Oct. 1 6, 1884] 



NA TURE 



597 



ing pressure to bear on one side or the other, and seeing whether 

 the action obtained is at all commensurate with the action found 

 by Mr. Hall. 



Prof. Hall then discussed an experiment by which Mr. Bidwell 

 had obtained a reversal of the effect, and showed that the 

 reversal was only apparent, and that when carefully examined 

 the results of Mr. bidwell's experiments were best satisfied by 

 the theory of the rotation of the equipotential surfaces about the 

 lines of magnetic force. 



Sir William Thomson spoke of the discovery of Mr. 

 Hall as being the most important made since the time of 

 Faraday. He favoured Mr. Hall's explanation, though he 

 considers Mr. Bidwell's suggestion as very important, and 

 thinks that it will very likely be found that both the Hall 

 effect and thermal effects have a common cause, rather than that 

 one is to be taken to explain the other. He showed also that 

 the mathematical examination of the subject indicates three 

 relations to be investigated, — first, the relation of thermal force 

 to the surfaces of equal rate of variation of temperature ; second, 

 the relation of electric current to the equipotential surfaces; 

 third, the relation of the thermal flow to isothermal surfaces. 

 The second of these is that investigated by Mr. Hall, who has 

 found that under the conditions mentioned the lines of flow are 

 not perpendicular to the equipotential surfaces. There remains, 

 therefore, "work for two more Halls," in either proving or dis- 

 proving the existence of the analogous actions in these other two 

 cases. Sir William Thomson also suggested the following ex- 

 ceedingly interesting mechanical illustration or analogue of Hall's 

 effect. Let us be living upon a table which rotates uniformly 

 for ever. A narrow circular canal is upon this table, concentric 

 with the axis of rotation of the table, and nearly full of water. 

 After a while the water will acquire the same velocity of rotation 

 as the table, and will come to a state of equilibrium. The outer 

 edge of the water in the canal will then stand a little higher than 

 the inner edge. Let us now apply a little motive force to the 

 water, and by means of a pump cause it to flow in the canal in 

 the same direction in which the table is already rotating : it is 

 evident that it wid stand higher on the outer edge, and 

 lower on the inner edge of the canal, than before. But, 

 should we cause it to flow in the opposite direction to the 

 motion of the table, it will stand lower on the outer edge, and 

 higher on the inner edge, than in its position of equilibrium. 

 The experiment made by Mr. Shelford Bidwell may also be 

 illustrated by putting a partition in the canal so as to divide it 

 into two circular concentric troughs, and make a little opening in 

 the partition at some point ; then taking two points near the 

 opening in the partition, one in one trough and one in the other, 

 if they are very close to the partition, the point in the outer 

 trough will be at a li/wer level than that in the inner one ; but if 

 they are not close to the partition, but one is taken close to the 

 outer edge of the outer trough and the other close to the inner 

 edge of the inner trough, then the point in the outer trough will 

 be at a higher level than that in the inner trough, though the 

 difference in level will be only about half of what it would have 

 been had there been no partition separating the canal into two 

 troughs. 



Prof. Forbes called attention to the fact that the classi- 

 fication of the metals according to their thermo-electric 

 qualities gives not only exactly the same division into positive 

 and negative, but that the very order obtained in that way cor- 

 responds to that obtained by classifying according to the Hall 

 effect, except possibly in the case of aluminium. 



In the Section of Mathematics and Astronomy the first paper 

 read was by Prof. E. C. Pickering, upon the colours of the 

 stars. The need of exact photometric measurement of different 

 parts of their spectra was first pointed out, and the author then 

 described a very ingenious method of accomplishing this. In 

 the telescope tube, a little beyond the focal plane, is a direct- 

 vision prism, so set as to give a spectrum extended in declina- 

 tion ; and on the preceding side of this prism is placed a piece 

 of plane glass, whose edges are so ground that, when a small 

 p rtion of the following side of the cone of rays falls upon it, 

 it gives a small white ghost, just preceding the spectrum and 

 always opposite the same wavedength. In the focal plane is 

 one of Prof. Pritchard's neutral-tint wedge photometers, and 

 behind it a thin metal diaphragm with four long, narrow slits 

 parallel to the equatorial motion ; so that, when the spectrum 

 transits behind them, four little stars — a red, yellow, blue, and 

 a violet — shine through these slits, and the tim; of the disap- 



pearance of each, as they move towards the thicker edge of the 

 wedge, measures its brightness. From these times may be 

 deduced the magnitude and colour curve of the star. To fix the 

 same wave-lengths for each observation, the little white ghost is 

 adjusted upon one of two parallel wires which project out beyond 

 the preceding side of the diaphragm. For a succeeding transit, 

 the ghost is adjusted upon the other wire, half a slit-interval 

 distant, and thus eight points of the spectrum are photometri- 

 cally measured. 



