6o 



NATURE 



[March 9, 191 1 



THE MANUFACTURE OF VARIETIES OF 

 COMMON SALT. 

 ^REAT interest has been aroused anionfj salt manu- 

 facturers by the announcement of the discovery of a 

 iK'w process capable of producinj^ every variety of com- 

 mercial salt in one plant and with jjrea't economy of fuel. 

 'Ihe inventor is Mr. James Hodgkinson, of the firm of 

 James Hodgkinson, Ltd., Pendleton, Manchester, makers 

 of mechanical stokers. It is said that the patent rights 

 for America have been sold to an American syndicate for 

 1,000,000/., and to the Canadian Pacific Railway Company 

 for Canada. 



Up to the present time, salt has been manufactured by 

 a process scarcely different from that used during the 

 Roman occupation, namely, boiling down the brine in 

 shallow open pans heated by coal fires. It is true that in 

 1839 Reynolds introduced the use of closed pans, in which 

 the steam was passed from the first pan to the others, and 

 a considerable economy in fuel efTectod, one ton of coal 

 yielding four tons of salt, as against two tons in the case 

 of open p;ms, which, however, still continued in use. 



The chief features in the Hodgkinson process are the 

 introduction of mechanical stokers for the fires, and the 

 working of a plant consisting of seven pans, three closed 

 and four open, heated by one fire, 'fhc quality and size 

 of the salt crystals largely depend on the temperature, 

 which can be so regulated in the new process as to pro- 

 duce every kind required. 



1 he first pan is covered, and produces, by crystallis.i- 

 tion alone, a table salt so fine that no grinding is required. 

 From this pan proceed waste gases, which pass under- 

 neath and heat all the remaining pans, while the steam 

 passes over to the succeeding pans and assists the pre- 

 cipitation of the salt. The second and third pans are also 

 covered, and produce a slightly coarser variety, known as 

 " dairy " salt. In the succeeding pans, all of which are 

 open, the gases are at a lower temperature, and coarser 

 salt in larger crystals is obtained. This is known as 

 " fishery " salt, and is used in fish-curing. It should be 

 added that there is an automatic flow of brine into the 

 pans, and an automatic discharge of the salt produced. 



The process, first successfully tried at Northwich, then 

 extended to Port Sunlight and"St. Helens, has resulted in 

 such complete consumption of smoke that several large 

 chimneys have been dispensed with. The success of the 

 process must be assured if the hopes of the inventor are 

 realised in practice. He claims that four or five times as 

 much salt can be produced in a given time as compared 

 with the old process, and that three tons of coal will be 

 saved out of every four. 



ARCH^OLOGICAL RESEARCH IN ARKANSAS^ 

 ]y[R. CLARENCE B. MOORE has issued another of 

 his valuable memoirs on the prehistoric pottery of 

 the United States. His last season's field work was on 

 the St. Francis, Little, White, and Black rivers in 

 .'\rkansas. Like all his other memoirs, this one is illus- 

 trated with maps and very numerous beautiful illustra- 

 tions, many in colours, and all of large size, so that every 

 detail is visible. Mr. Moore is still in the collecting stage 

 of his work ; generalisations we mav expect at a future 

 time. The principal sites along the St. Francis, although, 

 as a rule, having mounds in connection with them, are in 

 reality great dwelling-sites which have increased in height 

 gradually through long periods of occupancy, and the 

 aborigines, burying where they lived, have' formed in 

 course of time great cemeteries, all of which he believes 

 to be pre-Columbian, since no object was found in any way 

 indicating intercourse with Europeans excepting a' bone 

 comb, which, " though the shape is undoubtedly copied 

 from a European model, the decoration points to Indian 

 workmanship " ; he asserts it is prehistoric. 

 _ The St. Francis valley has yielded more examples of 

 Its ware than has any equal area in the United States, and 

 has been largely exploited by irresponsible collectors. The 

 earthenware is shell-tempered. Quantity rather than 

 quality seems to have been the aim of its 'makers, for the 

 1 Journal of the Academy of Natural Sciences of Philadelphia. 2nd series, 



Ari^^^Ll "^K V 15'^i'i"'"""^ °^^^^ ^'- ^^^""^' White, and Black Rivers 

 Arkansas, by C. B. Moore, pp. 255.364 (Philadelphia, 1910). 



