17. A charged body in air loses its charge in more than one way. The air, as is now known, has always present in it ions, some carrying a positive and others a negative charge, and those having the opposite sign to the charged body are attracted and tend to discharge it. The rate of loss of charge is thus largely dependent on the extent to which ions are present in the surrounding air. It depends, however, in addition on the natural mobility of the ions, and also on the opportunities for convection. Of late years many observations have been made of the ionic charges in air. The best-known apparatus for the purpose is that devised by Ebert. A cylinder condenser has its inner surface insulated and charged to a high positive or negative potential. Air is drawn by an aspirator between the surfaces, and the ions having the opposite sign to the inner cylinder are deposited on it. The charge given up to the inner cylinder is known from its loss of potential. The volume of air from which the ions have been extracted being known, a measure is obtained of the total charge on the ions, whether positive or negative. The conditions must, of course, be such as to secure that no ions shall escape, otherwise there is an underestimate. I+ is used to denote the charge on positive ions, I- that on negative ions. The unit to which they are ordinarily referred is 1 electrostatic unit of electricity per cubic metre of air. For the ratio of the mean value of I+to the mean value of I−, the letter Q is employed by Gockel (55), who has made an unusually complete study of ionic charges at Freiburg. Numerous observations were also made by Simpson (10)—thrice a day—at Karasjok, and von Schweidler has made a good many observations about 3P.M.at Mattsee (46) in 1905, and Seewalchen (38) in 1904. These will suffice to give a general idea of the mean values met with.Station.Authority.I+I−QFreiburgGockel0.340.241.41KarasjokSimpson0.380.331.17Mattseevon Schweidler0.350.291.19Seewalchenvon Schweidler0.450.381.17Gockel’s mean values of I+and Q would be reduced to 0.31 and 1.38 respectively if his values for July—which appear abnormal—were omitted. I+and I−both show a considerable range of values, even at the same place during the same season of the year. Thus at Seewalchen in the course of a month’s observations at 3P.M., I+varied from 0.31 to 0.67, and I−from 0.17 to 0.67.There seems a fairly well marked annual variation in ionic contents, as the following figures will show. Summer and winter represent each six months and the results are arithmetic means of the monthly values.Freiburg.Karasjok.I+I−QI+I−QWinter0.290.211.490.330.271.22Summer0.390.281.340.440.391.13If the exceptional July values at Freiburg were omitted, the summer values of I+and Q would become 0.33 and 1.25 respectively.18.Diurnal Variation.—At Karasjok Simpson found the mean values of I+and I−throughout the whole year much the same between noon and 1P.M.as between 8 and 9A.M.Observations between 6 and 7P.M.gave means slightly lower than those from the earlier hours, but the difference was only about 5% in I+and 10% in I−. The evening values of Q were on the whole the largest. At Freiburg, Gockel found I+and I−decidedly larger in the early afternoon than in either the morning or the late evening hours. His greatest and least mean hourly values and the hours of their occurrence are as follows:—Winter.Summer.I+I−I+I−Max.Min.Max.Min.Max.Min.Max.Min.0.3330.1930.2420.1300.4300.2440.3330.1922 PM7 PM2 PM8 PM4 PM9 to4 PM9 to10 PM10 PMGockel did not observe between 10P.M.and 7A.M.19. Ionization seems to increase notably as temperature rises. Thus at Karasjok Simpson found for mean values:—Temp. less than −20°−10° to −5°10° to 15°I+= 0.18, I−= 0.36I+= 0.36, I−= 0.30I+= 0.45, I−= 0.43Simpson found no clear influence of temperature on Q. Gockel observed similar effects at Freiburg—though he seems doubtful whether the relationship is direct—but the influence of temperature on I+ seemed reduced when the ground was covered with snow. Gockel found a diminution of ionization with rise of relative humidity. Thus for relative humidities between 40 and 50 mean values were 0.306 for I+and 0.219 for I−; whilst for relative humidities between 90 and 100 the corresponding means were respectively 0.222 and 0.134. At Karasjok, Simpson found a slight decrease in I−as relative humidity increased, but no certain change in I+. Specially large values of I+and I−have been observed at high levels in balloon ascents. Thus on the 1st of July 1901, at a height of 2400 metres, H. Gerdien (29) obtained 0.86 for I+and 1.09 for I−.20. In 1901 Elster and Geitel found that a radioactive emanation is present in the atmosphere. Their method of measuring the radioactivity is as follows (48): A wire not exceeding 1 mm. in diameter, charged to a negative potential of at least 2000 volts, is supported between insulators in the open, usually at a height of about 2 metres. After two hours’ exposure, it is wrapped round a frame supported in a given position relative to Elster and Geitel’s dissipation apparatus, and the loss of charge is noted. This loss is proportional to the length of the wire. The radioactivity is denoted by A, and A=1 signifies that the potential of the dissipation apparatus fell 1 volt in an hour per metre of wire introduced. The loss of the dissipation body due to the natural ionization of the air is first allowed for. Suppose, for instance, that in the absence of the wire the potential falls from 264 to 255 volts in 15 minutes, whilst when the wire (10 metres long) is introduced it falls from 264 to 201 volts in 10 minutes, then10A = (254 − 201) × 6 − (264 − 255) × 4 = 342; or A = 34.2.The values obtained for A seem largely dependent on the station.At Wolfenbüttel, a year’s observations by Elster and Geitel (56) made A vary from 4 to 64, the mean being 20. In the island of Juist, off the Friesland coast, from three weeks’ observations they obtained only 5.2 as the mean. On the other hand, at Altjoch, an Alpine station, from nine days’ observations in July 1903 they obtained a mean of 137, the maximum being 224, and the minimum 92. At Freiburg, from 150 days’ observations near noon in 1903-1904, Gockel (57) obtained a mean of 84, his extreme values being 10 and 420. At Karasjok, observing several times throughout the day for a good many months, Simpson (10) obtained a mean of 93 and a maximum of 432. The same observer from four weeks’ observations at Hammerfest got the considerably lower mean value 58, with a maximum of 252. At this station much lower values were found for A with sea breezes than with land breezes. Observing on the pier at Swinemünde in August and September 1904, Lüdeling (40) obtained a mean value of 34.Elster and Geitel (58), having found air drawn from the soil highly radioactive, regard ground air as the source of the emanation in the atmosphere, and in this way account for the low values they obtained for A when observing on or near the sea. At Freiburg in winter Gockel (55) found A notably reduced when snow was on the ground, I+being also reduced. When the ground was covered by snow the mean value of A was only 42, as compared with 81 when there was no snow.J.C. McLennan (59) observing near the foot of Niagara found A only about one-sixth as large as at Toronto. Similarly at Altjoch, Elster and Geitel (56) found A at the foot of a waterfall only about one-third of its normal value at a distance from the fall.21.Annual and Diurnal Variations.