502 



NA TURE 



[March 22, 1906 



This result was anticipated by the theory previously 

 given, and may be explained by considering the electrons 

 in the radiating substance to be accelerated in the direction 

 of electric displacement in the pulses passing over them. 



The intensities in the two principal directions were 

 approximately in the ratio 3:1. Considering the obliquity 

 of primary, secondary, and tertiary rays in the beams ex- 

 perimented upon, this result indicates fairly complete 

 polarisation in a narrow pencil of secondary radiation pro- 

 ceeding from the substance in a direction perpendicular to 

 that of propagation of the primary. 



When iron was used as the radiator in the secondary 

 beam, though the rates of deflection of the electroscopes 

 were of the same order of magnitude as before, there was 

 no appreciable variation as the direction of the primary 

 beam was changed. 



This result was what previous experiments on iron led 

 one to expect, and was the most conclusive proof of the 

 interpretation of the results obtained with carbon. The 

 independence of motion of the electrons disappears in the 

 heavier atoms, and each is subject to considerable forces 

 not directly due to the primary pulse (in this case the 

 secondary pulse) and not in the direction of electric dis- 

 placement in this pulse. Hence the variation in intensity 

 of the tertiary in different directions becomes inappreciable, 

 while the pulse thickness in the tertiary beam becomes 

 greater than in the secondary, and is consequently more 

 readily absorbed. 



Geological Society, February 16. — Annual General 

 meeting. — Dr. J. E. Marr, F.R.S., president, in the 

 chair. — Influence of the geological structure of English 

 Lakeland upon its present features. Anniversary 

 address : President. After an account of the light thrown 

 upon the structure of Lakeland by the writings of other 

 geologists, the president considered his subject under the 

 following heads : — Events prior to the uplift which pro- 

 duced the dome ; production of the dome ; initiation of the 

 drainage-lines ; effects of the three types of rocks upon 

 the scenery; modification of old drainage-lines; depression 

 of the outskirts ; effects of meteorological conditions, 

 (1) general, (2) the Glacial period. Of the events prior to 

 the dome-shaped uplift, he laid greatest stress upon the 

 movements of Devonian times, which had caused the 

 Lower Palaeozoic rocks to be affected by fractures forming 

 a roughly rhomboidal network, the fissures being marked 

 by belts of broken rock along their courses. He accepted 

 Hopkins's view of the formation of a dome comparable in 

 shape to a " caddy-spoon " with the short handle to the 

 east. He gave further reasons in support of the view that 

 the uplift of the dome and the final movements of the 

 Pennine Chain were of Tertiary date. After commenting 

 on the theory that rocks of New Red Sandstone age ex- 

 tended over the district, he discussed the nature of the 

 radial drainage impressed upon these newer rocks during 

 the uplift of the dome, and the removal of these rocks in 

 the district itself by denudation, producing a superimposed 

 drainage on the Lower Palaeozoic rocks. The changes 

 which took place in the valleys as the result of the imposi- 

 tion of the rivers upon the ancient rocks were then dis- 

 cussed, and it was maintained that diversion of the river- 

 courses had largely taken place owing to the easier erosion 

 along the shatter-belts. When discussing the effects of 

 meteorological conditions he commented on hill-outlines, 

 where the upper parts of hill-slopes presented a convex out- 

 line towards west and south, and a concave curve towards 

 east and north. This he attempted to explain as due to the 

 more profuse growth of vegetation on the slopes facing 

 n est and south. 



February 21. — Sir Archibald Geikie, Sec.R.S., president, 

 in the chair. — The constitution of the interior of the earth, 

 as revealed by earthquakes : R. D. Oldham. This paper 

 sets forth the information to be obtained from the records 

 of distant earthquakes. The record of a great earthquake 

 exhibits three phases, of which the third represents wave- 

 motion travelling along the surface of the earth, and can 

 give no information regarding the interior. The other two 

 phases form the preliminary tremors, and represent the 

 emergence of two forms of wave-motion propagated through 

 the earth. Up to a distance of 120 of arc from the origin, 



NO. 1899, VOL. 73] 



