Sept. 29, 1887] 



NATURE 



523 



pendent current is maintained in the same closed vessel. He 

 was now prepared to show the experiments in his laboratory. 



On the N'ature of the Photographic Star- Disks and the Removal 

 of a Diffuulty in Measurements for Parallax, by Prof. 

 Pritchard, F. R. S. — If the telescope could be made to follow the 

 star with perfect accuracy daring the whole time of exposure of 

 the photographic plate, the photographic image should be 

 circular. 



It is necessary, in order that photography may be of use, say 

 in measuring parallax, that this condition should be approxi- 

 mately fulfilled. For it is from the centre of the star-image that 

 the measurement must be made. 



Now the image of a star exposed to a photographic plate 

 driven by a clock having a small rate, and subject to small 

 periodic oscillations, as is generally the case with the majority of 

 driving-clocks, is not a simple linear trace, but a series of black 

 dots joined together by intervals less dense. By hand-driving, 

 those black dots, &c. , coalesce or are superimposed. If for the 

 purpose of measurement for parallax or otherwise a bright star 

 be covered over by a stop during the greater part of the duration 

 of the exp 3sure of the plate, and the stop be then removed for a 

 brief interval, it is shown by experimental m.asurement that the 

 bright star is accurately represented on the plate. 



Instruments for Stellar Photography, by Sir Howard Grubb, 

 F.R. S. — He said that in considering the best form of object- 

 glass for photograjihic pu.-poses, one important point was to 

 have as large a field as possible. An arrangement suggested by 

 Prof. Stokes by which the first and second surfaces of refraction 

 could be readily interchanged afforded the means of providing 

 amateurs with an instrument which could be used at will either 

 for ordinary or for photographic work. But by far the most 

 important point in stellar photography was the clockwork. It 

 was true that, no matter how perfect our clockwork might be, 

 we could never dispense with the guidance of the hand. But if 

 the clockwork be as good as it can be made, one is not tied down 

 to the telescope as one is with bad clock-work. 1 le thought it 

 Utopian to expect that it might be possible to construct a clock 

 accurate to 1/75 of a second, which some supposed to be neces- 

 sary, lie was at present able to correct any error greater than 

 xV or at any rate than |^ of a second, and he expected to be able 

 to reduce the error to 1/20 of a second, but not farther. He 

 did not believe that greater accuracy than this would be of any 

 use. 



In the course of a discussion which followed, Dr. Pritchard 

 said he had actually traced the origin of the doti in the trail of 

 his star-photographs to the periodicity of the screw. 



On the TurbuLnt Motion of Water betzveen Two Planes, by 

 Prof. Sir W. Thomson, F.R.S. — He said that one of the most 

 important questions in practical hydraulics was the flow or slip- 

 ping of a liquid on a solid. He supposed for simplicity that 

 there was nothing of finite slip of the fluid on the solid ; but 

 this was not essential to the reasoning. He considered the case 

 of water flowing between two parallel planes ; as an example of 

 which a river with a plane bottom and covered with a sheet 

 of ice might be taken. If the motion be laminar, i.e. free from 

 turbulence, then the line of flow is represented by a parabola, 

 supposing that there is no finite slip. The fluid moving in this 

 way under the influence of gravity and possessing viscosity is in 

 a state of stable equilibrium. If we suppose gravity and viscosity 

 both suddenly reduced to zero, the motion becomes one of un- 

 stable equilibrium. The smallest amount of viscosity gives 

 stability to the laminar motion, but the limits of stability are 

 narrowed by either diminishing the viscosity, or increasing the 

 effective component of gravity Osborne Reyn >lds made expe- 

 riments some years ago on the flow of water through tubes. 

 With great pressure there is always eddying, because the 

 limits between which stable equilibrium is possible are narrow, 

 and narrower the greater the pressure. He found that the 

 laminar flow continued in a central film of the water for a 

 certain distance, and then broke up suddenly into turbulence. 

 Froude had made experiments on the resistance which a veiy 

 smooth thin board met with in being moved at a uniform rate 

 through water. His results show that, if one of two infinite 

 planes (both at rest to begin with, and bounding a piece of water) 

 be suddenly set in uniform motion, the water will at first move 

 turbulently, and the turbulence will gradually pass into shearing 

 motion. In conclusion Sir W. Thomson expressed the hope that 

 candidates for the Adams prize would take up this subject. 