—At Wolfenbüttel, Elster and Geitel found A vary but little with the season. At Karasjok, on the contrary, Simpson found A much larger at midwinter—notwithstanding the presence of snow—than at midsummer. His mean value for November and December was 129, while his mean for May and June was only 47. He also found a marked diurnal variation, A being considerably greater between 3 and 5A.M.or 8.30 to 10.30P.M.than between 10A.M.and noon, or between 3 and 5P.M.At all seasons of the year Simpson found A rise notably with increase of relative humidity. Also, whilst the mere absolute height of the barometer seemed of little, if any, importance, he obtained larger values of A with a falling than with a rising barometer. This last result of course is favourable to Elster and Geitel’s views as to the source of the emanation.22. For a wire exposed under the conditions observed by Elster and Geitel the emanation seems to be almost entirely derived from radium. Some part, however, seems to be derived from thorium, and H.A. Bumstead (60) finds that with longer exposure of the wire the relative importance of the thorium emanation increases. With three hours’ exposure he found the thorium emanation only from 3 to 5% of the whole, but with 12 hours’ exposure the percentage of thorium emanation rose to about 15. These figures refer to the state of the wire immediately after the exposure; the rate of decay is much more rapid for the radium than for the thorium emanation.23. The different elements—potential gradient, dissipation, ionization and radioactivity—are clearly not independent of one another. The loss of a charge is naturally largely dependent on the richness of the surrounding air in ions. This is clearly shown by the following results obtained by Simpson (10) at Karasjok for the mean values ofa±corresponding to certain groups of values of I±. To eliminate the disturbing influence of wind, different wind strengths are treated separately.TableVIII.—Mean Values ofa±.WindStrength.I±0 to 0.1.0.1 to 0.20.2 to 0.30.3 to 0.40.4 to 0.50 to 10.450.601.262.043.031 to 20.651.081.852.923.832 to 3....2.703.885.33Simspon concluded that for a given wind velocity dissipation is practically a linear function of ionization.24. Table IX. will give a general idea of the relations of potential gradient to dissipation and ionization.TableIX.—Potential, Dissipation, Ionization.Potentialgradientsvolts permetre.qKarasjok (Simpson (10)).Kremsmünster (41).Freiburg (43).Rothhorn (43).a+a−I+I−Q0 to 50..1.12............50 to 1001.141.31..4.294.670.430.391.11100 to 1501.241.69..3.383.930.370.321.15150 to 2001.481.84..1.852.580.360.281.28200 to 300....3.211.371.580.260.191.42300 to 400....4.330.600.85......400 to 500....5.46..........500 to 700....8.75..........If we regard the potential gradient near the ground as representing a negative charge on the earth, then if the source of supply of that charge is unaffected the gradient will rise and become high when the operations by which discharge is promoted slacken their activity. A diminution in the number of positive ions would thus naturally be accompanied by a rise in potential gradient. Table IX. associates with rise in potential gradient a reduced number of both positive and negative ions and a diminished rate of dissipation whether of a negative or a positive charge. The rise inqand Q indicates that the diminished rate of dissipation is most marked for positive charges, and that negative ions are even more reduced then positive.At Kremsmünster Zölss (41) finds a considerable similarity between the diurnal variations inqand in the potential gradient, the hours of the forenoon and afternoon maxima being nearly the same in the two cases.No distinct relationship has yet been established between potential gradient and radioactivity. At Karasjok Simpson (10) found fairly similar mean values of A for two groups of observations, one confined to cases when the potential gradient exceeded +400 volts, the other confined to cases of negative gradient.At Freiburg Gockel (55,57) found that when observations were grouped according to the value of A there appeared a distinct rise in botha−and I+with increasing A. For instance, when A lay between 100 and 150 the mean value of a- was 1.27 times greater than when A lay between 0 and 50; while when A lay between 120 and 150 the mean value of I+ was 1.53 times larger than when A lay between 0 and 30. These apparent relationships refer to mean values. In individual cases widely different values ofa−or I+are associated with the same value of A.25. If V be the potential, ρ the density of free electricity at a point in the atmosphere, at a distance r from the earth’s centre, then assuming statical conditions and neglecting variation of V in horizontal directions, we haver−2(d/dr)(r² dV/dr) + 4πρ = 0.For practical purposes we may treat r² as constant, and replace d/dr by d/dh, where h is height in centimetres above the ground.We thus findρ = −(1/4π) d²V/dh².If we take a tube of force 1 sq. cm. in section, and suppose it cut by equipotential surfaces at heights h1and h2above the ground, we have for the total charge M included in the specified portion of the tube4πM = (dV/dh)h1− (dV/dh)h2.Taking Linke’s (28) figures as given in § 10, and supposing h1= 0, h2= 15 × 104, we find for the charge in the unit tube between the ground and 1500 metres level, remembering that the centimetre is now the unit of length, M = (1/4π) (125 − 25)/100. Taking 1 volt equal1⁄300of an electrostatic unit, we find M = 0.000265. Between 1500 and 4000 metres the charge inside the unit tube is much less, only 0.000040. The charge on the earth itself has its surface density given by σ = −(1/4π) × 125 volts per metre, = 0.000331 in e ectrostatic units. Thus, on the view now generally current, in the circumstances answering to Linke’s experiments we have on the ground a charge of −331 × 10−6C.G.S. units per sq. cm. Of the corresponding positive charge, 265 × 10−6lies below the 1500 metres level, 40 × 10−6between this and the 4000 metres level, and only 26 × 10−6above 4000 metres.There is a difficulty in reconciling observed values of the ionization with the results obtained from balloon ascents as to the variation of the potential with altitude. According to H. Gerdien (61), near the ground a mean value for d²V/dh² is −(1⁄10) volt/(metre)². From this we deduce for the charge ρ per cubic centimetre (1/4π) × 10−5(volt/cm²), or 2.7 × 10−9electrostatic units. But taking, for example, Simpson’s mean values at Karasjok, we have observedρ ≡ I+− I1= 0.05 × (cm./metre)3= 5 × 10−8,and thus (calculated ρ)/(observed ρ) = 0.05 approximately. Gerdien himself makes I+− I−considerably larger than Simpson, and concludes that the observed value of ρ is from 30 to 50 times that calculated. The presumption is either that d²V/dh² near the ground is much larger numerically than Gerdien supposes, or else that the ordinary instruments for measuring ionization fail to catch some species of ion whose charge is preponderatingly negative.26. Gerdien (61) has made some calculations as to the probable average value of the vertical electric current in the atmosphere in fine weather. This will be composed of a conduction and a convection current, the latter due to rising or falling air currents carrying ions. He supposes the field near the earth to be 100 volts per metre, or1⁄300electrostatic units. For simplicity, he assumes I+and I−each equal 0.25 × 10−6electrostatic units. The specific velocities of the ions—i.e.the velocities in unit field—he takes to be 1.3 × 300 for the positive, and 1.6 × 300 for the negative. The positive andnegative ions travel in opposite directions, so the total current is (1⁄300)(0.25 × 10−6)(1.3 × 300 + 1.6 × 300), or 73 × 10−8in electrostatic measure, otherwise 2.4 × 10−16amperes per sq. cm. As to the convection current, Gerdien supposes—as in § 25—ρ = 2.7 × 10−9electrostatic units, and on fine days puts the average velocity of rising air currents at 10 cm. per second. This gives a convection current of 2.7 × 10−8electrostatic units, or about1⁄27of the conduction current. For the total current we have approximately 2.5 × 10−16amperes per sq. cm. This is insignificant compared to the size of the currents which several authorities have calculated from considerations as to terrestrial magnetism (q.v.). Gerdien’s estimate of the convection current is for fine weather conditions. During rainfall, or near clouds or dust layers, the magnitude of this current might well be enormously increased; its direction would naturally vary with climatic conditions.27. H. Mache (62) thinks that the ionization observed in the atmosphere may be wholly accounted for by the radioactive emanation. If this is true we should have q = αn², where q is the number of ions of one sign made in 1 cc. of air per second by the emanation, α the constant of recombination, and n the number of ions found simultaneously by, say, Ebert’s apparatus. Mache and R. Holfmann, from observations on the amplitude of saturation currents, deduce q = 4 as a mean value. Taking for α Townsend’s value 1.2 × 10−6, Mache finds n = 1800. The charge on an ion being 3.4 × 10−10Mache deduces for the ionic charge, I+or I−, per cubic metre 1800 × 3.4 × 10−10× 106, or 0.6. This is at least of the order observed, which is all that can be expected from a calculation which assumes I+and I−equal. If, however, Mache’s views were correct, we should expect a much closer connexion between I and A than has actually been observed.28. C.T.R. Wilson (63) seems disposed to regard the action of rainfall as the most probable source of the negative charge on the earth’s surface. That great separation of positive and negative electricity sometimes takes place during rainfall is undoubted, and the charge brought to the ground seems preponderatingly negative. The difficulty is in accounting for the continuance in extensive fine weather districts of large positive charges in the atmosphere in face of the processes of recombination always in progress. Wilson considers that convection currents in the upper atmosphere would be quite inadequate, but conduction may, he thinks, be sufficient alone. At barometric pressures such as exist between 18 and 36 kilometres above the ground the mobility of the ions varies inversely as the pressure, whilst the coefficient of recombination α varies approximately as the pressure. If the atmosphere at different heights is exposed to ionizing radiation of uniform intensity the rate of production of ions per cc., q, will vary as the pressure. In the steady state the number, n, of ions of either sign per cc. is given by n = √(q/α), and so is independent of the pressure or the height. The conductivity, which varies as the product of n into the mobility, will thus vary inversely as the pressure, and so at 36 kilometres will be one hundred times as large as close to the ground. Dust particles interfere with conduction near the ground, so the relative conductivity in the upper layers may be much greater than that calculated. Wilson supposes that by the fall to the ground of a preponderance of negatively charged rain the air above the shower has a higher positive potential than elsewhere at the same level, thus leading to large conduction currents laterally in the highly conducting upper layers.29.Thunder.—Trustworthy frequency statistics for an individual station are obtainable only from a long series of observations, while if means are taken from a large area places may be included which differ largely amongst themselves. There is the further complication that in some countries thunder seems to be on the increase. In temperate latitudes, speaking generally, the higher the latitude the fewer the thunderstorms. For instance, for Edinburgh (64) (1771 to 1900) and London (65) (1763 to 1896) R.C. Mossman found the average annual number of thunderstorm days to be respectively 6.4 and 10.7; while at Paris (1873-1893) E. Renou (66) found 27.3 such days. In some tropical stations, at certain seasons of the year, thunder is almost a daily occurrence. At Batavia (18) during the epoch 1867-1895, there were on the average 120 days of thunder in the year.As an example of a large area throughout which thunder frequency appears fairly uniform, we may take Hungary (67). According to the statistics for 1903, based on several hundred stations, the average number of days of thunder throughout six subdivisions of the country, some wholly plain, others mainly mountainous, varied only from 21.1 to 26.5, the mean for the whole of Hungary being 23.5. The antithesis of this exists in the United States of America. According to A.J. Henry (68) there are three regions of maximum frequency: one in the south-east, with its centre in Florida, has an average of 45 days of thunder in the year; a second including the middle Mississippi valley has an average of 35 days; and a third in the middle Missouri valley has 30. With the exception of a narrow strip along the Canadian frontier, thunderstorm frequency is fairly high over the whole of the United States to the east of the 100th meridian. But to the west of this, except in the Rocky Mountain region where storms are numerous, the frequency steadily diminishes, and along the Pacific coast there are large areas where thunder occurs only once or twice a year.30. The number of thunderstorm days is probably a less exact measure of the relativeintensityof thunderstorms than statistics as to the number of persons killed annually by lightning per million of the population. Table X. gives a number of statistics of this kind. The letter M stands for “Midland.”TableX.—Deaths by Lightning, per annum, per million Inhabitants.Hungary7.7Upper Missouri and Plains15Netherlands2.8Rocky Mountains and Plateau10England, N. M.1.8South Atlantic8” E.1.3Central Mississippi7” S. M.1.2Upper ”7” York and W. M.1.1Ohio Valley7” N.1.0Middle Atlantic6Wales0.9Gulf States5England, S. E.0.8New England4” N. W.0.7Pacific Coast<1*” S. W.0.6North and South Dakota20London0.1California0* Note in case of Pacific coast, Table X., “<1” means “less than 1.”The figure for Hungary is based on the seven years 1897-1903; that for the Netherlands, from data by A.J. Monné (69) on the nine years 1882-1890. The English data, due to R. Lawson (70), are from twenty-four years, 1857-1880; those for the United States, due to Henry (68), are for five years, 1896-1900. In comparing these data allowance must be made for the fact that danger from lightning is much greater out of doors than in. Thus in Hungary, in 1902 and 1903, out of 229 persons killed, at least 171 were killed out of doors. Of the 229 only 67 were women, the only assignable explanation being their rarer employment in the fields. Thus,ceteris paribtis, deaths from lightning are much more numerous in a country than in an industrial population. This is well brought out by the low figure for London. It is also shown conspicuously in figures given by Henry. In New York State, where the population is largely industrial, the annual deaths per million are only three, but of the agricultural population eleven. In states such as Wyoming and the Dakotas the population is largely rural, and the deaths by lightning rise in consequence. The frequency and intensity of thunderstorms are unquestionably greater in the Rocky Mountain than in the New England states, but the difference is not so great as the statistics at first sight suggest.TableXI.—Annual Variation of Thunderstorms.Jan.Feb.March.April.May.June.July.Aug.Sept.Oct.Nov.Dec.Ediburgh1.81.41.43.812.320.828.219.17.02.31.10.8London0.60.51.66.612.718.325.519.29.33.11.70.9Paris0.20.42.37.514.921.622.017.09.93.50.40.4Netherlands2.21.83.76.514.014.715.614.710.310.13.82.5France2.22.84.18.413.818.714.613.510.06.33.12.4Switzerland0.20.30.54.911.922.929.918.09.81.10.30.2Hungary (a)0.00.11.65.720.925.023.215.95.71.30.40.2Hungary (b)0.00.01.03.211.820.630.725.36.90.50.00.0United States0.10.11.24.014.325.027.220.45.81.40.30.1Hong-Kong0.02.14.38.512.823.414.921.310.62.10.00.0Trevandrum3.23.813.120.918.64.91.23.52.512.912.03.3Batavia10.49.211.110.57.95.54.33.85.48.812.210.931. Even at the same place thunderstorms vary greatly in intensity and duration. Also the times of beginning and ending are difficult to define exactly, so that several elements of uncertainty exist in data as to the seasonal or diurnal variation. The monthly data in Table XI. are percentages of the total for the year. In most cases the figures are based on the number of days of thunder at a particular station, or at the average station of a country; but the second set for Hungary relates to the number of lightning strokes causing fire, and the figures for the United States relate to deaths by lightning. The data for Edinburgh, due to R.C. Mossman (64), refer to 130 years, 1771 to 1900. The data for London (1763-1896) are also due toMossman (65); for Paris (1873-1893) to Renou (66); for the Netherlands (1882-1900) to A.J. Monné (69); for France(71) (1886-1899) to Frou and Hann; for Switzerland to K. Hess (72); for Hungary (67) (1896-1903) to L. von Szalay and others; for the United States (1890-1900) to A.J. Henry (68); for Hong-Kong (73) (1894-1903) to W. Doberck. The Trevandrum (74) data (1853-1864) were due originally to A. Broun; the Batavia data (1867-1895) are from the BataviaObservations, vol. xviii.Most stations in the northern hemisphere have a conspicuous maximum at midsummer with little thunder in winter. Trevandrum (8° 31′ N.) and Batavia (6° 11′ S.), especially the former, show a double maximum and minimum.TableXII.