5IO 



NATURE 



[January 15, 1920 



"Basic or semi-immortal types" of vegetation are 

 represented by a horizontal line running across tlie 

 bottom of tlie diagram. Elsewhere Dr. Wieland has 

 lamented the depletion of the already small group of 

 palaeobotanists. If he could give us a general 

 treatise on the problems expressed so concisely in his 

 diagram, he might win a keen body of adherents. 



Mr. George Barrow, in a paper on " Some Future 

 Work for the Geologists' Association " (Proc. Geol. 

 Assoc., vol. XXX., p. I, 1919), revives in a remarlcable 

 degree the view that some of the features of the chalk 

 surface round London are due to marine erosion, 

 acting probably in Pliocene times. It is urged that 

 the quartz-pebbles of the pre-Glacial high-level gravels 

 were washed into the chalk basin by the waters of a 

 shallow sea, which cut passages in the escarpments 

 and thus originated many of the wind-gaps. The 

 chalk escarpments, together with those of the 

 Lower Greensand, were, in their first form, ridges left 

 by the marine denudation of the softer Gault and 

 Eocene strata. If the "beach deposits" (high-level 

 gravels) are of Pliocene age, no serious post-Pliocene 

 bending of the region can have occurred, since thev 

 lie at approximately the same levels. Mr. Barrow 

 suggests that they are little later than the Lenham 

 Beds, and the members of the Geologists' Associ.Ttion 

 are now invited to prove their age by a diligent search 

 for fossils. 



Dr. R. E. Slade and Mr. G. I. Higson, of the 

 British Photpgraphic Research .\ssociation, communi- 

 cate to the Journal of the Royal Photographic Society 

 {December, 1919) the results of their investigations, 

 which show that the shape of the characteristic curve 

 of a photographic plate depends not only on the thick- 

 ness and opacity of the film and the time and method 

 of development, but also on the relation of thfe 

 different sizes of grains in the film to each other and 

 the quantity of each size present. This new factor 

 they claim to be the most important. If the grains 

 are all of the same size, the curve is the steepest 

 possible. If the grains are of various sizes, the curve 

 is the sum of the curves due to each group of particles 

 of the same size. The larger grains are more sensi- 

 tive than the smaller grains. Uniformity of grain- 

 size is, therefore, desirable in plates for black-and- 

 white work, and the authors find that the steepness 

 of the curve can be foretold from photomicrograohs of 

 the grain. Photomicrographs and curves are given in 

 illustration. 



In May, 1914, we mentioned in, these columns two 

 methods which had been devised for reducing the 

 measurement of the horizontal component of the 

 earth's magnetic field to that of an electric current. 

 In the first — due to Prof. Hicks and tested in prac- 

 tice by Mr. W. A. Jenkins {Phil. Mag., October, 

 1913) — the earth's field was reversed by the current in 

 a coil, and the reversal determined by the time of 

 oscillation of a small magnet. In the second — due 

 to Sir .\. Schuster and tested at the National 

 Physical Laboratory by Mr. F. E. Smith {Terrestrial 

 Magnetism, March, 1914) — the earth's field was 

 annulled by that of the coil, and a small magnet set 

 itself at right angles to the field. In Terrestrial 



NO. 2620, VOL. 104] 



Magnetism for September, 1919, Prof. W. Uljanin, 

 formerly of the Kazan University (from which the 

 staff had to flee on the capture of the town by the 

 Bolsheviks in September, 1918), gives an abstract of 

 a paper he published in Russian in 1915 describing 

 a third method. It retains the sine principle of the 

 Kew magnetometer, but substitutes for the deflecting 

 magnet a pair of coils through which a standard 

 current measured by the potentiometer method is sent. 

 The method gives results at least as accurate as those 

 given by the magnetometer, and takes only a tenth 

 of the time. 



Two useful papers on three-clcctrode thermionic 

 valves have been published recently by the Bureau 

 of Standards, Washington. The first paper, by Mr. 

 J. M. Miller, discusses the connection between the 

 input impedance of the valve and the load in the 

 plate circuit. Theoretical relations are obtained which 

 enable us to calculate the input impedance when the 

 impedance in the plate-circuit is known. It is shown 

 that the results are in excellent accord with experi- 

 ment. It is interesting to notice that when the load 

 in the plate circuit is inductive, the impedance can be 

 represented as a negative resistance, in which cast; 

 the valve can act as a generator. The second paper, 

 by Mr. L. M. Hull, gives a partially successful attempt 

 to obtain a method of rating thermionic-valve genera- 

 tors. A clear theoretical statement of the problem 

 is given, and important theoretical conclusions are 

 deduced, but experimental work is still in progress. 

 The problem is one of considerable commercial im- 

 portance, as thermionic-valve generators are now the 

 standard source of supply for radio-telephone and 

 radio-telegraph systems, except in the few cases when 

 very high power is necessary. The present empirical 

 method used for rating these generators is of little 

 value. All interested in the subjects discussed in these 

 papers, the numbers of which are 351 and 355 respec- 

 tively, can obtain a copy of them by sending a request 

 to the Bureau. 



In the U.S. Bureau of Standards Scientific Paper 

 No. 350, entitled " Equilibrium Conditions in the 

 System Carbon, Iron Oxide, and Hydrogen in Rela- 

 tion to the Ledebur Method for Oxygen in Steel," it 

 is shown that mixtures of iron oxide and Acheson 

 graphite are not, and mixtures of iron oxide with 

 "cemented" iron or white iron (annealed or un- 

 annealed) are, reduced at 900° C. by the carbon in 

 them when hydrogen is passed over them at rates of 

 two litres per hour, or faster. Because of these facts 

 it is probably impossible to determine by the Ledebur 

 method more than 75 per cent, of the oxygen present 

 in steels as ferrous oxide. The effect of rate of 

 passage of hydrogen on the ledebur oxygen-content of 

 certain steels is shown. The paper can be obtained 

 on application to the Bureau. 



Scientific Paper No. 347 of the U.S. Bureau of 

 Standards describes an investigation carried out at 

 the Bureau on the heat-treatment of alloys of the 

 duralumin type, and the effect on the mechanical 

 properties observed of variations in the various heat- 

 treatment conditions. Conclusions are also drawn as 



