548 



NATURE 



{Oct. 6, 1887 



made by him seventeen years ago, as objection had been taken, 

 not to the accuracy of the observations themselves, but to the 

 conclusions he had drawn from them. Prof. Libby was to do 

 the photography. Photographs of the corona were to be taken ; 

 and they were anxious to determine whether there are true dark 

 lines in the corona or not. The place selected for the observa- 

 tions lay 150 or 160 miles to the north-west of Moscow. It is 

 needless to say that on the morning of the eclipse it rained, and 

 hardly anything could be done. They made an attempt to 

 determine the end of totality by the amount of light. The 

 diminution of the light was gradual, but after totality there was 

 a sudden burst. 



Prof. L. Weber, of Breslau, described photo-metric measure- 

 ments made during the eclipse at Breslau ; and then read a paper 

 on Observations of Atmospheric Electricity. Prof. Weber said that 

 the increase of potential seemed to be a linear function of the 

 height ; but the presence of dust in the air disturbed this rela- 

 lation. The earth represents a surface of equipotential, and the 

 other surfaces of equipotential are parallel, but come closer 

 together above the mountain-tops. 



Prof. Schuster said that, granting that the earth has a given 

 potential at any moment, the convection- currents in the air 

 would tend to reduce this, or to equalize the potential within the 

 earth itself. 



Prof. Everett remarked that wherever electricity is carried 

 down by raindrops, an inequality of potential will be caused ; 

 and evaporation would also cause inequalities. 



Prof. Rowland said that observations had been made during 

 the last four years at his laboratory by the U.S. Signal Service. 

 He did not see how the raindrops could disturb the distribution 

 of potential much. If the earth is electrified most of the elec- 

 tricity would be on the outride of the atmosphere. He therefore 

 looks for some other theory, and has given one in the Phil. Mag., 

 viz. that the earth would naturally be uniformly electrified if it 

 were not for currents of air in the upper atmosphere, which will 

 carry the electricity of the atmosphere towards the poles, making 

 auroras there. At the equator, therefore, a space must be left 

 which has to be filled up with electricity, and this takes place 

 by thunderstorms. Accordingly there is a circulation of electri- 

 city. In this connexion it is to be remembered that thunderstorms 

 are most common about the equator. 



The Hygrometry of Ben Nevis, by Mr. H. N. Dickson.— 

 This paper gives an account of observations which were under- 

 taken for the purpose of testing the applicability at high-level 

 stations of existing tables and formulae for calculating the dew- 

 point and humidity from the readings of wet- and dry-bulb 

 thermometers. The construction of the direct hygrometer used, 

 that of Prof. Chrystal, is described, and the action of the wet 

 and dry bulbs under dififerent meteorological conditions is exa- 

 mined in considerable detail ; the results showing that for in- 

 vestigations of this kind a great range of humidity is necessary, 

 the indications of the wet and dry bulbs being very uncertain 

 when the difference between them is small. 



The reduction of the observations is performed, in the first 

 place, by a graphic method, from which the following expression 

 is reduced : /' - /" = {t - t')k, f being the vapour-pressure 

 at the temperature t' of the wet bulb, /" that at the temperature 

 of the dew-point, and t the air tempc'-ature. The truth of the 

 above equation being assumed, the values of the quantity Jk are 

 next found by direct calculation from the observations. A 

 sudden large change takes place in its value at the freezing-point, 

 and a similar, though much smaller, discontinuity is shown to 

 occur when the wet bulb stands between 39° and 40°. 



The Different Varieties of Thunderstorms, and a Scheme for 

 their Systematic Obsei-vation in Great Britain, by the Hon. R, 

 Abercromby. — The writer said that there were three well-defined 

 types of thunderstorms in this country : (i) squall thunderstorms, 

 i.e. simply a squall associated with thunder and lightning ; (2) a 

 very common form which occurs in secondary cyclones : the 

 nature of this class needed investigation ; (3) far the most 

 curious class was that which might be called line-thunder- 

 storms, because their shape was a long narrow belt sometimes 

 200 or 300 miles long and only 4 or 5 broad. They move 

 broadside on, and are usually preceded by a squall of extreme 

 violence. He explained a scheme for the future systematic 

 study of thunderstorms, and invited the co-operation of volunteer 

 observers. 