—Diurnal Variation of Thunderstorms.Hour.0-2.2-4.4-6.6-8.8-10.10-12.0′-2′.2′-4′.4′-6′.6′-8′.8′-10′.10′-12′.Finland (76)2.32.02.23.04.612.118.919.216.110.16.13.4Edinburgh (64)1.72.01.41.74.714.222.423.711.99.25.12.0Belgium (77)3.02.91.71.82.06.412.921.619.415.88.44.1Brocken (78)1.62.51.31.34.23.112.128.622.410.17.25.6Switzerland (72)3.12.32.11.62.07.313.820.920.814.68.03.5Italy (77)1.31.61.42.03.08.519.526.516.69.88.31.5Hungary (i.) (67)2.11.91.92.12.911.518.122.017.910.76.22.8Hungary (ii.) (67)6.94.22.32.02.05.09.916.918.210.711.710.0Hungary (iii.) (75)2.31.92.02.42.77.916.122.119.112.77.63.2Hungary (iv.) (75)2.62.21.91.93.613.319.920.715.29.26.23.3Trevandrum (74)5.64.94.31.31.42.013.324.515.913.37.65.9Agustia (74)2.92.90.30.01.72.915.136.122.29.34.62.032.Daily Variation.—The figures in Table XII. are again percentages. They are mostly based on data as to the hour of commencement of thunderstorms. Data as to the hour when storms are most severe would throw the maximum later in the day. This is illustrated by the first two sets of figures for Hungary (67). The first set relate as usual to the hour of commencement, the second to the hours of occurrence of lightning causing fires. Of the two other sets of figures for Hungary (75), (iii.) relates to the central plain, (iv.) to the mountainous regions to north and south of this. The hour of maximum is earlier for the mountains, thunder being more frequent there than in the plains between 8A.M.and 4P.M., but less frequent between 2 and 10P.M.Trevandrum (8° 31′ N., 76° 59′ E., 195 ft. above sea-level) and Agustia (8° 37′ N., 77° 20′ E., 6200 ft. above sea-level) afford a contrast between low ground and high ground in India. In this instance there seems little difference in the hour of maximum, the distinguishing feature being the great concentration of thunderstorm occurrence at Agustia between noon and 6P.M.TableXIII.Year.Nether-lands.France.Hungary.U.S.A.Year.Nether-lands.France.Hungary.U.S.A.188298..141..18931022882332091883117..195..1894111300333336188495..229..1895119309280426188593..192..18961092662993411886102251319..1897119297350362188778292236..189895299386367188894286232..18991122993685631889126294258..1900108..401713189093299265..1901....502..1891983173022041902....322..1892863243502511903....256..33. Table XIII. gives some data as to the variability of thunder from year to year. The figures for the Netherlands (69) and France (71) are the number of days when thunder occurred somewhere in the country. Its larger area and more varied climate give a much larger number of days of thunder to France. Notwithstanding the proximity of the two countries, there is not much parallelism between the data. The figures for Hungary (67) give the number of lightning strokes causing fire; those for the United States (68) give the number of persons killed by lightning. The conspicuous maximum in 1901 and great drop in 1902 in Hungary are also shown by the statistics as to the number of days of thunder. This number at the average station of the country fell from 38.4 in 1901 to 23.1 in 1902. On the whole, however, the number of destructive lightning strokes and of days of thunder do not show a close parallelism.TableXIV.Decade ending1810.1820.1830.1840.1850.1860.1870.1880.1890.1900.Edinburgh4.95.77.76.75.76.55.410.69.49.2London9.58.311.511.810.511.99.615.713.0..Tilsit....12.512.116.115.311.917.621.8..Germany, South..........496691143175” West..........92106187244331” North..........124135245288352” East..........102143186210273” Whole..........9011618925431834. Table XIV. deals with the variation of thunder over longer periods. The data for Edinburgh (64) and London (65) due to Mossman, and those for Tilsit, due to C. Kassner (79), represent the average number of days of thunder per annum. The data for Germany, due to O. Steffens (80), represent the average number of houses struck by lightning in a year per million houses; in the first decade only seven years (1854-1860) are really included. Mossman thinks that the apparent increase at Edinburgh and London in the later decades is to some extent at least real. The two sets of figures show some corroborative features, notably the low frequency from 1860 to 1870. The figures for Germany—representing four out of six divisions of that country—are remarkable. In Germany as a whole, out of a million houses the number struck per annum was three and a half times as great in the decade 1890 to 1900 as between 1854 and 1860. Von Bezold (81) in an earlier memoir presented data analogous to Steffens’, seemingly accepting them as representing a true increase in thunderstorm destructiveness. Doubts have, however, been expressed by others—e.g.A. Gockel,Das Gewitter, p. 106—as to the real significance of the figures. Changes in the height or construction of buildings, and a greater readiness to make claims on insurance offices, may be contributory causes.35. The fact that a considerable number of people sheltering under trees are killed by lightning is generally accepted as a convincing proof of the unwisdom of the proceeding. When there is an option between a tree and an adjacent house, the latter is doubtless the safer choice. But when the option is between sheltering under a tree and remaining in the open it is not so clear. In Hungary (67), during the three years 1901 to 1903, 15% of the total deaths by lightning occurred under trees, as against 57% wholly in the open. In the United States (68) in 1900, only 10% of the deaths where the precise conditions were ascertained occurred under trees, as against 52% in the open. If then the risk under trees exceeds that in the open in Hungary and the United States, at least five or six times as many people must remain in the open as seek shelter under trees. An isolated tree occupying an exposed position is, it should be remembered, much more likely to be struck than the average tree in the midst of a wood. A good deal also depends on the species of tree. A good many years’ data for Lippe (82) in Germany make the liability to lightning stroke as follows—the number of each species being supposed the same:—Oak 57, Fir 39, Pine 5, Beech 1. In Styria, according to K. Prohaska (83), the species most liable to be struck are oaks, poplars and pear trees; beech trees again are exceptionally safe. It should, however, be borne in mind that the apparent differences between different species may be partly a question of height, exposure or proximity to water. A good deal may also depend on the soil. According to Hellmann, as quoted by Henry (82), the liability to lightning stroke in Germany may be put at chalk 1, clay 7, sand 9, loam 22.36. Numerous attempts have been made to find periodic variations in thunderstorm frequency. Among the periods suggested are the 11-year sun-spot period, or half this (cf. v. Szalay (67)). Ekholm and Arrhenius (84) claim to have established the existence of a tropical lunar period, and a 25.929-day period; while P. Polis (85) considers a synodic lunar period probable. A.B. MacDowall (86) and others have advanced evidence in favour of the view that thunderstorms are most frequent near new moon and fewest near full moon. Much more evidence would be required to produce a general acceptance of any of the above periods.37.St Elmo’s Fire.