210 



NA TURE 



[Jakuarv 2, 1896 



electric field between them, but only weak effects outside. That 

 some such things may possibly occur is indicated by observations 

 made in mountain districts, where violent electrical disturbances 

 are observed previous to the formation of clouds (Trabert, 

 Meteorologische Zeitschrift, 1889, p. 342). The cumulus cloud, 

 from which the lightning strikes out, is nearly always associated 

 with a cirrus layer above it, and the flash occurs more frequently 

 upwards or sidewards between the clouds than down to earth. 

 Under such circumstances it is clear that instruments on the 

 surface of the earth can only very partially indicate the nature 

 and distribution of electrical stress in the neighbourhood of the 

 cloud. 



Thunderstorms seem always to be connected with a vortex 

 motion, and meteorologists distinguish two kinds of thunder- 

 storms. The first kind forms in the outlying portions of a large 

 cyclonic system. The storms which occur in winter are mostly 

 of this nature, and the vortex necessary for its formation is of 

 the nature of a secondary disturbance. The thunderstorm which 

 forms in summer, on the other hand, makes its own vortex, and 

 is of a much more local character than that which is produced 

 round a previously established barometric depression. The 

 summer storm is much influenced by the character of a district. 

 There are certain configurations apparently favourable to its 

 ibrmation, as is clearly brought out by the charts which have 

 been made representing their frequency. 



The route travelled over by the storm is afifected by mountain 

 ridges, and rivers also seem to of^'er a peculiar impediment. 

 Many of them are brought to an end either along their whole 

 front, or only part of it, when they reach the banks of a large 

 river. ^ 



Some curious problems are presented by the detailed structure 

 of lightning flashes. Although these lie outside the range of the 

 present lecture, reference must be made to the very beautiful 

 photographs of lightning flashes taken both in this country and 

 abroad. The ordinary forms which lightning takes are familiar 

 to all ; but a good deal of mystery still surrounds the so-called 

 globular lightning. The manner in which this form appears is 

 best described in the words of eye-witnesses. 



Dr. A. Wartmann gives to the Physical Society of Geneva the 

 following account of what he saw : - — " At half-past six o'clock 

 in the evening I drove from Versoix to Genthoud. On the 

 Malagny road I heard the coachman say he did not know where 

 he was. His eyes were so much fatigued by the frequent and 

 intense lightning discharges, that he was blinded, and could not, 

 even in the intervals, see the road, in spite of the good lanterns 

 alongside. I stepped on to the box and took the reins. We 

 had barely passed the principal gate of the grounds of Dr. 

 Marcet, when I became conscious of a bright and lasting 

 luminosity behind me. Thinking it was a fire, I turned round, 

 and saw, at a distance of, roughly, 300 metres, a ball of fire of 

 about 40 cm. diameter. It travelled in our direction with a 

 velocity about equal to that of a bird of prey, and left no 

 luminous trail behind. Just as the ball had overtaken us, about 

 24 metres to our right, it burst with a terrific noise, and it seemed 

 to rne as if lines of fire started from it. We felt a violent 

 shaking, and remained blinded a few seconds. As soon as I 

 regained power of distinguishing objects, I saw that the horses 

 had turned at a right angle to the carriage, with their chests in 

 the hedge, with drooping ears and all signs of great terror. I 

 returned on the following day to the place where I had seen the 

 ball explode, but could find no sign of any damage. At a dis- 

 tance of 100 metres I found that a group of three trees, border- 

 ing a wood, had their upper branches singed, but it is not possible 

 to say whether this was due to the discharge which I had 

 seen." 



The following is a translation of an account given by Mr. 

 H. W. Roth (Meteorologische Zeitschrift, 1889, p. 231):— 

 " During the thunderstorm of May 19, 1888, at about 6 p.m., a 

 flash of lightning took effect which seems to me remarkable from 

 a physiological point of view. The dealer Werner, from EUer- 

 bruch, and his son (sixteen years old), with a one-horse convey- 

 ance containing rags, were on the road which leads from here to 

 the village of Ottensen, about three miles away in a south- 

 westerly direction. The father had been left a little behind, 

 and the son was occupied in giving bread to the horses, when 

 he found himself suddenly surrounded by light, and noticed a 



3 Bebber, " Meteorologie, ' p. 255 ; Bornstein, Archiv der Seewart viii 

 1885. 



2 Arch, dcs Sci. Phys. et Nat. (3) vol. xxi., 1889. The above account is 

 translated from the Meteorologische Zeitschrijt, 1889. 



