1 66 



NA TURE 



[June i8, 1896 



was shown, and some of the results which have been 

 obtained were exhibited. All attempts to obtain true reflection 

 have failed, although it appears as if most bodies, including air, 

 are capable of giving diffuse reflection. — Dr. .Shettle, who was 

 announced to give a paper on Rontgen rays, explained that he 

 had just discovered that the effects he had intended to describe 

 were due to red light which had penetrated his dark room. 

 — Prof, du Bois said that Galitzine had found that Rontgen rays 

 were polarised by tourmaline, a special form of developer being 

 employed. The behaviour of tourmaline to light waves presents 

 some curious features, for if the wave-lengtli is increased a point 

 is at length reached where the ordinary and extraordinary rays are 

 equally absorbed. P"or greater wave-lengths the ordinary con- 

 ditions are reversed. If the Rontgen rays are not homogeneous, 

 the contradictory results obtained by different observers might 

 be due to the fact that they were working with rays which were 

 differently absorbed by tourmaline. — Mr. Ssvinton said he had 

 tried the effect of heating the kathode, and had obtained results 

 which were similar to those obtained by the author. Mr. 

 Swinton further said that he had found that the blue lumin- 

 escence sometimes observed depended on the size of the kathode. 

 With tubes in which the kathode was almost a complete hemi- 

 sphere it was impossible to eliminate this blue luminescence. — 

 Mr. Appleyard suggested the performance of the experiments 

 under the surface of a dielectric. — Prof. Gray said he had 

 obtained some indication of regular reflection, but nothing 

 definite. The author in his reply said that it had been found 

 that if the Rontgen rays are reflected from a surface of sodium 

 in vacuo the amount reflected is a minimum for normal incidence, 

 and increases at oblique incidence. Comparing this behaviour 

 with that of ultra-violet light, it supports the idea that the 

 Rontgen rays consist of transverse vibrations. The Society 

 then adjourned till June 26. 



Geological Society, May 27. — Dr. flenry Hicks, F.R.S., 

 President, in the chair. — The President announced that a por- 

 trait in oils of the late Prof. Huxley had been presented to the 

 Society by Sir John Evans, K.C.B., F.R.S. — On the Pliocene 

 deposits of Holland, and their relation to the English and Bel- 

 gian crags, with a suggestion for the establishment of a new 

 zone " Amstelien," and some remarks on the geographical con- 

 ditions of the Pliocene epoch in Northern Europe, by F. W. 

 Harmer. The author drew attention to some papers by Dr. J. 

 Lorie, of Utrecht, describing the strata met with in some deep 

 borings in Holland, which showed that the Newer Pliocene is in 

 that country nearly 500 feet thick, and that it had been depressed 

 more than 1000 feet below its original position. He inquired 

 whether this subsidence could be connected with the elevation 

 of the Older Pliocene in Belgium and Kent, and how far these 

 earth-movements could be traced in East Anglia and influenced 

 the deposition of the English crag. He gave particulars of the 

 alterations in level which have taken place during and since the 

 Crag period in England and on the continent, showing that the 

 two movements of upheaval and subsidence have much in 

 common, and especially that they regularly increase in degree to 

 the north and south respectively. — The /,?«i'-«/a-flagsand igneous 

 rocks of the neighbourhood of Dolgelly, by Philip Lake and 

 S. H. Reynolds. — The Kildare inlier, by S. H. Reynolds and 

 C. I. Gardiner. The area described in this paper is occupied 

 by four prominent hills composed of lower paleozoic rocks 

 rising as an inlier from beneath carboniferous beds. The authors 

 gave the following succession of rocks in descending order. (6) 

 Green and grey micaceous grits and shales of Dunmurry. (5) 

 Red and black shales. Gap : no exposure seen. (4) Limestones of 

 the chair of Kildare. (3) Contemporaneous igneous rocks. (2) 

 Fossiliferous ash of Grange Hill House, (i) Green gritty shales 

 (unfossiliferous). 



Cambridge. 



Philosophical Society, May 25. — [a) On the spectroscope 

 used in connection with the 25-inch refractor ; ((i) on a sugges- 

 tion for a form of spectroheliograph, by Mr. H. F. Newall. 

 On the period of the earth's free Eulerian precession, by Mr. 

 J. Larmor. The following general proposition is easily estab- 

 lished ; it has been suggested by the recent memoirs of Prof. 

