Sept. 7, 1882] 



NATURE 



465 



quake waves, the author described the " time-take," which is a 

 clock which is an automatic arrangement causing dots to be 

 made and the time of wave-motion to be indicated without 

 stopping the clock. He describes shocks observed by him in 

 Japan as travelling at 10,000 feet per second, decreasing as it 

 went on to 4500 feet, getting slower and slower as it 

 went on. The waves last from thirty seconds to four minutes. 

 The author describes the result of experiments carried out by 

 himself and Mr. Gray as to artificial earthquakes, explosions of 

 2 lbs. to 5 lbs. of dynamite in boreholes 10 feet in depth, fired 

 by electricity, and the effect of lettiug a heavy iron ball fall on 

 the ground to a depth of 20 to 30 feet in height. The effect of 

 shocks is communicated along the surface, gradually decreasing as 

 it proceeded from the point of propagation, but at a less rate as 

 the distance increases. 



SECTION A — Mathematical and Physical 



On the Absolute Measurement af Electric Currents, by Prof. 

 Lord Rayleigh. — The absolute measurement of current is more 

 difficult than that of resistance. All the methods hitherto 

 employed requite either accurate measurements of the horiz rntal 

 intensity of the earth's magnetism or of coils of small radius and 

 many turns. This latter is difficult to evaluate, as it is im- 

 possible to measure the length of the wire wound, as the tension 

 necessary to make the wire lie evenly, stretches it very consider- 

 ably, whilst it is mo--t important to determine the mean radius 

 accurately, as an err >r therein doubles itself in the final result. 

 The method of Kohlrausch is free from this objection, but it 

 requires a knowledge of the moment of inertia, a quantity not 

 ia>y accurately to determine. When the electromagnetic action 

 is a simple force, it can be determined directly. In 

 recent determination, a large solenoid i? suspended vertically 

 in a balance, and is acted on by a flat co-axal coil of much larger 

 radius. This is simple to think about, but not calculated to 

 secure precise results. The appearance of accuracy is illusory, 

 can be assumed that the distribution of wire is abso- 

 lutely uniform. It would appear that all the turns of the sus- 

 pended coil should operate a, much as possible, that is, that the 

 suspended coil should be compact, and should be placed in the 

 position of maximum effect. There is a further incidental advan- 

 tage in this arrangement. The expression for the attraction 

 involves as factors the product of the number of turn^, the 

 square of the current, and a function of the mean radii of the 

 two coil-i and .if the distance between their mean planes. This 

 function is of no dimen ions. When the position is such that 

 the function for two given coils is a maximum, the result is 

 practically dependent only on the two mean radii, and the func- 

 tion being of no dimensions, can involve the-e mean radii only 

 in the form of a rati ■. This can be obtained electrically with 

 full precision by dividing a current between them in such a way 

 that no effect is produced on a small magnet at their common 

 centre. In practice it will be desirable to duplicate the lixed 

 coil, placing the suspended coil midway between two similar 

 fixed ones, through v. hich the current passes in opposite direc- 

 ti ins. 



11 the Duralii u of Free Electric Currents in a Conducting 

 Cylinder, by L01 —This paper was devoted to con- 



sidering the rate of decay of currents of electricity circulating 

 round a conducting cylinder. The tine in which the intensity 

 sinks from <■ to 1 is called the " time of subsidence." For a 

 copper cylinder of r centimetres radius, this is equal to >-'■' ooo. 

 That this may be one second, the diameter of the cylinder must 

 be two feet. 



On the Equilibrium of Liquid-conducting Surfaces charged 

 with Electricity, by Lord Rayleigh.— This was a mathematical 

 paper in which was investigated the condition of stability of a 

 sphere of fluid charge! with electricity. If Q be the charge, T 

 tlie surface tension of the fluid, and a the radius of the sphere, 

 then the condition of stability is tint T>Q i /l6ira 3 . 



Preliminary Account of Results obtained during the late Total 

 ipse, by Prof. Schuster and Capt. Abney. — Three 

 fisof the corona were obtained with different exposures. 

 The comet Tewfik, discovered during the eclipse, appears on 

 the photographs, and the change of its position in successive 

 plates shows that it was moving away from the sun. The corona 

 is seen to extend over a solar diameter away from the sun. A 

 plate exposed in a camera which had a prism in front of the 

 ens shows the spectra of different prominences, which are not 

 ound to be identical, but in every case the lines II and K are 



the strongest. A photograph obtained in a complete spectro- 

 scope shows (1) a complicated prominence spectrum; (2) a 

 strong continuous spectrum in the lower parts of the corona ; 

 (3) a reversal of the solar line G in the upper regions ; (4) a 

 series of coronal lines, different from the prominence lines. 