Prof. Young of Princeton spoke very highly of the ingenuity 

 and effectiveness of the device, especially for the systematic 

 measurement of a large number of stars. He pointed out, 

 however, what might be a source of error, viz. the different 

 sensitiveness of different observers' eyes to different colours, so 

 that they would probably observe the times of disappearance 

 of the four coloured stars slightly relatively different. 



The next paper, by Prof. Daniel Kirkwood, discussed the 

 question whether the so-called "temporary stars" may be 

 variables of long period, referring to the sometimes-claimed 

 identity of the temporary stars 945 and 1264, with the well- 

 known Tycho Brahe's star, which blazed forth in Cassiopeia in 

 1572, and whose position is pretty closely known from his mea- 

 sures. The conclusion reached was, that on account of the 

 sudden apparition of the temporary stars, the short duration of 

 their brightness, and the extraordinary length of their supposed 

 periods, they should be considered as distinct from variables. 



Prof. Mansfield Merriman, the author of the well-known 

 treatise on "Least Squares," proposed a criterion for the rejec- 

 tion of doubtful observations, founded upon Wagen's demon- 

 stration of the law of frequency of error, which was simpler 

 than Pierce's or Chauvenet's. It involves, however, a deter- 

 mination of what is the unit of increment between errors of 

 different sizes, a thing difficult to determine in very many cases. 



Prof. Pickering then read another paper upon systematic 

 errors in stellar magnitudes, showing, without any question, 

 that the magnitudes of all the star-catalogues, from that of 

 Ptolemy down to the great work of Argelander in the Dunh- 

 mustirun? — all depending upon eye-estimates— are systematic- 

 ally affected by being in, or close to, the Milky Way ; they all 

 being estimated too faint, and the error amounting to about half 

 a magnitude in the Milky Way itself. This arises from the 

 brightness of the background upon which the star is viewed. In 

 the Harvard photometry measures, this source of error is 

 avoided, since, in the comparison of each" star with the Pole- 

 Star, the two fields are superposed, and their added brightness 

 affects both stars alike. 



Prof. M. W. Harrington, Director of the Ann Arbor Ob- 

 servatory, read a paper upon the asteroid ring. He showed 

 that the representative average orbit would be an ellipse of 

 small eccentricity, with semi major axis equal to about 27 times 

 that of the earth, and inclined to the plane of the ecliptic about 

 1° ; and that, in the progressive discovery of these small bodies, 

 the average mean distance had gradually increased, but now 

 seemed to have reached its limit. On the assumption that the 

 surfaces of all the asteroids have the same reflecting power as 

 Vesl 1, Prof. Harrington reaches the conclusion that the volume 

 of Vc sta is about 5/17 that of all these 230 bodies put together, 

 and that Vesta and Ceres together form almost one-half the total 

 volume. 



Prof. Rogers, of the Harvard College Observatory, read two 

 papers. The first, upon the magnitude of the errors which may 

 be introduced in the reduction of an observed system of stellar 

 co-ordinates to an assumed normal system by graphic methods, 

 showed a great amount of laborious research, and was a good 

 illustration of the vast amount of monotonous work necessary in 

 the present stage of astronomical observation in order to reach 

 the highest degree of accuracy attainable by the search for and 

 elimination of minute systematic errors. His next paper was 

 upon the original graduation of the Harvard College meridian 

 circle in situ. This described a method of turning a meridian 

 circle through any desired constant arc up to about 30 without 

 any dependence upon the circle and reading microscopes, 

 effected by means of an arm swinging between fixed stops, and 

 clamping to a circular ring on the axis by an electro-magnetic 

 clamp. With this Prof. Rogers claimed to be able to set off a- 

 constant arc through as many as five thousand successive move- 

 ments of the clamping arm. The ingenious method suggested' 

 and carried out by Mr. George B. Clark, of the firm of Alvan^ 

 Clark and Sons, of grinding the clamping circle to a perfect 

 circular form while the telescope was swung round in its Y's was; 



