, for which it has an intense liking. 

 All the suitable inks are freely soluble in glycerine. Some, 

 too, contain acetic acid either in the free state or in com- 

 bination with bases as in rosaniline acetate. The acetic 

 acid exerts a solvent action on the gelatin, so that it will 

 be found that after taking off some impressions with an 

 acetic acid ink, as the "multiplex," the jelly will be 

 etched wherever the ink has come into contact with it. 

 As long as any of the ink remains on the jelly, the 

 glycerine will come out of the pores to keep it moist, but 

 when the whole of the ink has been removed the flow of 

 glycerine ceases, and the parts become quite dry. If the 

 ink is not entirely removed by taking a sufficient number 

 of impressions, and the jelly left, after a lapse of twenty- 

 four hours the remaining ink will be absorbed by the 

 jelly. It is necessary, therefore, that the copies should be 

 taken off as soon as possible, so as to avoid the defect 

 caused by the spreading of the ink. 



Most of the makers suggest, that directly the slab is 

 done with, the type should be washed off. The hekto- 

 graph and most others require that the water should 

 be warm, but the finely divided barium sulphate in the 

 chromograph, renders the surface less tenacious, and the 

 impression may be removed with cold water. 



Where practicable, it is better in all cases to leave the 

 slab for twenty-four hours, when the old impression will 

 be quite absorbed, and not interfere with a new one. 



This gelatin copying process has been received with so 



1 A very paent and easily prepared ink which will yield a hundred copies, 

 may be made by dissolving rosanil.ne in a c U-salurated solution of oxalic 

 acid. It must be allowed to do' s l ontaneonsly. 



» a cz. gelatin dissolved in 6 I z. water, and 20 oz glycerine, sp. gr. 126, 

 previously wanned, stirred in. Any air bubbles in the gelatin are rem oved 

 Wore the addition of the glycerine. A cheaper compound which answers 

 equally well, but is rather darker, a nsists of Scotch glue 6 01., water8 01., 

 glycerine 20 01. These quantities make a slab 10 X 13 X J. 



