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 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. 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.

.—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). References in the text:—(6) M. Z., vol. 4, 1887, p. 352; (7) T. M., vol. 4, 1899, p. 213; (8) P. Z., vol. 4, p. 661; (9) M. Z., vol. 23, 1906, p. 114; (10) P. T., vol. 205 A, 1906, p. 61; (11) P. Z., vol. 5, p. 260; (12) C. Chree, P. T., vol. 206 A, p. 299; (13) Annual volumes, Greenwich Magnetical and Meteorological Observations; (14) M. Z., vol. 8, 1891, p. 357; (15) M. Z., vol. 7, 1890, p. 319 and vol. 8, 1891, p. 113; (16) Annual volumes, ''Annaes do Obs. do Infante D. Luiz; (17) Annual Reports, Central Meteorological Observatory of Japan; (18) Observations made at the Mag. and Met. Obs. at Batavia, vol. 18, 1895; (19) J. D. Everett, P. T., vol. 158, 1868, p. 347; (20) M. Z., vol. 6, 1889, p. 95; (21) A. B. Chauveau, Ann. bureau central météorologique, Paris, année 1900, “Mémoires,” p. C1; (22) V. Conrad, S., 113, p. 1143; (23) P. B. 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 Königl. 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. Report for 1898, p. 808, also Electricity in Agriculture and Horticulture'' (London, 1904).

ATMOSPHERIC RAILWAY. About 1840–1845 great interest was excited by a method of propelling railway trains through the agency of atmospheric pressure. Various inventors worked at the realization of this idea. On the system worked out in England by Jacob Samuda and S. Clegg, a continuous pipe or main was laid between the rails, and in it a partial vacuum was maintained by means of air pumps. A piston fitting closely in it was connected to the leading vehicle of the train by an iron plate which passed through a longitudinal groove or aperture running the whole length of the pipe. This aperture was covered by a valve consisting of a continuous strip of leather, strengthened on each side with iron plates; one edge was fastened, while the other was free to rise, and was closed against a composition of beeswax and tallow placed in the groove, the surface of which was slightly melted by a heater, carried on each train, in order to secure an air-tight joint. Connected behind the piston was a frame carrying four wheels which lifted and sustained the continuous valve for a distance of about 15 ft. Thus the piston having atmospheric pressure on one side of it and a vacuum equal to 15 or 16 in. of mercury on the other, was forced along the tube, taking the train with it. Various advantages were claimed by the advocates of the system, including cheapness of operation as compared with steam locomotives, and safety from collision, because the main was divided into sections by separating valves and only one train could be in each section at a given time. It was installed on about 2 m. of line between Kingstown and Dalkey (Ireland) in 1843 and worked till 1855; it was also tried on the London and Croydon and on the South Devon lines, but was soon abandoned. The same principle is applied in the system of (q.v.) to the transmission of small parcels in connexion with postal and telegraph work.

ATOLL (native name atollon in the Maldive Islands), a horse-shoe or ring shaped coral reef enclosing a lagoon. The usual shape is that of a partly submerged dish with a broken edge, forming the ring of islands, standing upon a conical pedestal. The dish is formed of coral rock and the shells of various reef-dwelling mollusca, covered, especially at the seaward edges, with a film of living coral polyps that continually extend the fringe, and enlarge the diameter of the atoll. The lagoon tends to deepen when the land is stationary by the death of the coral animals in the still water, and the patchy disintegration of the “hard” coral, while waves and storms tear off blocks of rock and pile them up at the margin, increasing the height of the islands, which become covered by vegetation. The lagoon entrance in the open part of the horse-shoe is always to leeward of prevailing winds, since the coral growth is there slower than where the waves constantly renew the polyps’ food supply. The conical pedestal rising from the depths is frequently a submarine volcanic cone or island, though any submerged peak may be crowned by an atoll. For the theory of atoll formation see. ATOM (Gr. , indivisible, from privative, and  , to cut), the term given in physical science to the ultimate indivisible particle of matter, and so by analogy to something minutely small in size. If we examine such a substance as sugar we find that it can be broken up into fine grains, and these again into finer, the finest particles still appearing to be of the same nature as sugar. The same is true in the case of a liquid such as water; it can be divided into drops and these again into smaller drops, or into the finest spray the particles of which are too small to be detected by our unaided vision. In fact, so far as the direct evidence of our senses tells us, matter appears to be indefinitely divisible. Moreover, small particles do not seem to exist in the water until it is broken up; so far as we can see, the material of the water is continuous not granular. This conception of matter, as infinitely divisible and continuous, was taught by