Some Matters relating to the Sun, by Prof. Schuster. — Ob- 

 servations of the shape assumed by the solar corona in suc- 

 cessive eclipses during the last fifteen years have shown remark- 

 able changes coincident with the sun-spot period. The corona 

 of sun-spot minimum is characterised by a certain symmetry 

 about an axis not far removed from the sun's axis of rotation, but 

 very likely not quite coincident either with it or with the per- 

 pendicular to the ecliptic plane. Some apparent irregularities in 

 the symmetry seem to be due to differences in the position of the 

 earth in its annual orbit. Changes in the spectroscopic and 

 polariscopic properties of the corona which are coincident and 

 connected with the changes of form seem to point to partly 

 meteoric origin of the corona. 



On a Misprint in the Tidal Report for 1872, by Mr. G. H. 

 Darwin. — Mr. Darwin has recently been carrying out a laborious 

 reduction, by the Method of Least Squares, of the observations 

 of the tides of long period at a number of stations. The results, 

 which seem to have an important bearing on the question of the 

 rigidity of the earth's mass, will appear a> § S4S in the new 

 edition of Thomson and Tait's Natural Philosophy, now in the 

 press. Subsequently to the completion of the calculations, Prof. 

 J. C. Adams disc jvered a misprint in the Tidal Report of 1872, 

 which firms the bads for the method of harmonic analysis, 

 which has been applied to the tidal observations. On inquiry 

 of Mr. Roberts, who has superintended the original computa- 

 tions, Prof. Adams learnt that the erroneous formula has been 

 used in all the reductions of the long period tides. The erroneous 

 formula occurs near the middle of p. 471 of the Report of the 

 British Ass icia 1 n for 1872, in the instructions for clearing the 

 diurnal means f r >m the undue influence of the sh <rt period tides ; 

 in the first of the two formulae for that purpose, the factor 

 sin 12 n -in \ n should obviously be replaced by sin 24 njsAn n. 

 of long period are evaluated by the following pro- 

 cess : — A mean is taken of the twenty-four heights of the water 

 above the datum line at each mean solar hour during the twenty- 

 four hours. The 365 diurnal means form the results of tidal 

 for the whole year, and these are to be treated by 

 harmonic analysis ; but the continuous integrals which arise in 

 Fourier's method are of course replaced by finite integrals. This 

 method of procedure introduces an undue influence of the short 

 period tides on the values deduced for the long tides, and a cor- 

 rection t . each diurnal mean is neces:-ary to get rid of this influ- 

 ence. It is in the formula for the correction to be applied in 

 the case of the semi-diurnal tides that the error occurs. This 

 paper is an evaluation of the maximum effect which can have 

 been exercised on the results by the error. The analysis shows 

 that all the values assigned to the long period tides in the Tidal 

 Reports and Tide Tables innst have been more or le.^s vitiated. 

 The lunar fortnightly declinational tide, the semi-annual and the 

 annual tide have suffered comparatively little. The monthly 

 elliptic tide has suffered more, and the synodic fortnightly tide 

 will in many years have been utterly worthless. The paper con- 

 tains suggestions of a new method of procedure in the harmonic 

 analy-is of the tides of long period, and abo discusses a remark- 

 able result of the procedure by diurnal means in consequence of 

 which there is an exaggeration of the undue influence exercised 

 by the short-period tides on those of long period, in which either 

 the sum or difference of the speeds is exactly 15 or 30° per mean 

 solar hour. 



elocityof White and Coloured Light, by Mr. G. Forbes. 

 — The author gave an account of experiments made by him in con- 

 junction with Dr. James Young, F.R.S., with a view to deter- 

 mining the veli city of light. This research has been published 

 in the Transactions of the Royal Society. The chief p lint of 

 interest is that it appears that the velocity of bine light is greater 

 than that if red, the difference being between 1 and 2 per cent, 

 of the whole velocity. 



;ive no other p issible explanation of 

 the phenomena described by Mr. Forbes, but he had great 

 hesitation in accepting them from considerations on other sides, 

 altogether repudiated them, and Lord Rayleigh 

 thought that Foucault's method, that used by Michelson, was 

 better suited to bring out results, if such existed, than Fizeau's, 

 for it would produce a spectrum of considerable length. H e 

 would refer to some other points which he noticed in a letter 

 to NATURE about twelve months ago, especially as to what is 



