Jan. 2, 1890] 



NATURE 



213 



large number of experiments with various anemometers on the 

 whirling machine at Hersham. Twelve of these were made 

 with the friction of the Kew anemometer artificially increased, 

 seven with a variable velocity, and fourteen with the plane of 

 the cups inclined at an angle to the direction of motion. In dis- 

 cussing the results the following points are taken into considera- 

 tion, viz. the possibility of the existence of induced eddies, the 

 effect of the increased friction due to the centrifugal force and 

 gyroscopic action, and the action of the natural wind. The 

 conclusion that the instrument is greatly affected by the vari- 

 ability of the wind to which it is exposed seems to be irresistible, 

 and if so, the exact value of the factor must depend upon the 

 nature of the wind as well as upon the mean velocity. There 

 is evidence to show that during a gale the variations of velocity 

 are sometimes of great extent and frequency, and there can be 

 but little doubt that in such a case the factor is less than 2 "15. 

 The one point which does seem clear is, that for anemometers 

 of the Kew pattern the value 3 is far too high, and consequently 

 that the registered wind velocities are considerably in excess of 

 the true amount. — On testing anemometers, by Mr. W. H. 

 Dines. The author describes the various methods employed 

 in the testing of anemometers, points out the difficulties that have 

 to be encountered, and explains how they can be overcome. — 

 On the rainfall of the Riviera, by Mr. G. J. Symons, F. R. S. 

 The author has collected all the available information respecting 

 rainfall in this district, which is very scanty. He believes that 

 the total annual fall along the Riviera from Cannes to San 

 Remo is about 31 inches, and that any difference between 

 the several towns has yet to be proved. — Report on the pheno- 

 logical observations for 1889, by Mr. E. Mawley. This is a 

 discussion of observations on the flowering of plants, the appear- 

 ance of insects, the song and nesting of birds, &c. Taken as 

 a whole, 1889 was an unusually gay and bountiful year. 



Physical Society, Dec. 6, 1889. — Prof. Reinold, President, 

 in the chair. — The following communications were read : — On 

 the electrification of a steam jet, by Shelford Bid well, F.R. S. 

 The author showed that the opacity of steam issuing from a 

 nozzle is greatly increased by bringing electrified points near it, 

 and that its colour is changed to orange-brown. Electrified balls 

 and disks when placed in the steam produce similar effects, and 

 when these are connected with an iiifluence machine at work, 

 the decoloration of the jet rapidly responds to each spark. On 

 examining the absorption spectrum of the unelectrified jet, little 

 or no selective absorption was detected, but on electrification, the 

 violet disappeared, the blue and green were diminished, and the 

 orange and red remained unchanged. From these results the 

 author concludes that electrification causes an increase in the 

 size of the water particles in the steam, from something small 

 compared with the wave-length of light, to about 1/50000 of an 

 inch in diameter. Allied phenomena with water jets have been 

 observed by Lord Rayleigh, who found that a straggling water jet 

 is rendered much more coherent by bringing a rubbed stick of seal- 

 ing wax near it. These observations are of considerable meteoro- 

 logical interest, for the steam jet phenomena go far towards ex- 

 plaining the cause of the intense darkness of thunderclouds, and of 

 the lurid yellow light with which that darkness is frequently 

 tempered. After making his experiments the author learnt that 

 similar observations had recently been made by the late Robert 

 Helmholtz, who viewed the steam jets by reflected light against 

 a dark background. On electrification the jets became much 

 better defined, and presented diffraction colours. Luminous 

 flames also produced similar effects, and Mr. Bidwell has found 

 that glowing touch paper is equally efficient. Helmholtz con- 

 jectures that the sudden condensation may be due to molecular 

 tremors or shock imparted by the electrification upsetting the 

 unstable equilibrium of the supersaturated vapour, just as a 

 supersaturated saline solution is suddenly crystallized when dis- 

 turbed. Another hypothesis suggests that condensation is caused 

 by the introduction of solid matter into the jet by the exciting 

 cause, thus producing nuclei upon which the vapour may con- 

 dense. On reading Helmholtz's paper, the author tried the 

 effect of gas-flames on water jets, and found that when luminous 

 they influenced the jet considerably, whereas non-luminous 

 flames had no appreciable effect. He also found that luminous 

 flames are positively electrified, and demonstrated this before the 

 meeting. Prof. Riicker thought the surface tension of the films 

 surrounding the water jets might be lowered by the presence of 

 a burning substance, and that the smoke from the to^ich paper 

 used in some of the experiments on steam jets would introduce 



solid particles and facilitate condensation. Mr. Richardson in 

 quired whether a red-hot iron had any effect. Dr. Fisonsaid he 

 had made experiments on the electrification of flame, and found 

 that potentials varying from + 2 volts to - i^ volts could be 

 obtained in the region within and surrounding a Bunsen flame. 