—Luminous discharges from masts, lightning conductors, and other pointed objects are not very infrequent, especially during thunderstorms. On the Sonnblick, where the phenomenon is common, Elster and Geitel (87) have found St Elmo’s fire to answer to a discharge sometimes of positive sometimes of negative electricity. The colour and appearance differ in the two cases, red predominating in a positive, blue in a negative discharge. The differences characteristic of the two forms of discharge are described and illustrated in Gockel’sDas Gewitter. Gockel states (l.c. p. 74) that during snowfall the sign is positive or negative according as the flakes are large or are small and powdery. The discharge is not infrequently accompanied by a sizzling sound.38. Of late years many experiments have been made on the influence of electric fields or currents on plant growth. S. Lemström (88), who was a pioneer in this department, found an electric field highly beneficial in some but not in all cases. Attempts have been made to apply electricity to agriculture on a commercial scale, but the exact measure of success attained remains somewhat doubtful. Lemström believed atmospheric electricity to play an important part in the natural growth of vegetation, and he assigned a special rôle to the needles of fir and pine trees.Bibliography.—The following abbreviations are here used:—M.Z.,Meteorologische Zeitschrift; P.Z.,Physikalische Zeitschrift; S.,Sitzungsberichte k. Akad. Wiss. Wien, Math. Naturw. Klasse, Theil ii. 2; P.T., “Philosophical Transactions Royal Society of London”; T.M.,Terrestrial Magnetism, edited by Dr L.A. Bauer.Text-books:—(1) G. le Cadet,Étude du champ électrique de l’atmosphère(Paris, 1898); (2) Svante A. Arrhenius,Lehrbuch der kosmischen Physik(Leipzig, 1903); (3) A. Gockel,Das Gewitter(Cologne, 1905).Lists of original authorities:—(4) F. Exner,M.Z., vol. 17, 1900, p. 529 (especially pp. 542-3); (5) G.C. Simpson,Q.J.R. Met. Soc., vol. 31, 1905, p. 295 (especially pp. 305-6). 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Zölss,P.Z., vol. 5, p. 260; (24)T.M., vol. 7, 1902, p. 89; (25)Revue générale des sciences, 1906, p. 442; (26)T.M., vol. 8, 1903, p. 86. and vol. 9, 1904, p. 147; (27)S., 93, p. 222; (28)M.Z., vol. 22, 1905, p. 237; (29)P.Z., vol. 4, p. 632; (30)Phil. Mag., vol. 20, 1885, p. 456; (31)Expédition polaire finlandaise, vol. 3 (Helsingfors, 1898); (32) A. Paulsen,Bull. de l’Acad. ... de Danemarke, 1894, p. 148; (33)Wied. Ann., vol. 46, 1892, p. 584; (34)P.T., vol. 191 A, p. 187; (35)M.Z., vol. 5, 1888, p. 95;S., 99, p. 421;T.M., vol. 4, 1899, p. 15; (36)Camb. Phil. Soc. Proc., vol. 11, p. 428, and vol. 12, pp. 17 and 85; (37)P.Z., vol. 4, pp. 267 and 873; (38) E.R. v. Schweidler,S., 113, p. 1433; (39)S., 111, July 1902; (40)Veröffentl. des Kg. Preuss. Met. Inst., 1904; (41)P.Z., vol. 5, p. 106; (42)S., 114, p. 198; (43)P.Z., vol. 4, p. 871; (44)P.Z., vol. 4, p. 93; (45)M.Z., vol. 23, 1906, p. 229; (46)S., 114, p. 1705; (47)S., 114, p. 399; (48)P.Z., vol. 4, p. 522; (49)S., 113, p. 1455; (50)P.Z., vol. 4, p. 627; (51)P.Z., vol. 4, p. 90; (52)S., 114, p. 151; (53)M.Z., vol. 23, 1906, p. 253; (54)P.Z., vol. 5, p. 749; (55)M.Z., vol. 23, 1906, pp. 53 and 339; (56)P.Z., vol. 5, p. 11; (57)P.Z., vol. 5, p. 591; (58)T.M., vol. 9, 1904, p. 49; (59)P.Z., vol. 4, p. 295; (60)P.Z., vol. 5, p. 504; (61)T.M., vol. 10, 1905, p. 65; (62)S., 114, p. 1377; (63)Camb. Phil. Soc. Proc., vol. 13, p. 363; (64)Trans. R.S. Edin., vol. 39, p. 63, and vol. 40, p. 484; (65)Q.J.R. Met. Soc., vol. 24, 1898, p. 31; (66)M.Z., vol. 11, 1894, p. 277; (67)Jahrbücher der Konigl. Ung. Reichsanstalt für Met. und Erdmag., vol. 33, 1903, III. Theil with appendix by L. von Szalay; (68) U.S. Dept. of Agriculture,Weather Bureau Bulletin, No. 30, 1901; (69)M.Z., vol. 19, 1902, p. 297; (70)Q.J.R. Met. Soc., vol. 15, 1889, p. 140; (71)M.Z., vol. 20, 1903, p. 227; (72)M.Z., vol. 20, 1903, p. 522; (73)M.Z., vol. 23, 1906, p. 367; (74)M.Z., vol. 22, 1905, p. 175; (75) J. Hegyfoky,M.Z., vol. 20, 1903, p. 218; (76)M.Z., vol. 22, 1905, p. 575; (77) S. Arrhenius,M.Z., vol. 5, 1888, p. 348; (78) G. Hellmann,M.Z., vol. 22, 1905, p. 223; (79)M.Z., vol. 11, 1894, p. 239; (80)M.Z., vol. 23, 1906, p. 468; (81)Berlin Sitz., 1889, No. 16; (82) A.J. Henry,U.S. Dept. of Agriculture Bull., No. 26, 1899; (83)M.Z., vol. 16, 1899, p. 128; (84)K. Sven. Vet. Akad. Hand., Bd. 19, No. 8, Bd. 20, No. 6, Bd. 31, Nos. 2 and 3; (85)M.Z., vol. 11, 1894, p. 230; (86)Nature, vol. 65, 1902, p. 367; (87)M.Z., vol. 8, 1891, p. 321; (88)Brit. Assoc. Reportfor 1898, p. 808, alsoElectricity in Agriculture and Horticulture(London, 1904).
17. A charged body in air loses its charge in more than one way. The air, as is now known, has always present in it ions, some carrying a positive and others a negative charge, and those having the opposite sign to the charged body are attracted and tend to discharge it. The rate of loss of charge is thus largely dependent on the extent to which ions are present in the surrounding air. It depends, however, in addition on the natural mobility of the ions, and also on the opportunities for convection. Of late years many observations have been made of the ionic charges in air. The best-known apparatus for the purpose is that devised by Ebert. A cylinder condenser has its inner surface insulated and charged to a high positive or negative potential. Air is drawn by an aspirator between the surfaces, and the ions having the opposite sign to the inner cylinder are deposited on it. The charge given up to the inner cylinder is known from its loss of potential. The volume of air from which the ions have been extracted being known, a measure is obtained of the total charge on the ions, whether positive or negative. The conditions must, of course, be such as to secure that no ions shall escape, otherwise there is an underestimate. I+ is used to denote the charge on positive ions, I- that on negative ions. The unit to which they are ordinarily referred is 1 electrostatic unit of electricity per cubic metre of air. For the ratio of the mean value of I+to the mean value of I−, the letter Q is employed by Gockel (55), who has made an unusually complete study of ionic charges at Freiburg. Numerous observations were also made by Simpson (10)—thrice a day—at Karasjok, and von Schweidler has made a good many observations about 3P.M.at Mattsee (46) in 1905, and Seewalchen (38) in 1904. These will suffice to give a general idea of the mean values met with.
Gockel’s mean values of I+and Q would be reduced to 0.31 and 1.38 respectively if his values for July—which appear abnormal—were omitted. I+and I−both show a considerable range of values, even at the same place during the same season of the year. Thus at Seewalchen in the course of a month’s observations at 3P.M., I+varied from 0.31 to 0.67, and I−from 0.17 to 0.67.
There seems a fairly well marked annual variation in ionic contents, as the following figures will show. Summer and winter represent each six months and the results are arithmetic means of the monthly values.
If the exceptional July values at Freiburg were omitted, the summer values of I+and Q would become 0.33 and 1.25 respectively.
18.Diurnal Variation.—At Karasjok Simpson found the mean values of I+and I−throughout the whole year much the same between noon and 1P.M.as between 8 and 9A.M.Observations between 6 and 7P.M.gave means slightly lower than those from the earlier hours, but the difference was only about 5% in I+and 10% in I−. The evening values of Q were on the whole the largest. At Freiburg, Gockel found I+and I−decidedly larger in the early afternoon than in either the morning or the late evening hours. His greatest and least mean hourly values and the hours of their occurrence are as follows:—
Gockel did not observe between 10P.M.and 7A.M.
