526 



NA TURE 



[September 28, 11593 



minutes are required. Tlie readings were Cilibrated by com- 

 jiirison wilh an iron-copper junction, heated in paraffin oil and 

 balanced against the aciinometer couple. i° F. was found to be 

 equal to about thirty->ix divisions. Another Committee gave 

 detailed reports of magnetic work at the Falmouth Observatory. 

 The other Committees submitted formal reports asking for re- 

 appointment, in some cases with small grants of money. 



Prof. G. F. Fitzgerald gave an interesting communication on 

 " The period of vibration of Disturbances of Electrificatibn of 

 ilie earth." The period of oscillation of a simple sphere of the 

 size of the earth, supposed charged with opposiie charges of 

 electricity at its ends, would be abjut Vr'h of a second ; but the 

 hypothesis that the earth is a conducting body surrounded by a 

 non-conductor, is not in accordance with the fact. Probably 

 the upper regions of our atmosphere are fairly good conductors. 

 In a Geissler tube air is a good conductor, and we know that 

 when part of a gas is transmitting an electrical disturbance the 

 rest of the gas in its neighbourhood becomes capable of trans- 

 milting such as well. Extending the analogy, we may assume 

 that during a thunderstorm the air becomes capable of trans- 

 mitting small disturbances. If the earth is surrounded by a 

 conducting shell its capacity may be regarded as that of two 

 concentric spheres,and is accordingly greater than that of a simple 

 sphere, which would produce a corresponding change in the rate 

 of oscillation. But at the same time the presence of currents in 

 the outer air would alter the self-induction ; and calculation 

 shows that the net result is a comparatively slight change in the 

 period of oscillation. If we assume the height of the region of 

 the aurora to be 60 miles or 100 kilometres, we get a period of 

 oscillation of O'l second. Assuming it to be 6 miles(or 10 km.) 

 the period becomes 03 second. On the sun we might e.'cpec: 

 very much greater periods of oscillai ion, but ihese oscillations 

 would not give rise to radiations. If alternaiingcurrentsof the 

 kind referred to leally travelled north and south around the 

 earth they would give rise to east and west alternating magnetic 

 forces of periods between iV and xg of a second. Dr. Lodge 

 has already looked for evidence of such magnetic forces, but on 

 the assumption that the perioJ would be ,'» second. The .author 

 has calculated what magnetic disturbances would be produced by 

 given charges. A disturbance equal 10 ]',; part of the horizontal 

 force of the earth would correspond to an eleclroslalic charge of 

 80 C.G.S. units per sq. cm. Such a charge would reduce the 

 superhcial pressure on the earth by an amount C)rres|)onding to 

 a weight of 40 gm. per sq. cm. This does not sound probable, 

 but we must remember that it would correspond to a most fear- 

 ful magnetic storm. A charge of S C.G.S. units per sq. cm. 

 would produce a variation of , J^p of H, and -would not sensibly 

 aflect the barometer. The records of existing magnetic ob- 

 servatories are not sufficiently complete to admit of testing the 

 other suggestions made in the paper. Prof. O. Lodge thought 

 that the detection and observation of such magnetic disturbances 

 was work that could only be done in a National Physical 

 Laboratory. If the sun were a conducting body surrounded by 

 a non-conductor, the period of an electrical oscillation upon 

 it would be 6J seconds. He had hung up in his laboratory a 

 needle and watched it for hours, but the only disturbances ob- 

 served were due to trains and traffic. He pointed out that the 

 electric vibrations of a molecule, calculated from its size, were 

 more rapid than those required to produce light. He suggested 

 the addition of a jacket like that which Prof. Fitzgerald assumed 

 to exist around the earth ; but would this prevent radiation ? 



TJic Moon's AtiiwspltcrcaTid the Kinetic Theory of Gases. — Sir 

 Robert Ball has suggested that the absence of any atmosphere 

 investing the moon is a simple and necessary con.sequence of 

 the kinetic theory ofgases. I'rof. Liveing has applied this theory 

 to interplanetary and interstellar space, with reference to the 

 chemical constitution of planetary atmospheres. According to 

 Sir Robert Ball the mean molecular speed of oxygen and 

 nitrogen is less than the speed with which a body would have 

 to be projected in order to leave the moon without ever return- 

 ing ; but in the course of collisions between the molecules they 

 frequently attain speeds sufficiently great to enable them to 

 overcome the moon's attraction, and thus escape from the 

 moon's atmosphere. On the other hand, the speed required to 

 permanently leave the earth is one which "it would seem that 

 the molecules of oxygen and nitrogen do not generally ever 

 reach," and therefore the earth retains a copious atmosphere. 

