TKANSACTIONS OF THE SECTIONS. 41 



waves (either ot excess or deficiency) are propagated from it through the medium, — 

 a result to which a rude analogy may be found in imagining a bucket of water 

 poured into (or taken out of) one end of a long narrow channel of still water. The 

 previous equilibrium is thereby overthrown, and a wave is propagated either from 

 or towards the site of the disturbance. 



Faits divers de Physique Terrestre. Par Dr. Janssew. 



The Rainfall of Natal, South Africa. By Dr. Mann, F.E.G.S. Sfc. 



The British colony of Natal, situated on the south-east coast of Africa, 800 

 miles beyond the Cape of Good Hope, lies between the 27th and -Slst parallels of 

 south latitude, and the 29th and 32nd meridians of east longitude, looks out 

 upon the Indian Ocean by a coast-line of 150 miles broad, and slopes down from a 

 height of 6000 feet, where the inland frontier is formed by the rim of the great 

 tableland of the Continent. The general set of the air-current is from the moist 

 spaces over the ocean, up this rapid land slope, and the rainfoll of the colony is in 

 the main due to this cause. The result is a very remarkable climate, in which 

 tropical productions grow luxuriantly in the lower districts, and temperate produc- 

 tions thrive in the higher regions. The mean temperature of the year is 64°'7 at 

 a height of 2000 feet, and 69-'-2 on the coast. 



The year is divided into a wet and dry season, rather than into a summer and 

 winter season. The six months from October to March are cloudy and moist, and 

 the six months from April to September are months of bright and rarely inter- 

 rupted simshine. The entire rainfall, for a period of ten years, at an elevation of 

 2095 feet, and 45 miles inland, was 302 inches, giving a mean yearly fall of 30-22 

 inches ; of this 25-o3 inches fall during the six summer months, and 5-09 during 

 the six winter months. The mean monthly fall for the summer period is 4-2 inches ; 

 the same for the winter period is 0-87 of an inch. The increase and diminution 

 of the fall is singularly regular, as becomes apparent when the progression is repre- 

 sented in a diagram traced out pictorially upon paper. 



Tlie months of April and September are properly intermediate months in regard 

 to rainfall. Thej' have each a mean fall of one inch and a half, while the mean 

 monthly fall for the rigidly dry period from May to August barely exceeds half 

 an inch. 



The heaviest rainfall in one year during this period was 37-31 inches ; the least 

 rainfall in one year 22-34 inches. Upon eight months out of the 120 comprised 

 in the period of observation, no rain fell on 33 months only. The monthly fall 

 was less than one inch, on one occasion onty. There were 107 days in succession 

 without rain. Eight other intervals ga\-e rainless periods of between 40 and 68 

 days, and 13 intervals of between 20 and 33 days. 



There were 19 months on which the monthly fall reached 5 inches, 9 months 

 on which it reached 6 inches, 2 months on which it reached 7 inches, and 1 

 month when it reached 8-95 inches. There were 49 days on which the daily fall 

 exceeded 1 inch, 13 days on which it reached 14 inch, only 5 days on which it 

 exceeded 2 inches. The heaviest daily fall was 2^ inches. The heaviest fall at 

 Maritzburg on consecutive rainy days was 10-81 inches. 



The rivers in Natal are low, iDut not dry in the winter season, and are swollen 

 during the six months of heavier rain. The coast district alone is subject to 

 occasional devastating floods. In 1857 there was one in which the river Unr>-eni 

 near its mouth in the neighbourhood of the seaport of Durban, rose 28 feet above 

 its ordinary level, and overflowed a considerable space of low groimd near the 

 port, carr\-ing down cattle, large deposits of reeds and large trees, to the sea. In 

 1868 another coast-flood took place, in which a fine iron-girder bridge, 900 feet 

 long, and recently erected over the same river at a cost of £19,000, was carried 

 away, and in which damage to the extent of £100,000 was inflicted on public 

 works and private property. 



The rainfall on the first occasion was 21 inches on the coast, and 11 inches at 



