Oct. lo, 1889] 



NATURE 



585 



in iron, but in steel, brass, and copper wires. He has not yet 

 examined other metals. The experiment consisted in observing, 

 with much optical magnification, the extension of a very long 

 piece of wire, directly loaded with lead weights. The wire was 

 hung from a rigid support in a testing flue or recess, built in the 

 wall of the laboratory, and extending up through four stories. 

 At a distance of 806 cms. below the top a small clamp was fixed 

 on the wire, and this formed the support of the back foot of a 

 little tripod, the feet of which consisted of three needle points 

 about an inch apart. The two front feet rested in a slot and 

 hole in a fixed shelf which stood in front of the long wire. The 

 tripod carried a plane mirror which became tilted forward or 

 backward as the wire extended or retracted. Readings were 

 taken by a telescope of the reflected scale of a levelling staff" 

 placed vertically at a distance of some 5 metres from the mirror. 

 The staff was graduated to i/ioo of a foot, and it was easy to 

 read by estimation to i/iooo of a foot, which corresponded to 

 o"ooooooio2 of the length of the wire. At first a fixed shelf 

 was used to support the two front feet of the mirror, but the 

 effects of temperature were found to be excessive, although the 

 greatest care was taken to shield the wire from draughts ; and 

 the plan was resorted to of hanging the shelf from two adjacent 

 wires of the same material, suspended from the same support as 

 the wire which was to be stretched. In this form the arrange- 

 ment for optical multiplication was nearly the same as one used 

 by Mr. Bottomley in recent experiments on the extension of 

 loaded wires by heat. 



Prof. Henry Stroud, in a paper on the E.M.F. produced by 

 an abrupt variation of temperature at the point of contact of 

 two portions of the same metal, gave the results of experiments 

 on the E.M.F. produced when two portions of the same metal 

 are in contact, and one portion is kept at a high temperature and 

 the other at a low temperature. The precautions necessary in 

 the performance of such experiments are discussed in the paper, 

 and the results obtained are given in the case of copper and 

 iron. The E.M.F. establishea in the case of copper is from 

 hot to cold across the junction, while in the case of iron it is 

 from cold to hot, and of far greater magnitude. These experi- 

 ments are being continued with a view to obtaining the relation 

 between the E.M.F. and the difference of temperature. 



Prof. McLeod, F.R.S., in a paper on the black-bulk thermo- 

 meter in vacuo, concludes the black bulbs should be as small as 

 possible, and very little of the stem blackened, and also that 

 the case should be as thin as is consistent with strength. 



Sir W. Thomson, F.R. S., communicated a paper, by Magnus 

 Maclean and Makita Goto, on electrification of air by combus- 

 tion. A series of experiments was made, under the instructions 

 of Sir William Thomson, for the purpose of endeavouring to 

 find : (l) the state of the air inside a room as regards electrifica- 

 tion ; (2) the relation between electrification of air within a 

 building and the atmospheric potential in its neighbourhood 

 outside ; and (3) the causes which produce or change the elec- 

 trification of any given mass of air. It was found that an in- 

 closed mass of air is electrified negatively by the burning of a 

 paraffin lamp, of coal gas, of sulphur, of magnesium, and of 

 several other substances ; and, on the other hand, the burning 

 of charcoal electrified a room positively. 



Prof. C. Michie Smith read some notes on atmospheric elec- 

 tricity and the use of Sir W. Thomson's portable electrometer. 

 Recent observations fully confirm the author's previous con- 

 clusions that, with a dry west wind, the potential of the air for 

 some distance above the ground is usually negative. This seems 

 always to be associated with dusty air. In using the portable 

 electrometer in hot moist climates, special precautions have to 

 be taken in drying it. As much sulphuric acid must be used as 

 the pumice can absorb, and the pumice should be dried at least 

 once a fortnight. The charging-rod itself must be very carefully 

 dried, and, alter charging, should be lifted out with a piece of 

 warm silk. 



Mr. A. W. Clayden read a paper on dark flashes of lightning. 

 The author exhibited a negative taken on June 6 last, which 

 shows several reversed images of lightning-flashes. He de- 

 scribed a series of experiments which he had carried out with 

 the object of discovering whether the phenomenon could be 

 imitated in the laboratory. The steps in the investigation were 

 illustrated by a series of negatives showing photographs of elec- 

 tric sparks. The conclusions arrived at are, that photographic 

 images of electric sparks can be reversed by the action of dif- 

 fused light. Reversal is only produced when the exposure to 

 diffused light is subsequent to (or possibly simultaneous with) 



exposure to the image of the spark. If the plate is first acted 

 upon by diffused lignt, the sparks give a direct image unless the 

 action has been considerable, in which case they seem to make 

 no impression. 



