498 



NATURE 



\April20, 1876 



in manuscript. Indeed, Dr. Wild would almost lay it down as 

 a rule that unpublished observations should be deemed scientifi- 

 cally useless. As to conpjresses, he does not think that they 

 should be occupied with discussions on the laws of meteorology. 

 The derivation of laws from observations should be looked lor 

 in the undisturbed thoughtfulness of individuals. Experience 

 shows, however, that private persons do not employ themselves 

 as much as formerly in working out observations, and it seems 

 to be absolutely necessary for the advancement of meteorology 

 that every official observer should be given sufficient time, beyond 

 that required for mechanical work, for developing the science as 

 far as his powers will permit, and that the central institutions 

 should be adequately endowed for this purpose. — The next paper 

 is a review, by Dr. Hann, of the publications of Messrs. Fjord 

 and Paul la Cour on the climate of Denmark, which contain very 

 valuable statistics in the decennial means of fourteen stations. As 

 in other similarly situated countries, both the heat of summer and 

 the cold of winter are more intense inland than on the coasts, and 

 in July the most easterly stations are the warmest. The mild 

 weather of spring seems to advance from S.W. to N.E., and 

 the cool weather of October from N. W. to S.E. Thus the mean 

 temperature of 8° is reached in N.W. Jiitland on the nth, 

 in Bornholm and the northern extremity of Riigen between the 

 23rd and 24th of October. The mean monthly range of tem- 

 perature is greatest in May, least in January ; from April to 

 August the maxima rise higher above the monthly mean than the 

 minima sink below it ; from September to March the relation is 

 converse. The absolute maximum range was registered in July, 

 the absolute minimum in November. The average number of 

 days on which frost occurs is ninety-two ; time of maximum rain- 

 fall the latter end of August and beginning of September ; of 

 minimum, the beginning of April. Yearly mean rainfall — in 

 Denmark, 604 mm. ; on the west coast of Jutland, 670 mm. ; at 

 Copenhagen, 587 mm. ; Bornholm, 580 mm.. A small table in- 

 serted here by Dr. Hann gives a great deal of information as to days 

 on which rain lell, and on which thunderstorms, hailstorms, fog, 

 and cloud occurred. January is the cloudiest. May and July are the 

 least cloudy of months. Tables showing the frequency per cent, 

 of the diffisrent winds and the monthly barometric pressure close 

 Dr. Hann's summary of the valuable work under review. — In 

 the Kleinere Mittheilungen, Dr.Gustav Hellmann states the chief 

 results of his inquiries into the distribution of thunderstorms in 

 Northern Germany. In general the annual mean number of 

 thunderstorms in Germany increases from N.E. to S.W. It is 

 least on the coasts of the Baltic, particularly in East Prussia 

 greatest in the district of the Upper Rhine. On the eastern 

 coast of the Baltic about twelve are observed in the year ; on the 

 western coast of the Baltic, sixteen ; and on the coast of the 

 North Sea, fifteen. Inland, the number averages twenty. They 

 increase in number with increasing altitude, up to about 1,400 m., 

 then decrease rapidly. Winter thunderstorms are much more 

 common in Northern Germany than in Austria and Hungary. 



SOCIETIES AND ACADEMIES 

 London 



Royal Society, March 30. — *' An Experiment on Electro- 

 Magnetic Rotation." By W. Spottiswoode, M.A., Treas. and 

 V.P.R.S. 



The phenomena of. the rotation of movable conductors, carry- 

 ing currents, about lines of magnetic force, are well known. 

 One form of experiment, commonly called the rotating spark, 

 presents, beside the actual rotation, some peculiar features 

 which do not appear to have been noticed in detail. 



The spark, when carefully observed, is seen to assume a spiral 

 form ; and the spiral is right-handed or left-handed according to 

 both the direction of the current and the magnetic polarity. 

 This effisct is particularly noticeable if the magnetic pole be in- 

 serted only a short distance beyond the ring. The discharge is 

 then seen to spread itself out sheetwise on the ring in the direc- 

 tion in which rotation would take place. The edge of the sheet 

 is in the form of a helix. 



The object of the following observations is to bring out the 

 character of this phenomenon by making it a principal instead 

 of a secondary feature of the experiment. 



