+----------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| Decade ending | 1810| 1820| 1830| 1840| 1850| 1860| 1870| 1880| 1890| 1900|
+----------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
| Edinburgh | 4.9| 5.7| 7.7| 6.7| 5.7| 6.5| 5.4| 10.6| 9.4| 9.2|
| London | 9.5| 8.3| 11.5| 11.8| 10.5| 11.9| 9.6| 15.7| 13.0| .. |
| Tilsit | .. | .. | 12.5| 12.1| 16.1| 15.3| 11.9| 17.6| 21.8| .. |
| Germany, South | .. | .. | .. | .. | .. | 49 | 66 | 91 | 143 | 175 |
| " West | .. | .. | .. | .. | .. | 92 | 106 | 187 | 244 | 331 |
| " North | .. | .. | .. | .. | .. | 124 | 135 | 245 | 288 | 352 |
| " East | .. | .. | .. | .. | .. | 102 | 143 | 186 | 210 | 273 |
| " Whole | .. | .. | .. | .. | .. | 90 | 116 | 189 | 254 | 318 |
+----------------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+
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's _Das 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. Lemstrom (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. Lemstrom
believed atmospheric electricity to play an important part in the
natural growth of vegetation, and he assigned a special role to the
needles of fir and pine trees.
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