Prof. Cleveland Abbe, in a paper on the determination of the 

 amount of rainfall, gave a rhume of the general results of in- 

 vestigations on this subject. The deficit in catch by a gauge due 

 to the eddies of wind is shown, on general reasoning, to be 

 proportional to the velocity of the wind and to the relative 

 percentage of small and slowly-falling drops. 



Prof. C, Piazzi Smyth, late Astronomer- Royal for Scotland, 

 read a paper entitled " Hygrometry in the Meteorological Jour- 

 nal." After noting the superior value officially attached to 

 determinations of the mean temperature by observations of 

 self-registering thermometers, recording its maximum and mini- 

 mum every twenty-four hours, the author inquires why the still 

 more difficult problem of ascertaining the mean daily moisture 

 of the atmosphere is thrown over to a different principle of ob- 

 serving, long since condemned for simple temperature. Believ- 

 ing that the want of good self-registering dry- and wet-bulb 

 thermometers for the purpose was the chief obstacle, and having 

 alighted on some recent makes of Sixe's thermometers with 

 several very recommendable qualities, he invested two out of 

 four of them with the peculiar fittings for wet-bulb hygrometers, 

 after having made a table of index corrections for them all, as 

 compared with standard thermometers. But as soon as hygro- 

 metric observations began, the depression of the wet, below the 

 dry, bulb always came out at only two-thirds of what a standard 

 but non-registering Glaisher arrangement gave out, for the hori- 

 zontal form of Sixe, and no more than half for a vertical form. 

 These differences of hygrometric statement, though rather puz- 

 zling for a time, were traced up to the wet bulbs of the Sixe's 

 thermometers being in contact on one side with the scale-plate. 

 For when that plate in the horizontal Sixe form was cut entirely 

 away from the bulb, and a new vertical form of Sixe was made 

 [per Mr. James Bryson, Edinburgh) with its long thin bulb 

 wholly outside the rest of the instrument, the indications of all 

 three varieties of wet- bulb hygrometers became practically the 

 same ; and hygrometry was relieved of the old drawback on its 

 registrations of maximum and minimum quantities of moisture 

 in every twenty-four hours, 



Mr. F. T. Trouton read a paper on some experiments on 

 radiation with Prof. Hertz's mirrors. These experiments were 

 described in Nature, vol. xxxix. p. 391, and vol. xl. p. 398. 

 Certain of them were repeated by Mr. Trouton in the Physical 

 Laboratory of the Durham College of Science. 



Profs. A. W. Riicker, F.R.S., and T. E. Thorpe, F.R.S., 

 read a paper, on the relation between the geological constitution 

 and the magnetic stale of the United Kingdom, before a joint 

 meeting of Sections A and C. The authors have, during the 

 last five summers, determined the magnetic elements at 200 sta- 

 tions, distributed over the whole of the United Kingdom, and 

 have employed the results of their observations in a study of the 

 disturbing magnetic forces in play in various districts. If all 

 disturbing causes were removed, an observer travelling due west 

 from London would find that the declination increased by about 

 half a degree for each degree of longitude. In fact, the rate of 

 increase, though equal on an average to this amount, is irregular. 

 Between London and Windsor it is considerably larger, and 

 between Windsor and Reading smaller than the mean. The 

 forces which produce these abnormal variations depend upon the 

 geological character of the district. Two principal theories have 

 been proposed to account for these disturbances : (i) the theory 

 of direct action of magnetic rocks ; (2) the other explanation 

 associates the deflection of the needle with disturbances of the 

 earth currents of electricity produced by irregularities in the 

 geological constitution of the country, and especially with geo- 

 logical faults. On the whole, the authors think that the theory 

 of the direct action of magnetic rocks agrees best with the 

 observed facts, and they have shown that the kingdom can be 

 divided into a small number of magnetic districts in which the 

 directions of the disturbing forces are evidently closely connected 

 with the geological constitution. 



In a paper on the passage of electricity through gases. Prof. 

 Arthur Schuster, F.R. S., gave an acccount of his investigations 

 during the last two years on the distribution of potential in the 

 neighbourhood of the negative pole of discharge of electricity 

 through rarefied gases. Knowing the rate of fall of potential, it 

 can be determined whether there is any bodily electrification in 

 any part of the negative glow. It was found that the kathode is 