19. Ionization seems to increase notably as temperature rises. Thus at Karasjok Simpson found for mean values:—
Simpson found no clear influence of temperature on Q. Gockel observed similar effects at Freiburg—though he seems doubtful whether the relationship is direct—but the influence of temperature on I+ seemed reduced when the ground was covered with snow. Gockel found a diminution of ionization with rise of relative humidity. Thus for relative humidities between 40 and 50 mean values were 0.306 for I+and 0.219 for I−; whilst for relative humidities between 90 and 100 the corresponding means were respectively 0.222 and 0.134. At Karasjok, Simpson found a slight decrease in I−as relative humidity increased, but no certain change in I+. Specially large values of I+and I−have been observed at high levels in balloon ascents. Thus on the 1st of July 1901, at a height of 2400 metres, H. Gerdien (29) obtained 0.86 for I+and 1.09 for I−.
20. In 1901 Elster and Geitel found that a radioactive emanation is present in the atmosphere. Their method of measuring the radioactivity is as follows (48): A wire not exceeding 1 mm. in diameter, charged to a negative potential of at least 2000 volts, is supported between insulators in the open, usually at a height of about 2 metres. After two hours’ exposure, it is wrapped round a frame supported in a given position relative to Elster and Geitel’s dissipation apparatus, and the loss of charge is noted. This loss is proportional to the length of the wire. The radioactivity is denoted by A, and A=1 signifies that the potential of the dissipation apparatus fell 1 volt in an hour per metre of wire introduced. The loss of the dissipation body due to the natural ionization of the air is first allowed for. Suppose, for instance, that in the absence of the wire the potential falls from 264 to 255 volts in 15 minutes, whilst when the wire (10 metres long) is introduced it falls from 264 to 201 volts in 10 minutes, then
10A = (254 − 201) × 6 − (264 − 255) × 4 = 342; or A = 34.2.
The values obtained for A seem largely dependent on the station.At Wolfenbüttel, a year’s observations by Elster and Geitel (56) made A vary from 4 to 64, the mean being 20. In the island of Juist, off the Friesland coast, from three weeks’ observations they obtained only 5.2 as the mean. On the other hand, at Altjoch, an Alpine station, from nine days’ observations in July 1903 they obtained a mean of 137, the maximum being 224, and the minimum 92. At Freiburg, from 150 days’ observations near noon in 1903-1904, Gockel (57) obtained a mean of 84, his extreme values being 10 and 420. At Karasjok, observing several times throughout the day for a good many months, Simpson (10) obtained a mean of 93 and a maximum of 432. The same observer from four weeks’ observations at Hammerfest got the considerably lower mean value 58, with a maximum of 252. At this station much lower values were found for A with sea breezes than with land breezes. Observing on the pier at Swinemünde in August and September 1904, Lüdeling (40) obtained a mean value of 34.
Elster and Geitel (58), having found air drawn from the soil highly radioactive, regard ground air as the source of the emanation in the atmosphere, and in this way account for the low values they obtained for A when observing on or near the sea. At Freiburg in winter Gockel (55) found A notably reduced when snow was on the ground, I+being also reduced. When the ground was covered by snow the mean value of A was only 42, as compared with 81 when there was no snow.
J.C. McLennan (59) observing near the foot of Niagara found A only about one-sixth as large as at Toronto. Similarly at Altjoch, Elster and Geitel (56) found A at the foot of a waterfall only about one-third of its normal value at a distance from the fall.
21.Annual and Diurnal Variations.—At Wolfenbüttel, Elster and Geitel found A vary but little with the season. At Karasjok, on the contrary, Simpson found A much larger at midwinter—notwithstanding the presence of snow—than at midsummer. His mean value for November and December was 129, while his mean for May and June was only 47. He also found a marked diurnal variation, A being considerably greater between 3 and 5A.M.or 8.30 to 10.30P.M.than between 10A.M.and noon, or between 3 and 5P.M.
At all seasons of the year Simpson found A rise notably with increase of relative humidity. Also, whilst the mere absolute height of the barometer seemed of little, if any, importance, he obtained larger values of A with a falling than with a rising barometer. This last result of course is favourable to Elster and Geitel’s views as to the source of the emanation.
22. For a wire exposed under the conditions observed by Elster and Geitel the emanation seems to be almost entirely derived from radium. Some part, however, seems to be derived from thorium, and H.A. Bumstead (60) finds that with longer exposure of the wire the relative importance of the thorium emanation increases. With three hours’ exposure he found the thorium emanation only from 3 to 5% of the whole, but with 12 hours’ exposure the percentage of thorium emanation rose to about 15. These figures refer to the state of the wire immediately after the exposure; the rate of decay is much more rapid for the radium than for the thorium emanation.
23. The different elements—potential gradient, dissipation, ionization and radioactivity—are clearly not independent of one another. The loss of a charge is naturally largely dependent on the richness of the surrounding air in ions. This is clearly shown by the following results obtained by Simpson (10) at Karasjok for the mean values ofa±corresponding to certain groups of values of I±. To eliminate the disturbing influence of wind, different wind strengths are treated separately.
TableVIII.—Mean Values ofa±.
Simspon concluded that for a given wind velocity dissipation is practically a linear function of ionization.
24. Table IX. will give a general idea of the relations of potential gradient to dissipation and ionization.
TableIX.—Potential, Dissipation, Ionization.
If we regard the potential gradient near the ground as representing a negative charge on the earth, then if the source of supply of that charge is unaffected the gradient will rise and become high when the operations by which discharge is promoted slacken their activity. A diminution in the number of positive ions would thus naturally be accompanied by a rise in potential gradient. Table IX. associates with rise in potential gradient a reduced number of both positive and negative ions and a diminished rate of dissipation whether of a negative or a positive charge. The rise inqand Q indicates that the diminished rate of dissipation is most marked for positive charges, and that negative ions are even more reduced then positive.
At Kremsmünster Zölss (41) finds a considerable similarity between the diurnal variations inqand in the potential gradient, the hours of the forenoon and afternoon maxima being nearly the same in the two cases.
No distinct relationship has yet been established between potential gradient and radioactivity. At Karasjok Simpson (10) found fairly similar mean values of A for two groups of observations, one confined to cases when the potential gradient exceeded +400 volts, the other confined to cases of negative gradient.
At Freiburg Gockel (55,57) found that when observations were grouped according to the value of A there appeared a distinct rise in botha−and I+with increasing A. For instance, when A lay between 100 and 150 the mean value of a- was 1.27 times greater than when A lay between 0 and 50; while when A lay between 120 and 150 the mean value of I+ was 1.53 times larger than when A lay between 0 and 30. These apparent relationships refer to mean values. In individual cases widely different values ofa−or I+are associated with the same value of A.
25. If V be the potential, ρ the density of free electricity at a point in the atmosphere, at a distance r from the earth’s centre, then assuming statical conditions and neglecting variation of V in horizontal directions, we have
r−2(d/dr)(r² dV/dr) + 4πρ = 0.
For practical purposes we may treat r² as constant, and replace d/dr by d/dh, where h is height in centimetres above the ground.
We thus find
ρ = −(1/4π) d²V/dh².
If we take a tube of force 1 sq. cm. in section, and suppose it cut by equipotential surfaces at heights h1and h2above the ground, we have for the total charge M included in the specified portion of the tube
4πM = (dV/dh)h1− (dV/dh)h2.