NO. 1366, VOL. 53] 



fiery ball, about the size of his fist, moving towards him along the 

 back of the horse. Then he lost consciousness. He felt no 

 concussion. The father, on approaching, saw the horse's limbs 

 still contracting, and at first he thought his son was dead, but 

 succeeded, after considerable efiforts, in bringing him back to life 

 in about three-quarters of an hour. The horse was dead." 



Some curious statistics have been collected, especially in 

 Germany, as to the damage done by lightning flashes. That 

 damage seems to have increased to an enormous extent within 

 the last fifty years, and although in cases of this kind statistics 

 may easily be at fault, there seems no doubt about the reality of 

 the fact, which may find an explanation in the partial cutting 

 down of forests in those parts where thunderstorms chiefly 

 occur. When lightning strikes into forests, it selects certain 

 trees by preference. Thus, in the principality of Lippe, taking 

 the percentage of beeches struck by lightning as unity, that for 

 other trees is as follows : — Oak 48, spruce fir 5, Scotch fir 33. 



The St. Elmo's fire, a continuous discharge from points and 

 sharp angles, is often observed on board ship and in mountain 

 districts during a storm. Its appearance was considered a sign 

 of the approaching end of the lightning, and was looked upon 

 with favour by the ancient sailors in the Mediterranean Sea, 

 who gave to it the name of Castor and Pollux. There was 

 another appearance called Helena, a bad omen, which by many 

 is believed to have been another form of the St. Elmo's fire, and 

 the present name has been stated to be a corruption of the word • 

 Helena. Some support is given to this view by the fact that the 

 Emperor Constantine built a castle in the Pyrenees, which he 

 named after his mother, Helena, and this castle seems to be 

 referred to occasionally as St. Elne or St. Elme. But it is 

 much more probable, as argued by Dr. F. Piper {Pogg. 

 Ami., vol. Ixxxii. p. 317), that the word is derived from St. 

 Erasmo, a bishop who came from Antiochia, and suffered a 

 martyr's death at the beginning of the fourth century. He 

 seems to have been specially considered the patron of Italian 

 sailors. Churches and castles in Naples and Malta were called 

 St. Erasmo and St. Ermo, and Ariosto describes St. Elmo's 

 fires as St. Ermo's fires. The electric discharge which goes 

 under this name has a different appearance according as it is the 

 positive or negative electricity which escapes, and both kinds 

 occur with about equal frequenc)'. 



Although we have not yet arrived at any satisfactory theory of 

 atmospheric electricity, some progress has been made, and this 

 account would not be complete without a short account of the 

 views taken by men of science on the suV)ject. The number of 

 theories proposed is very considerable. Dr. Suchsland,' in a 

 pamphlet published in 1886, gives an account of twenty-four, to 

 which he adds one — his own. The year 1884 alone haS given 

 birth to four theories. 



We may group the theories according to the origin they assign 

 to the source of energy which is involved in the formation of the 

 electric field. All the work we can perform is either derived 

 from the sun or from the earth's rotation. There is, as far as I 

 know, only one theory — that of Edlund — which makes the 

 earth's rotation in space responsible for the separation of electri- 

 cities in the atmosphere. But Edlund's views are not tenable 

 in theory, and, even granting his deductions, the normal fall of 

 potential should, according to the views of the author, have a 

 different sign in the polar and equatorial regions, which is con- 

 trary to the observed fact. This theory does not, however, 

 exhaust the possibility of explaining atmospheric electricity as 

 a phenomenon of electromagnetic induction, and it is not dis- 

 proved that in some form or other the rotation of the earth's 

 magnetic field may play a part in the origin of the electric field. 

 The theories which take solar radiation as the source of the 

 energy divide themselves into several groups. We may think of 

 a direct thermo-electric or actinic action, but there is, so far, no 

 experimental support to such views. One of the earliest and 

 most natural suppositions is the belief in evaporation as a source 

 of electrification. This was Volta's theory, and experiments 

 have at various times been produced in its support ; but, so 

 far, no one has been able to invalidate Faraday's conclusion 

 that whenever electrification seemed to appear as a consequence 

 of evaporation, it was really due to secondary causes, such as 

 the friction of the liquid spray against the sides of the containing 

 vessel. Rejecting Volta's theory, there is nothing left but 

 the belief in some form of contact or frictional electricity either 

 between drops of water and air, or water and ice, or any two of 



1 " Die Gemeinschaftliche Ursache der Elektrischen Meteore und des 

 Hagels," H. W. Schmidt, Halle-a.-S. 



