472 



NATURE 



[February 14, 19 18 



a total depth of 2071 ft. A useful educational section is 

 given, showing our knowledge of the floor of eroded 

 Carboniferous rocks that underlies eastern Kent. 



Mr. C. A. CoTTOX, of Wellington, N.Z., contributes to 

 the American ]ournal of Science (yo\. xiiv., p. 249, 1917) 

 a paper illustrated by numerous diagrams on " Block 

 Mountains in New Zealand." This is in part a sum- 

 mary of his previous work, and is accompanied by an 

 important bibliography. The expository methods of 

 Prof. W. M. Davis are utilised, and fault-scarps, in 

 various stages of maturity, are traced as the margins 

 of block-masses throughout central Otago. The im- 

 portance of tors as measures of the amount of erosion 

 of a land-surface is usefully pointed out. 



Several changes have been instituted in the Monthly 

 Meteorological Chart of the Atlantic Ocean beginning 

 with the January number for this year. The part 

 which refers to the Mediterranean has been discon- 

 tinued and replaced by inset maps showing the mean 

 annual rainfall and the mean rainfall of the current 

 month over Nigeria. To the coast line of the great 

 American lakes the results of observations for pressure, 

 air, and sea temperature and currents are added. 

 Among other changes and additions there is a map 

 showing the distribution of specific gravity. The simi- 

 lar monthly chart of the Indian Seas has also under- 

 gone some changes, and now includes a large-scale 

 map of the China Sea, showing the distribution of 

 pressure, air, and sea temperature. 



The rainfall of 1917 in the British Isles was about 

 the average, but large areas of deficient rainfall 

 occurred in all parts of the country. According to 

 Symons's Meteorological Magazine for January (vol. 

 lii.. No. 624) the most important of these areas were 

 in the centre, part of the north, and the south-west of 

 England, all of which had deficiencies of more than 

 10 per cent. The east midlands of Scotland were also 

 dry, the deficiency exceeding 20 per cent, over an area 

 extending from the Firth of Forth to the Grampians. 

 The southern half of Ireland and the extreme north and 

 the south of Wales had a rainfall below the average. 

 Unusually wet regions included the west and north of 

 Scotland, the north of Ireland, the Yorkshire Wolds, 

 Cardigan Bay, and the London district. August, Octo- 

 ber, and November showed a general excess of rainfall 

 over the country. May was rather wet in Ireland and 

 June in England, especially locally. February and 

 December were unusually dry, and there was, on the 

 whole, a general deficiency of rainfall during the first 

 seven months of the year. 



Part i of vol. xxx. of the Proceedings of the Physical 

 Society of London is exceptionally strong in optical 

 papers. Mr. T. H. Blakesley points out the conveni- 

 ence of representing • a simple lens by a point on a 

 plane diagram in w^hich the co-ordinates are the 

 quotients of the two radii of curvatures of the two 

 bounding surfaces by the thickness of the lens at its 

 middle point. Lenses having some particular property 

 are then represented on the diagram by the points on 

 some line which in many cases turns out to be straight. 

 Mr. T. Smith and Miss Dale, of the National Physical 

 Laboratory, show that the mechanically strong triple- 

 cemented objective may with advantage be substituted 

 for the non-cemented doublet of flint and crown glass 

 at present usual in small telescopes. Such triple objec- 

 tives, it is shown, can be designed with the first-order 

 spherical aberration and coma zero and the second 

 order small, and these conditions do not necessarily 

 limit the lens surfaces to those of small curvature. 



With reference lo the possible risk involved in the 

 use and transportation of celluloid articles, an inves- 



j tigation into the elfects of heat upon such articles was- 

 carried out by the U.S. Bureau o^ Standards in 1907. 

 In view of the present interest in nitro-cellulose pro- 

 ducts it has recently been thought desirable to publish 



I the results, which are now given in Technologic 



' Paper No. 98, issued by the bureau. The chief con- 

 clusions arrived at were that when celluloid is exposed 

 to heat, decomposition commences at temperatures in 

 the neighbourhood of 100° C, and above 170° the 

 decomposition takes place with explosive violence. If 

 loss of heat by radiation is prevented, the heat of 

 decomposition at temperatures of about 120° to 135° 



I may rais'e the temperature of the mass to the ignition 

 point; and momentary contact with bodies having a 

 temperature of 430° — below visible red-iheat — mav 

 ignite celluloid articles. The rate of combustion was 

 found to be from five to ten times that of paper, pine- 

 wood, or poplar wood of the same dimensions and 

 burning under the same conditions. Nitro-cellulose 

 exists and reacts as such in celluloid, and th^ rate of 

 its decomposition when heated is not diminished bv 

 admixture with zinc oxide (a common ingredient of 

 celluloid products) in proportions up to 20 per cent. 

 There appears to be no good evidence that celluloid 

 articles often inflame spontaneously, or that thev are 

 directly explosive under any conditions. The vapours 

 evolved by decomposition are poisonous and extremely 

 combustible, and may be ignited by the heat of decom- 

 position of the celluloid itself. The decomposition is 

 autocatalytic, and while not necessarily explosive, it 

 may readily approach that condition as a limit. 



In the Journal of Geology, vol. xxv., p. 629, 1917^ 

 Prof. L. V. King, of McGill University, discusses the 

 internal friction and limiting strength of rocks under 

 conditions of stress such as exist within the earth. 

 Taking his data from Adams and Bancroft's experi- 

 ments on the effect of intense end pressures applied 

 to small rock specimens enclosed in nickel-steel cylin- 

 drical jackets, he shows that a simple theoretical treat- 

 ment of the elastic stage suffices to explain the mode 

 of shearing rupture observed in the rock and the en- 

 closing jackets. His main purpose, however, is to test 

 Navier's modification of Tresca's theory that a stressed 

 solid would commence to flow (without rupture) as soon 

 as the maximum shearing stress exceeded a limiting 

 value K characteristic of the solid; Navier's hypothesis 

 replaced K by K+yuN, where N is the stress normal 

 to the shearing plane, while p. is a coefficient of internal 

 friction. Prof. King concludes, from the work of 

 Adams and Bancroft, that for some kinds of rocks con- 

 stants K and /x do exist, although the theory does not 

 lead to very accordant values from different sets of 

 experiments. In some cases, while the limit of plas- 

 ticity certainly increases with the hydrostatic pressure, 

 the internal friction does not seem to be simply propor- 

 tional to the normal stress; this was particularly so. 

 for some of the hardest rocks, such as dolomite, which 

 appear to possess great internal friction. These con- 

 clusions have an important bearing on questions of 

 geology and geodynamics. Sir G. Darwin estimated 

 that under the continents of Africa and America the . 

 strain must be so great that marble would break under 

 it, though strong granite would stand. This was based 

 on the limiting stresses found from ordinary crushing 

 tests, but it now appears that the limiting stress 

 will be much greater at considerable depths, owing to 

 the great hydrostatic pressure. It is suggested that 

 great movements of the earth's crust have mainly pro- 

 ceeded by slow and gradual adjustment, rather than 

 by series of cataclvsmal collaoses. • 



NO. 2520, VOL. 100] 