Taking Linke’s (28) figures as given in § 10, and supposing h1= 0, h2= 15 × 104, we find for the charge in the unit tube between the ground and 1500 metres level, remembering that the centimetre is now the unit of length, M = (1/4π) (125 − 25)/100. Taking 1 volt equal1⁄300of an electrostatic unit, we find M = 0.000265. Between 1500 and 4000 metres the charge inside the unit tube is much less, only 0.000040. The charge on the earth itself has its surface density given by σ = −(1/4π) × 125 volts per metre, = 0.000331 in e ectrostatic units. Thus, on the view now generally current, in the circumstances answering to Linke’s experiments we have on the ground a charge of −331 × 10−6C.G.S. units per sq. cm. Of the corresponding positive charge, 265 × 10−6lies below the 1500 metres level, 40 × 10−6between this and the 4000 metres level, and only 26 × 10−6above 4000 metres.
There is a difficulty in reconciling observed values of the ionization with the results obtained from balloon ascents as to the variation of the potential with altitude. According to H. Gerdien (61), near the ground a mean value for d²V/dh² is −(1⁄10) volt/(metre)². From this we deduce for the charge ρ per cubic centimetre (1/4π) × 10−5(volt/cm²), or 2.7 × 10−9electrostatic units. But taking, for example, Simpson’s mean values at Karasjok, we have observed
ρ ≡ I+− I1= 0.05 × (cm./metre)3= 5 × 10−8,
and thus (calculated ρ)/(observed ρ) = 0.05 approximately. Gerdien himself makes I+− I−considerably larger than Simpson, and concludes that the observed value of ρ is from 30 to 50 times that calculated. The presumption is either that d²V/dh² near the ground is much larger numerically than Gerdien supposes, or else that the ordinary instruments for measuring ionization fail to catch some species of ion whose charge is preponderatingly negative.
26. Gerdien (61) has made some calculations as to the probable average value of the vertical electric current in the atmosphere in fine weather. This will be composed of a conduction and a convection current, the latter due to rising or falling air currents carrying ions. He supposes the field near the earth to be 100 volts per metre, or1⁄300electrostatic units. For simplicity, he assumes I+and I−each equal 0.25 × 10−6electrostatic units. The specific velocities of the ions—i.e.the velocities in unit field—he takes to be 1.3 × 300 for the positive, and 1.6 × 300 for the negative. The positive andnegative ions travel in opposite directions, so the total current is (1⁄300)(0.25 × 10−6)(1.3 × 300 + 1.6 × 300), or 73 × 10−8in electrostatic measure, otherwise 2.4 × 10−16amperes per sq. cm. As to the convection current, Gerdien supposes—as in § 25—ρ = 2.7 × 10−9electrostatic units, and on fine days puts the average velocity of rising air currents at 10 cm. per second. This gives a convection current of 2.7 × 10−8electrostatic units, or about1⁄27of the conduction current. For the total current we have approximately 2.5 × 10−16amperes per sq. cm. This is insignificant compared to the size of the currents which several authorities have calculated from considerations as to terrestrial magnetism (q.v.). Gerdien’s estimate of the convection current is for fine weather conditions. During rainfall, or near clouds or dust layers, the magnitude of this current might well be enormously increased; its direction would naturally vary with climatic conditions.
27. H. Mache (62) thinks that the ionization observed in the atmosphere may be wholly accounted for by the radioactive emanation. If this is true we should have q = αn², where q is the number of ions of one sign made in 1 cc. of air per second by the emanation, α the constant of recombination, and n the number of ions found simultaneously by, say, Ebert’s apparatus. Mache and R. Holfmann, from observations on the amplitude of saturation currents, deduce q = 4 as a mean value. Taking for α Townsend’s value 1.2 × 10−6, Mache finds n = 1800. The charge on an ion being 3.4 × 10−10Mache deduces for the ionic charge, I+or I−, per cubic metre 1800 × 3.4 × 10−10× 106, or 0.6. This is at least of the order observed, which is all that can be expected from a calculation which assumes I+and I−equal. If, however, Mache’s views were correct, we should expect a much closer connexion between I and A than has actually been observed.
28. C.T.R. Wilson (63) seems disposed to regard the action of rainfall as the most probable source of the negative charge on the earth’s surface. That great separation of positive and negative electricity sometimes takes place during rainfall is undoubted, and the charge brought to the ground seems preponderatingly negative. The difficulty is in accounting for the continuance in extensive fine weather districts of large positive charges in the atmosphere in face of the processes of recombination always in progress. Wilson considers that convection currents in the upper atmosphere would be quite inadequate, but conduction may, he thinks, be sufficient alone. At barometric pressures such as exist between 18 and 36 kilometres above the ground the mobility of the ions varies inversely as the pressure, whilst the coefficient of recombination α varies approximately as the pressure. If the atmosphere at different heights is exposed to ionizing radiation of uniform intensity the rate of production of ions per cc., q, will vary as the pressure. In the steady state the number, n, of ions of either sign per cc. is given by n = √(q/α), and so is independent of the pressure or the height. The conductivity, which varies as the product of n into the mobility, will thus vary inversely as the pressure, and so at 36 kilometres will be one hundred times as large as close to the ground. Dust particles interfere with conduction near the ground, so the relative conductivity in the upper layers may be much greater than that calculated. Wilson supposes that by the fall to the ground of a preponderance of negatively charged rain the air above the shower has a higher positive potential than elsewhere at the same level, thus leading to large conduction currents laterally in the highly conducting upper layers.
29.Thunder.—Trustworthy frequency statistics for an individual station are obtainable only from a long series of observations, while if means are taken from a large area places may be included which differ largely amongst themselves. There is the further complication that in some countries thunder seems to be on the increase. In temperate latitudes, speaking generally, the higher the latitude the fewer the thunderstorms. For instance, for Edinburgh (64) (1771 to 1900) and London (65) (1763 to 1896) R.C. Mossman found the average annual number of thunderstorm days to be respectively 6.4 and 10.7; while at Paris (1873-1893) E. Renou (66) found 27.3 such days. In some tropical stations, at certain seasons of the year, thunder is almost a daily occurrence. At Batavia (18) during the epoch 1867-1895, there were on the average 120 days of thunder in the year.
As an example of a large area throughout which thunder frequency appears fairly uniform, we may take Hungary (67). According to the statistics for 1903, based on several hundred stations, the average number of days of thunder throughout six subdivisions of the country, some wholly plain, others mainly mountainous, varied only from 21.1 to 26.5, the mean for the whole of Hungary being 23.5. The antithesis of this exists in the United States of America. According to A.J. Henry (68) there are three regions of maximum frequency: one in the south-east, with its centre in Florida, has an average of 45 days of thunder in the year; a second including the middle Mississippi valley has an average of 35 days; and a third in the middle Missouri valley has 30. With the exception of a narrow strip along the Canadian frontier, thunderstorm frequency is fairly high over the whole of the United States to the east of the 100th meridian. But to the west of this, except in the Rocky Mountain region where storms are numerous, the frequency steadily diminishes, and along the Pacific coast there are large areas where thunder occurs only once or twice a year.
30. The number of thunderstorm days is probably a less exact measure of the relativeintensityof thunderstorms than statistics as to the number of persons killed annually by lightning per million of the population. Table X. gives a number of statistics of this kind. The letter M stands for “Midland.”
TableX.—Deaths by Lightning, per annum, per million Inhabitants.
The figure for Hungary is based on the seven years 1897-1903; that for the Netherlands, from data by A.J. Monné (69) on the nine years 1882-1890. The English data, due to R. Lawson (70), are from twenty-four years, 1857-1880; those for the United States, due to Henry (68), are for five years, 1896-1900. In comparing these data allowance must be made for the fact that danger from lightning is much greater out of doors than in. Thus in Hungary, in 1902 and 1903, out of 229 persons killed, at least 171 were killed out of doors. Of the 229 only 67 were women, the only assignable explanation being their rarer employment in the fields. Thus,ceteris paribtis, deaths from lightning are much more numerous in a country than in an industrial population. This is well brought out by the low figure for London. It is also shown conspicuously in figures given by Henry. In New York State, where the population is largely industrial, the annual deaths per million are only three, but of the agricultural population eleven. In states such as Wyoming and the Dakotas the population is largely rural, and the deaths by lightning rise in consequence. The frequency and intensity of thunderstorms are unquestionably greater in the Rocky Mountain than in the New England states, but the difference is not so great as the statistics at first sight suggest.