Sir W. Thon»s< n said that the natural history of thunderstorms 



was less known than any other part of meteorology, and that 

 Mr. Abercromby's scheme would be likely to give much informa- 

 tion on the subject. 



On the Magnetization of HadfieWs Manganese .Steel in Strong 

 Fields, by Prof. J. A. Ewing, F.R.S., and William Low.— 

 Messrs. Hadfield, of Sheffield, manufacture a steel containing 

 about 12 per cent, of manganese and 08 per cent, of carbon, 

 which possesses many remarkable qualities. Prominent amongst 

 these, as the experiments of Hopkinson, Bottomley, and Barrett 

 have shown, is a singular absence of magnetic susceptibility. 

 Hopkinson, by applying a magnetic force, |^, of 244 C.G.S. 

 imits to a specimen of this metal produced a magnetic induction, 

 ?3, of only 310 C.G.S. units ; in other words, the permeability/* 

 was I "27, and the intensity of magnetization 3 was a little over 

 5 units. 



The experiments made it clear that even under magnetic 

 forces extending to io,oao C.G.S. units the resistance which 

 this manganese steel offers to being magnetized suffers no 

 break-down in any way comparable to that which occurs in 

 wrought iron, cast iron, or ordinary steel at a very early stage 

 in the magnetizing process. On the contrary, the permeability 

 is approximately constant under large and small forces. 



The conclusion has some practical interest. It has been sug- 

 gested that this steel should be used for the bed-plates of 

 dynamos and in other situations where a metal is wanted that 

 will not divert the lines of induction from neighbouring 

 spaces. In such cases the magnetic forces to which man- 

 ganese steel would be subjected would certainly lie below the 

 limit to which the force has been raised in these experiments. 

 We may therefore conclude that in these uses of the material it 

 may be counted upon to exhibit a magnetic permeability only 

 fractionally greater than that of copper, or brass, or air. 



On the Influence of a Plane of Transverse Section on the 

 Magnetic Permeability of an Iron Bar, by Prof. J. A. Ewing, 

 F. R. S., and William Low. — It has been remarked by Prof. 

 J.J. Thomson and Mr. H. F. Newall that when an iron bar is 

 cut across and the cut ends are brought into contact, the 

 magnetic permeability is notably reduced (Cambridge Phil. Soc. 

 Proc, February 1887). The attention of the authors was 

 directed to the matter by finding the same phenomenon present 

 itself in experiments on the magnetization by the "isthmus" 

 method ; and they proceeded to examine the effect by an appli- 

 cation of the method Hopkinson has used to measure magnetic 

 permeability (" Magnetization of Iron," Phil. Trans., Part 2, 

 1885), A round bar, nearly half a square centimetre in section, 

 and 13 centimetres long, had its ends united by a massive 

 wrought-iron yoke to reduce it to a condition approximating to 

 endlessness, and its magnetization by various magnetic forces 

 was examined, both when free from stress and when compressed 

 by a load of 226 kilos, per square centimetre. It was then 

 cut in the lathe and the halves placed in contact, and the 

 magnetization again examined with and without load. It was 

 next cut into four parts, and finally into eight parts, and magnet- 

 ized in each case. Every new plane of section caused a notable 

 loss of permeability. The following are the maximum values of 

 the permeability in each case : — 



Solid bar ... ... 1220 



Bar cut in two ... 980 



Bar cut in four ... 640 



Bar cut in eight ... 400 

 Next another bar was tested, first when solid, next with one 

 cut finished in the lathe, and finally with the cut surfaces faced 

 true by scraping and comparing them with a Whitworth i 

 plane. So long as the bar was not compressed its magnetic | 

 permeability was nearly the same, whether the ends were left 

 roughly finished or were faced true. But when load was applied 

 the effect of facing the ends was remarkable : the faced bar then 

 behaved as a solid bar would, while the bar with rough cut ends 

 still showed a decided defect of permeability as compared with 

 a solid bar. 



This made it seem highly probable that the whole effect was 

 due to a film of air between the cut faces. Applying Hopkin- 

 son's method to calculate the thickness this film would need to 

 have in order to account for the observed increase of magnetic 

 resistance, the authors find its thickness is only about 1/35 o^ ^ 

 millimetre when the magnetic force is 10 C.G.S. units, and 

 diminishes to about 1/70 of a millimetre when the force is 

 50 C.G.S. In the case of the bar cut into four or eight parts, 

 each cut has an effect equivalent to the introduction of a film 01 



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