 Mr. G. H. Bryan, in reading his paper on this subject, stated 

 that no statistical calculations had hitherto been made with the 

 object of testing these questions ; he was not aware until his 



NO. 1248, VOL. 48] 



paper had been printed that explanations based on the kinetic 

 theory had been suggested as far back as 1878 by Mr. S. Tolver 

 Preston and Mr. Johnstone Stoney. Mr. Bryan has applied 

 the theory to inve>ligate that elfect of varying temperature! 

 upon the relative densities of oxygen and hydrogen in a 

 permanent distribution under various conditions ; he has 

 also calculated the aver.age number of molecules of gaj 

 to every one whose speed is sufficiently great to over- 

 come the attraction of given bodies in the solar system, 

 and gives tables showing the results. Thus for oxvgen at 

 o" C, rather over one molecule in every three biUion it 

 moving fast enough to fly off permanently from the moon, and 

 only one in every 2'3 :■: lO''-" is moving fast enough to escape 

 from the earth's atmosphere ; while the sun's attraction, even 

 at the distance of the earth, prevents more than one in every 

 2 X 10"""" from escaping. In the discussion which followed. 

 Sir Robert Ball stated that the suggestion really did not originate 

 with himself, but were familiar to him as having been discussed 

 many years ago in a paper by Mr. Johnstone Stoney. Among 

 celestial bodies the moon is unique in having no atmosphere. 

 In the earth's atmosphere there is no free hydrogen. Stoney'» 

 theory accounts for these effects. On the other hand, in the 

 case of big bodies like Sirius it is hydrogen, and essentially 

 hydrogen alone, which forms their atmosphere. 



Grinding an J polishing of glass surfaces. — Lord Rayleigh 

 stated that he had been investigating the nature of these pro- 

 cesses, and gave a most interesting description of the results. 

 He first pointed out that th'.; process of grinding with emery is 

 not, as is commonly supposed, a scratching process. The 

 normal effect is the production of isolated detached pits — not 

 scratches. The glass gives way under the emery ; at the same 

 time the emery gives way under the glass and suffers abrasion. 

 An image seen through glass which has been finely ground (but 

 not yet polished) has perfect definition. And so when the sun 

 is viewed through a cloud the image is sharp as long as there is 

 an image ; even when the cloud thickens, the edge appears to 

 be sharp until we lose the image altogether. A glass lens linely 

 ground gives very good definition, but there is great loss of light 

 by irregular reflection. To obviate this the lens is polished, and 

 examination under a microscope shows that in the process of 

 polishing with pilch and rouge the polishing goes on entirely 

 on the surface or plateau, the bottom of each pit being led un- 

 touched until the .adjoining surface is entirely worked down to 

 it. It a|)peared interesting to investigate the amount or" glass 

 removed during the process of polishing. This was done oolh 

 by weighing and interfeience methods, and the amount reinovtd 

 was found to be surprisingly small. A sufficiently gooti polish 

 was obt.iiiied when a thickness corresponding to 2h wave-lengthi 

 of sodium light was removed, and the polishing was cornpleie 

 when a thickness corresponding 104 wave-lengths was removed. 

 Lord Rayleigh is of opinion that the process of polishing is not 

 continuous with that of grinding, but that it consists in a re;' 

 moval of molecular layers of the surface of the glass. Grinding 

 is easy and rapid, whereas polishing is tedious and ditliculi. 

 The action of hydrofluoric acid in dissolving glass was also in- 

 vestigated, and was found to be much more regular — that is, 

 has geneially been assumed to be by chemists. It was found 

 to be easy to remove a layer corresponding in thickness to half 

 a wave-length of sodium light ; and wan due precautions as 

 little as one-tenth of a wave-length. 



Mr. W. B. Croft exhibited simple apparatus for observing and 

 photographing interference and tbffraction phenomena. Ivo 

 bench was used, but the various pieces of apparatus were 

 mounted on the usual stands used for holding lenses, &c. 

 One of these contained a thin aluminium plate with a needle- 

 hole, or the slit of a spectroscope. On this the light of a lamp 

 was focussed by means of a lens. As an observing eye- 

 piece, the eye-piece of a Beck microscope was used aud was 

 placed about 2 ft. from the slit or point, the object being 

 introduced between. The stands should be adjusted so that 

 the light proceeds straight into the eye-piece. The whole of 

 the special apparatus required need only cost a few shillings, 

 and with this the usual pnenomena of Fresnel's bi-prism, sharp 

 edges, perforated zinc, Ovtc, can be both seen and photographed. 

 Mr. Croft exhibited an admirable series of slides photographed 

 direct with the aid of his apparatus, including interesting ex- 

 amples of the bright central spot in the shadow of a small 

 opaque screen (shot). 



On Sun-spots and Solar Envelopes. — The Rev. F. Hewlett 

 gave an account of observations and records made by him 