 Prof S. P. Thompson commented on the contrast between Mr. 

 Bidwell's experiments and those of Dr. Lodge on the dissipation, 

 of fogs by electricity, and also asked whether the colour of the 

 jet depended on the length of the spark produced by the 

 machine. Prof. Forbes thought a crucial test between the two 

 hypotheses of Helmholtz could be obtained by trying the experi- 

 ment in a germless globe. The President said he had recently 

 noticed that gas flames were electrified. Mr. Bidwell in reply 

 said he ought to have mentioned that the effect of flames on jets 

 may be due to dirt, for if soap or milk be added to the water in 

 the steam generator, no effect is produced by electrification or 

 flame. As to change of colour with spark-length, little (if any) 

 variation is caused thereby. He had not tried whether a red-hot 

 iron produced any effect on a steam jet, — Notes on geometrical 

 optics. Part 2, by Prof S. P. Thompson. Three notes were 

 presented, the first of which dealt with the geometrical use of 

 "focal circles " in problems relating to lenses and mirrors, and 

 to single refracting surfaces. By "focal circles" the author 

 means the circles having the principal foci as centres, and whose 

 radii are equal to the focal lengths. By their use the point con- 

 jugate to any point on the principal axis is readily determined. 

 One construction for a mirror is to draw a tangent to the focal 

 circle from a point p on the axis ; the foot of the perpendicular 

 to the axis drawn through the point of contact gives the point 

 conjugate to r. When applied to a thin lens, a tangent is drawn 

 as above to one focal circle, and the line joining the point of 

 contact with the centre of the lens is produced to meet the other 

 focal circle ; a perpendicular to the axis from the remote point of 

 intersection gives the conjugate point. Modifications applicable 

 to thick lenses and single refracting surfaces were also given. In 

 his second note the author treated similar problems by the aid 

 of squares drawn on the principal focal distances, the construc- 

 tions being remarkably simple, as will be seen from the figure, in. 



-M7 Mi 



which Mj Mj represent the principal planes of a thick lens, Fj,. 

 Fj, its principal foci, and p and Q are conjugate points. The 

 line B c is drawn parallel to p A. In the third note, the paths of 

 rays through prisms are determined by the aid of imaginary- 

 planes representing the apparent position of the plane bisecting 

 the dihedral angle of the prism when viewed through its two 

 faces. Just as in problems on thick lenses in which the part be- 

 tween the principal planes may be supposed removed, so when 

 dealing with prisms, the part between the imaginary planes 

 above referred to may be supposed non-existent. In another 

 method of treatment, the apparent positions of points outside the 

 prism when viewed from inside the prism are made use of, and 

 their application to illustrate dispersion was pointed out. Mr. 

 C. V. Boys asked whether the latter construction could be used 

 to show why the slit of a spectroscope appears curved. — On the 

 behaviour of steel under mechanical stress, by Mr. C. H. Carus- 

 Wilson. This is an inquiry into the properties of steel as illus- 

 trated by the stress-strain curves given in automatic diagrams 

 from testing machines, and by magnetic changes which take 

 place during testing. After pointing out that the permanent 

 elongation of a bar under longitudinal stress consists of a sliding 

 combined with an increase of volume, the author showed that 

 the "yield" is caused by the limit of elastic resistance (p) 

 parallel to one particular direction in the bar (generally at 45° 

 to the axis) being less than along any other direction. When this 

 lower limit is reached, sliding takes place in this direction until 

 the hardening of the bar caused thereby raises the limit of 

 elastic resistance (in the direction referred to) to that of the rest 

 of the bar, after which the stress must be increased to produce 

 further permanent set. From considerations based on the stress- 