The arrangement here described consisted in using the poles 

 of an electro-magnet as the terminals of a discharge from an 

 induction-coil, and in observing the effect on the form of the 

 discharge caused by exciting the electro-magnet. For this pur- 

 pose the movable poles were insulated from the main body of 



the magnet by interposing a sheet of ebonite thick enough to 

 prevent the passage of the discharge. The discharge was then 

 effected either in the open air or in a closed chamber. The 

 latter was constructed of a short cylinder of glass, say 3 inches 

 in length and 2 in diameter, having conical ends pointed in- 

 wards, so as to receive the poles of the magnet. The chamber 

 was also furnished with a pipe and stop-cock for the purposes 

 of exhaustion. 



The discharge from an induction-coil taken in air or other gas 

 at atmospheric pressure, consists, as is well known, primarily of 

 the spark proper or bright line, irregular in form and instanta- 

 neous in duration. But beside this, when the primary wire is 

 thick and the tattery-current strong, the spark is enveloped in a 

 bright cloud, or rather flame, which is capable of being throw n 

 on one side, although not entirely detached from the spark by a 

 current of air. This, when examined in a revolving mirror, i h 

 found to be subsequent in time to the spark proper, and may Iji: 

 considered to be due to the gas in the neighbourhood of the 

 spark becoming sufficiently heated to conduct part of the dis- 

 charge, and to the consequent combustion of any extraneouj 

 matter floating in the medium. Such a view is supported by 

 the fact that the colour of this flame depends partly, upon the 

 nature of the gas in which the discharge takes place, and partly 

 upon that of any volatilisable matter which may be introduced 

 near the poles. 



The exciting of the magnet produces upon 'the] spark prop ;r 

 no appreciable effect ; but as soon as the flame is submitted to 

 its action it is spread out into a sheet, which arranges itself in a 

 hehcoid right-handed or left-handed according to the direct 

 of the current and of the magnetic polarity in obeJience 

 Ampere's law. j 



Effects substantially the same are produced whether the difl 

 charge be taken in gas at atmospheric or at a less pressure. Bm 

 in the former case the helix has a lower, in the latter a steeper 

 gradient ; that is to say, in the former case it presents a greater, 

 in the latter a less number of turns, for a given interval betwc 

 the poles. 



Various gases were tried — atmospheric air, carbonic aciJ, 

 ether, chloroform, coal-gas, hydrogen. Of these the first two 

 succeeded best. With air the illumination of the flame-shr *■ 

 was rather greater ; but with carbonic acid greater steadines- 

 position was obtained. With both ether and chloroform, occ 

 sional flashes, brilliantly illuminated, were seen ; but some 

 chemical action appeared to take place militating against the | 

 steady development of the flame-sheet. With coal-gas there] 

 was an inconvenient deposit of carbon upon the sides of the! 

 chamber. With hydrogen the cloud was not sufficiently dev 

 loped. 



The success which attended the experiment^with air may pi., 

 sibly be partly due, as suggested above, to the combustion of the 

 extraneous matter floating therein ; and in fact the brilliancy 

 and extent of the sheet may be increased by attaching a piece of 

 metallic sodium to the negative terminal, or by causing a stream 

 of any of the chlorides in powder, e.g., of strontium, lithium, 

 &c. , to flow across the field of action. 



When a piece of sodium (or better still of soda) is attached to 

 one of the terminals, two effects may be noticed. When that 

 terminal is negative the whole of the flame is bright yellow, 

 showing that the sodium is not only detached but even carried 

 across the field and deposited on the positive terminal. When, 

 however, the terminal, to which the sodium is attached is posi- 

 tive, it is found that the flame, when observed through a red 

 glass, appears yellow to a certain distance from the (positive) 

 terminal to which the sodium is attached, but red beyond ; and 

 also that the pitch of the helix is less near the position than near 

 the negative terminal. These effects may be attributed to the 

 presence of metallic vapour evolved by the heat at the positive 

 terminal, but not carried across the field as when the terminal in 

 question is negative. 



The following explanation "of the phenomenon, from whici 

 the mathematical part is omitted, is due to Prof. Stokes. Sup- 

 posing the magnetic field to be uniform, the lines of force will 

 be straight lines from pole to pole. In such a condition every- 

 thing being symmetrical no rotation would take place. But if 

 through any local circumstance the path of the current be dis- 

 torted and displaced, then each element will be subject to two 

 forces, one tending to turn the current round the axis, the other 

 tending to make it follow the shortest path so as to diminish the 

 resistance. 



And the general nature of the phenomenon may be described 

 as follows: — "First, we have the bright spark of no sensible 