these waves are propagated at a rate which increases with 

 the depth of the wave-path, and reaches an average of more 

 than 10 km. sec. for the first-phase, and more than 

 6 km. sec. for the second-phase waves. The increase may 

 be attributed to the effect of increased pressure and 

 temperature. Beyond this limit the first-phase waves show 

 a reduction in the mean rate of transmission, while the 

 second-phase waves are found, not where they would be 

 expected, but at about 11' later. Two interpretations are 

 given. Either alternative leads to the conclusion that, after 

 the outermost crust of the earth is passed, there is no 

 indication of any rapid change of physical and chemical 

 properties until a depth of about six-tenths of the radius 

 is reached. — The Tarannon series of Tarannon : Dr. Ethel 

 M. R. Wood. The Tarannon strata are well developed in 

 the Llanbrynmair-Tarannon district, and the present paper 

 gives the results of a detailed survey of the entire Tarannon 

 series as there exhibited. Lists of the contained graptolites 

 are given, and the species are paralleled with those from 

 the corresponding beds of the south of Scotland, the Lake 

 District, North Wales, central Wales, and Sweden, 

 demonstrating the similarity of the graptolitic succession in 

 all these districts. The Tarannon series in this district 

 has a maximum thickness of 3500 feet, but thins somewhat 

 as it is traced north-westward. It rests conformably on 

 Llandovery rocks below, and passes up without a break 

 into Wenlock beds above. This rock-series is stratU 

 graphically continuous from base to summit. The strata 

 of the overlying Wenlock series present all the characters 

 of the Denbigh Grits and Flags of North Wales. The 

 Llandovery series, which underlies the Tarannon series, 

 has, at present, been recognised only in the western part 

 of the district, namely, in the valley of the Twymyn, and 

 its rocks are brought to the surface by an anticlinal fold. 

 A comparison of the graptolitic lists shows that the 

 Tarannon series, as here defined, corresponds almost exactly 

 with the Gala or Queensberry group of the south of Scot- 

 land, includes all the palaeontological zones hitherto 

 assigned to the Tarannon, and fills up the whole period 

 intervening between the Llandovery below and the Wenlock 

 above. 



Physical Society, February 23. — Pro f . J. Peiry, F.R.S., 

 president, in the chair. — A note on Talbot's lines: J. 

 Walker. The diffraction-pattern of a line of mono- 

 chromatic light seen in locus, due to a rectangular aperture 

 with its sides parallel to the line, is characterised by dark 

 bands arranged at equal intervals on either side of the geo- 

 metrical image of the line. The effect of covering half 

 the aperture with a retarding plate is to displace the bands 

 of an odd order towards the covered side by an amount 

 proportional to the retardation introduced, those of an 

 even order remaining fixed. Suppose that the light is 

 white and that its monochromatic constituents have been 

 made by spectral analysis to occupy different angular posi- 

 tions in the field. Owing to the dispersion, the bands of 

 an even order are obliterated ; but in the case of those of 

 an odd order the dispersing power of the plate itself pro- 

 duces a dispersion of the bands, and consequently these 

 bands will be seen, provided the plate have a suitable thick- 

 ness and be so placed that the dispersion of the bands 

 produced by it acts in opposition to the primitive dispersion 

 of the light. — Secondary Rontgen radiation : Dr. C. G. 

 Barkla. In previous papers the author has shown that 

 the secondary X-rays from certain gases and light solids 

 subject to Rontgen radiation may be fully explained by 

 considering the corpuscles or electrons constituting the 

 atoms to be accelerated in the direction of electric dis- 

 placement in each primary Rontgen pulse as it passes 

 through such substances, and that the interaction between 

 the electrons affects only to a slight extent the character 

 of the secondary radiation. Experiments on the absorption 

 of rays proceeding from thick plates of a large number of 

 elements showed that beyond the region of atomic weights 

 in which the character of the secondary radiation is almost 

 independent of the nature of the radiation, the absorb- 

 ability is a periodic function of the atomic weight of the 

 radiator, and that, so far as these experiments have gone, 

 different periods are represented by curves of similar form. 

 The theory which has been found to explain all the pheno- 