NO. 2158, VOL. 861 



ware is often insufficiently fired, and the vessels are fre- 

 quently thick and lop-sided. A high polish is almost 

 absent. A very large proportion of the vessels are un- 

 decorated or with trivial decoration. Incised decoration i> 

 scarcely ever .seen, the inierior surface of most of the ware 

 being unsuited to incised decoration of excellent quality, 

 even had it been attempted. Practically none . of th^ 

 vessels obtained by him are of types new to the pottery 

 of the Middle Mississippi valley. The types of pottery 

 have been well described by Prof. W. H." Holmes in his 

 " Aboriginal Pottery of the Eastern United States " 

 (20th Ann. Rep. Bur. Am. Eth). The most noteworthy 

 find was a ceremonial *' spear-head " of sheet-copper. 

 Several vessels in the form of heads with fairlv 

 well-modelled faces were obtained, and some in the form 

 of human figures. One burial of a very young infant had 

 near by it a small bowl in which, upright, was a little 

 effigy bottle. 



Unlike the St. Francis river sites, which are on high 

 ground, the sites on the White river and its upp^r 

 reaches, the Black river, are on low, overflow ground, 

 where arch.cological research is not likely to be rewarded ; 

 indeed, with a few trifling exceptions, no aboriginal objects 

 had previously been obtained from this district. With onr- 

 exception, no site of interest was found along the White 

 river, and hut three vessels of earthenware wen- 

 encountered along the stream. Along the Black river, 

 while some vessels were found, not one was of a character 

 to warrant its transportation home. .Although burials 

 were fairly numerous, they were almost invariably un- 

 accompanied by artifacts, a remarkable fact considering 

 the custom of lavishing deposits upon the dead as practised 

 by aborigines of neighbouring regions. The best find was a 

 narrow, well-made ceremonial axe-head of green quartzite, 

 212 mm. in length. A. C. Haddon. 



THE COEFFICIENT OF SKIN FRICTION I\ 

 AIR AT MODERATELY HIGH VELOCITIES. 

 'T'HE object of this study is to find a coefficient of turbu- 

 lent friction readily applicable to the design of 

 aerodynamic appliances such as aeroplanes and windmills. 



It is assumed throughout that: — 



(i) The surfaces on which the friction occurs are of 

 reasonably smooth finish, and present no marked h ud 

 resistance due to irregularities of surface. 



(2) The velocities of the air relative to the surface arf> 

 well above the critical values at which pure viscous shear- 

 ing resistance is superseded by the generation of 

 momentum in the contacting film or fluid, so that the 

 skin resistance varies as the square of the velocity. 



(3) The length of the surface in the direction of motion 

 bears so small a relation to the dimensions perpendicular 

 thereto that the diminution of the coefficient with the 

 length is inappreciable. 



The study is divided into two parts : — 



(a) An analysis of the various methods of determining 

 the skin-friction coeflficient already known, with a tabular 

 record of the numerical results already attained or thereby 

 deducible, leading up to the most probable value of the 

 said coefiicient. 



[(b) An experimental study made with the purpose of 

 confirming the accuracy or otherwise of the said probable 

 value. This is now in progress, the apparatus (an epicvclic 

 differential dynamometer) being made. .As this will' not 

 be ready for some considerable time, it mav be useful to 

 publish the comparative analysis beforehand.] 



Symbols Employed. 



British engineer's units — Pounds, feet, seconds. Force in 

 lbs. wt. 



/. Coefficient of skin friction. 



C = about 07. being the coefliicient of normal pressure on 

 unit square surface, with unit velocity, and unit mass of 

 air. 



p. Mass of unit volume of air at normal temperature. 

 About 008 pound per cubic foot. 



A,. Area subject to skin friction. 



A.. -Area of mid-section or normal surface of a dirigible. 



k. Ratio of skin friction on a dirigible to normal resist- 

 ance on an area equal to the cross-section. 