 Newcomb and Mr. Hough. Consider any solid body, for 

 example the earth, in rotation about its axis of greatest moment 

 of inertia : when the body is not absolutely rigid, the period of 

 the small free precessional motions of the axis of rotation will 

 depend in part on its elastic yielding to the centrifugal force ; 

 but in all such cases, whether the body is homogeneous or not, 

 whether the elasticity is perfect or imperfect, this precessional 



NO. 1390, VOL. 54] 



motion will be the same as that of a body absolutely rigid, with 

 its materials distributed in the configuration which the actual 

 body would assume, on the supposition that it remains perfectly 

 elastic, were it relieved of the centrifugal force of rotation. 

 Taking the case of the earth, in which the equatorial moments 

 of inertia are all equal to ,/, while the axial one is C, the 

 ordinary forced astronomical precessions give the value of 

 (C-A)jC; while knowledge of the variation of terrestrial gravity 

 gives C-A; so that C and A are separately known. The 

 period of the free Eulerian precession gives (C - A'}/A', where 

 C and A' are the moments of inertia which the earth would 

 have were the strain corresponding to centrifugal force removed. 

 In so far then as this free period can be reliably disentanj;led 

 from the actual observations of changes of latitude, which are 

 also affected by unknown irregular variations due to meteor- 

 ological causes, and so more or less of an annual character, we 

 derive from it a knowledge of C - A'; thereby obtaining an 

 additional datuin for discussions relating to the constitution of 

 the earth's interior. This is on the supposition that the earth 

 is wholly solid. The influence of the surface waters can, how- 

 ever, be estimated by the same principle, as they are in the 

 main deep enough to make an equilibrium theory applicable. 

 It appears that, if the actual earth were absolutely rigid, and 

 wholly covered by an ocean, the mobility of this ocean would 

 lengthen the period of free precession by about 14 per cent. 

 But this superior limit is reduced both by the limited extent of 

 the ocean and by the yielding of the solid earth ; so that, on an 

 outside estimate, not more than 6 or 8 of the actual 40 per 

 cent, of lengthening of the period can be due to mobiUty ot 

 the surface waters. On this equilibrium theory, an amplitude 

 of a third of a second of arc in the Eulerian precession would 

 produce a tidal component, of the same period, whose ampli- 

 tude would in middle latitudes be about half an inch ; 

 which is just the kind of result that has been derived from 

 examination of the tidal observations in Holland and on the 

 east and west coasts of North America. The influence of pos- 

 sible fluidity of a portion of the interior has been fully developed 

 liy Mr. Hough, the results agreeing with indications virtually 

 given by Lord Kelvin so long ago as 1S76, and published in the 

 British Association Report for that year. The conclusion drawn 

 by Mr. Hough from the Chandler period, that, for the small 

 stresses involved, the interior of the earth is in the main jjer- 

 fectly elastic and about as rigid as steel, is in accord with the 

 recent observations by seismologists of what is probably the 

 time of propagation of earthquake disturbances from Japan to 

 Europe in a direct line across the earth's interior. — Note on a 

 point in theoretical dynamics, by Sir Robert Ball. Let a be a 

 screw about which a free rigid body is made to twist in con- 

 sequence of an impulsive wrench administered on some other 

 screw 7). Except in the case where a and tj are reciprocal it 

 will always be possible (in many different ways) to design and 

 place a rigid body so that two arbitrarily chosen screws a and jj 

 will possess the required relation. Let now /8 and f be two 

 other screws (not reciprocal) ; we may consider the question as 

 to whether a rigid body can be designed and placed so that a 

 shall be the instantaneous screw corresponding to i) as an im- 

 pulsive screw, while /3 bears the same relation to f. It is easy 

 to see that it will not generally be possible for a, ^, t), i to 

 stand in the required relations ; they must in some way be 

 restricted. It is the object of the author's note to show that the 

 restrictions are two in number, and to set down what they are. 



Edinburgh. 

 Royal Society, June i. — Prof. Copeland in the chair. — 

 Prof Tait read a paper on the linear and vector function. We 

 speak of fluid motion as being " differentially irrotational " when 

 there is a velocity potential, and as " rotational " when there is 

 a vortex. In the first case, the strain involved is pure, i.e. 

 there are three rows of particles, at right angles to one another, 

 whose directions are momentarily unchanged. In the second 

 case, one such row of particles alone exists. But there is, when 

 we look at the matter from the point of view of the roots of the 

 strain-cubic, a third case— where there are three rows of particles, 

 not generally at right angles to one another. Prof. Tait showed 

 that such a strain is, in general, the residt of the superposition of 

 two successively applied, but different, pure strains. Thus, 

 comparing the non-vortex states of a small element of a fluid at 

 three successive instants, a portion, cubical at the instant A, 

 may be found, such as to be brick-shaped, without change of 

 direction of its edges, at B. Similarly from B to C. But to 