TableXI.—Annual Variation of Thunderstorms.
31. Even at the same place thunderstorms vary greatly in intensity and duration. Also the times of beginning and ending are difficult to define exactly, so that several elements of uncertainty exist in data as to the seasonal or diurnal variation. The monthly data in Table XI. are percentages of the total for the year. In most cases the figures are based on the number of days of thunder at a particular station, or at the average station of a country; but the second set for Hungary relates to the number of lightning strokes causing fire, and the figures for the United States relate to deaths by lightning. The data for Edinburgh, due to R.C. Mossman (64), refer to 130 years, 1771 to 1900. The data for London (1763-1896) are also due toMossman (65); for Paris (1873-1893) to Renou (66); for the Netherlands (1882-1900) to A.J. Monné (69); for France(71) (1886-1899) to Frou and Hann; for Switzerland to K. Hess (72); for Hungary (67) (1896-1903) to L. von Szalay and others; for the United States (1890-1900) to A.J. Henry (68); for Hong-Kong (73) (1894-1903) to W. Doberck. The Trevandrum (74) data (1853-1864) were due originally to A. Broun; the Batavia data (1867-1895) are from the BataviaObservations, vol. xviii.
Most stations in the northern hemisphere have a conspicuous maximum at midsummer with little thunder in winter. Trevandrum (8° 31′ N.) and Batavia (6° 11′ S.), especially the former, show a double maximum and minimum.
TableXII.—Diurnal Variation of Thunderstorms.
32.Daily Variation.—The figures in Table XII. are again percentages. They are mostly based on data as to the hour of commencement of thunderstorms. Data as to the hour when storms are most severe would throw the maximum later in the day. This is illustrated by the first two sets of figures for Hungary (67). The first set relate as usual to the hour of commencement, the second to the hours of occurrence of lightning causing fires. Of the two other sets of figures for Hungary (75), (iii.) relates to the central plain, (iv.) to the mountainous regions to north and south of this. The hour of maximum is earlier for the mountains, thunder being more frequent there than in the plains between 8A.M.and 4P.M., but less frequent between 2 and 10P.M.Trevandrum (8° 31′ N., 76° 59′ E., 195 ft. above sea-level) and Agustia (8° 37′ N., 77° 20′ E., 6200 ft. above sea-level) afford a contrast between low ground and high ground in India. In this instance there seems little difference in the hour of maximum, the distinguishing feature being the great concentration of thunderstorm occurrence at Agustia between noon and 6P.M.
TableXIII.
33. Table XIII. gives some data as to the variability of thunder from year to year. The figures for the Netherlands (69) and France (71) are the number of days when thunder occurred somewhere in the country. Its larger area and more varied climate give a much larger number of days of thunder to France. Notwithstanding the proximity of the two countries, there is not much parallelism between the data. The figures for Hungary (67) give the number of lightning strokes causing fire; those for the United States (68) give the number of persons killed by lightning. The conspicuous maximum in 1901 and great drop in 1902 in Hungary are also shown by the statistics as to the number of days of thunder. This number at the average station of the country fell from 38.4 in 1901 to 23.1 in 1902. On the whole, however, the number of destructive lightning strokes and of days of thunder do not show a close parallelism.
TableXIV.
34. Table XIV. deals with the variation of thunder over longer periods. The data for Edinburgh (64) and London (65) due to Mossman, and those for Tilsit, due to C. Kassner (79), represent the average number of days of thunder per annum. The data for Germany, due to O. Steffens (80), represent the average number of houses struck by lightning in a year per million houses; in the first decade only seven years (1854-1860) are really included. Mossman thinks that the apparent increase at Edinburgh and London in the later decades is to some extent at least real. The two sets of figures show some corroborative features, notably the low frequency from 1860 to 1870. The figures for Germany—representing four out of six divisions of that country—are remarkable. In Germany as a whole, out of a million houses the number struck per annum was three and a half times as great in the decade 1890 to 1900 as between 1854 and 1860. Von Bezold (81) in an earlier memoir presented data analogous to Steffens’, seemingly accepting them as representing a true increase in thunderstorm destructiveness. Doubts have, however, been expressed by others—e.g.A. Gockel,Das Gewitter, p. 106—as to the real significance of the figures. Changes in the height or construction of buildings, and a greater readiness to make claims on insurance offices, may be contributory causes.
35. The fact that a considerable number of people sheltering under trees are killed by lightning is generally accepted as a convincing proof of the unwisdom of the proceeding. When there is an option between a tree and an adjacent house, the latter is doubtless the safer choice. But when the option is between sheltering under a tree and remaining in the open it is not so clear. In Hungary (67), during the three years 1901 to 1903, 15% of the total deaths by lightning occurred under trees, as against 57% wholly in the open. In the United States (68) in 1900, only 10% of the deaths where the precise conditions were ascertained occurred under trees, as against 52% in the open. If then the risk under trees exceeds that in the open in Hungary and the United States, at least five or six times as many people must remain in the open as seek shelter under trees. An isolated tree occupying an exposed position is, it should be remembered, much more likely to be struck than the average tree in the midst of a wood. A good deal also depends on the species of tree. A good many years’ data for Lippe (82) in Germany make the liability to lightning stroke as follows—the number of each species being supposed the same:—Oak 57, Fir 39, Pine 5, Beech 1. In Styria, according to K. Prohaska (83), the species most liable to be struck are oaks, poplars and pear trees; beech trees again are exceptionally safe. It should, however, be borne in mind that the apparent differences between different species may be partly a question of height, exposure or proximity to water. A good deal may also depend on the soil. According to Hellmann, as quoted by Henry (82), the liability to lightning stroke in Germany may be put at chalk 1, clay 7, sand 9, loam 22.
36. Numerous attempts have been made to find periodic variations in thunderstorm frequency. Among the periods suggested are the 11-year sun-spot period, or half this (cf. v. Szalay (67)). Ekholm and Arrhenius (84) claim to have established the existence of a tropical lunar period, and a 25.929-day period; while P. Polis (85) considers a synodic lunar period probable. A.B. MacDowall (86) and others have advanced evidence in favour of the view that thunderstorms are most frequent near new moon and fewest near full moon. Much more evidence would be required to produce a general acceptance of any of the above periods.
37.St Elmo’s Fire.—Luminous discharges from masts, lightning conductors, and other pointed objects are not very infrequent, especially during thunderstorms. On the Sonnblick, where the phenomenon is common, Elster and Geitel (87) have found St Elmo’s fire to answer to a discharge sometimes of positive sometimes of negative electricity. The colour and appearance differ in the two cases, red predominating in a positive, blue in a negative discharge. The differences characteristic of the two forms of discharge are described and illustrated in Gockel’sDas Gewitter. Gockel states (l.c. p. 74) that during snowfall the sign is positive or negative according as the flakes are large or are small and powdery. The discharge is not infrequently accompanied by a sizzling sound.
38. Of late years many experiments have been made on the influence of electric fields or currents on plant growth. S. Lemström (88), who was a pioneer in this department, found an electric field highly beneficial in some but not in all cases. Attempts have been made to apply electricity to agriculture on a commercial scale, but the exact measure of success attained remains somewhat doubtful. Lemström believed atmospheric electricity to play an important part in the natural growth of vegetation, and he assigned a special rôle to the needles of fir and pine trees.
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