Lord Rayleigh mentioned experiments which he had made on 



jets of coloured liquids and of smoky air. In the case of a jet 

 projected into air the motion is unstable from the first, but the 

 instability only shows itself at a certain distance. This distance 

 diminishes with the velocity. He thought it possible that in 

 Reynolds's experiments the instability was in like manner present 

 from the first in the central film, and that the film remained dis- 

 tinct for a certain distance only in virtue of the purchase it had 

 obtained. 



On the Theory of Electrical Endosmose and Allied Phenotnena, 

 and on the Existen.e of a Sliding Coefficient for a Fluid in 

 Contact zuith a Solid, by Prof. Lamb, F.R.S, — This paper 

 deals with the laws governing the electric transport of conduct- 

 ing liquids through the walls of porous vessels or along capillary 

 tubes, and other related phenomena, which have been investi- 

 gated experimentally by Wiedemann and Quincke, and explained 

 by the latter writer on the assumption of a contact diff'erence of 

 potential between the fluid and its solid boundaries. This ex- 

 planation has been developed mathematically by Von Helmholtz. 

 Applying the known laws of viscous fluids, he finds that the 

 calculated results, so far as they depend on quantities which 

 admit of measurement, are in satisfactory agreement with the 

 experiments, and that the values which it is necessary to assign 

 to the contact diff'erence above spoken of are in all cases com- 

 parable with the E.M.F. of a Daniell's cell. Incidentally he 

 arrives at the conclusion that in the cases considered there is no 

 slipping of the fluid over the surface of the solid with which it 

 is in contact. In the present paper a slightly different view is 

 taken, and it is assumed that a certain finite (though possibly 

 very minute) amount of slipping takes place, and that it forms 

 an essential feature in the phenomena. The various cases con- 

 sidered by Von Helmholtz are treated on this assumption, and 

 in some respects extended. In all cases the results differ from 



those obtained by Von Helmholtz by a factor -, where / is a 



d 



linear magnitude measuring the " slip," viz. j3 = nil, and d de- 

 notes the distance between the plates of an air-condenser whose 

 capacity per unit area is the same as that of the apposed surfaces 

 of solid and fluid. For example, by coaiparison with Wiede- 

 mann's experiments, Von Helmholtz infers that, for a certain 

 solution of CUSO4 in the pores of a clay vessel, E/D = 1 77, 

 where D is the E.M.F. of a Daniell's cell. On the modified 

 hypothesis adopted in the present paper, the inference would be 



F / 

 that _ . 1 = 177. 

 T> d 

 As this involves two unknown ratios, no such definite conclusion 

 can be drawn, bat it is evident that the phenomena are con- 

 sistent even with very small values of E/D, provided / be a 

 sufficient multiple of d. Since d is a quantity of molecular 

 order of magnitule (comparable probably with 10"^ centimetres) 

 the value of / may still be so minute as to render the effects of 

 slipping qiute insensible in such experiments as those of Poiseilie. 

 They come to be of importance in the cases at present under 

 consideration only in consequence of the relatively great forces, 

 due to the fall of potential along the course of the current, 

 which act on the outer electrified layer of fluid and drag it over 

 the surface of the solid. 



On the Ratio of the Tivo Elasticities of Air, by Prof. S. P. 

 Thompson, D.Sc. — Pro.''. Thompson said his paper would be 

 chiefly interesting to those who had to teach thermo-dynamics to 

 beginners. It was important to have some simple form of ex- 

 periment for determining the ratio of the two elasticities of air, 

 ^uch as might readily be shown to a student. He described a 

 simple form of apparatus which he considered more suitable 

 than the usual one. 



A Null Method in Elect ro-Caloritnetry, by Prof. Stroud, D.Sc, 

 and Mr. W. W. Haldane Gee, B.Sc. — ^The method is a modifi- 

 cation of Joule's method for determining the specific heats of 

 liquids and solids, possessing, however, the unique advantages 

 of eliminating the correction for radiation as well as that for the 

 thermal capacity of calorimeter and stirrer. The liquids for 

 comparison in the two calorimeters are heated by wires carrying 

 electric currents in such a way that the rises in temperature are 

 the same in each case as tested thermo-electrically. The adjust- 

 ment is effected by shunting the current through one of the 

 calorimeters. 



Prof. H. A. Rowland gave a description of a map of the 

 sjlar s^;ectrum. He said that for several years he has wo;ked 



