z64 OBSEEVATIONS ON THE WINDS. 



It must be remarked that, even when the Tornado is dry, it is always 

 followed by a fall in the temperature, very sensible to the thermometer. 

 This proves that it is not formed at the level of the soil or of the sea, but 

 in the higher regions of the atmosphere, and that the axis of its gyratory 

 movement is not vertical, or that the motion of the air is rather spiral 

 than circular. 



Some remarks on Tornados, as experienced off the West Coast of Africa, 

 are given previously, on pages 158 — 163. 



10.— WATERSPOUTS. 



(210.) The well-known phenomenon, called a Waterspout, which is fre- 

 quently seen on the Atlantic, proceeding from black dense clouds, always 

 ippears in warm weather, generally in calms, or with little wind; but they 

 aave been seen during a fresh gale. It has been shown, by the celebrated 

 Dr. Franklin, and other writers, that a Whirlwind on land, and a Water- 

 spout at sea, arise from the same general causes, and may be considered 

 as one and the same. At sea they are commonly harmless, unless ships 

 happen to be immediately under them ; but if, in the progressive motion 

 of the whirl, it passes from the sea over the land, and there suddenly 

 breaks, violent and mischievous torrents are the consequence. At sea, 

 after the spout breaks, the water descends in the form of very heavy rain. 

 In the vicinity of a spout, the wind commonly flies round in sudden gusts, 

 and all ships should therefore take in their square sails. 



That a Waterspout and Whirlwind are identical has been amply demon- 

 strated by those who have seen this meteor pass from the sea to land, and 

 the contrary. They have both a progressive as well as circular motion ; 

 they usually appear after calms and great heats, and mostly happen in 

 the warmer latitudes. 



(211.) The general principles underlying the formation of Whirlwinds, 

 Tornados, and Waterspouts, are described as follows by Lieutenant E. 

 Hayden, U.S.N. 



A layer of warm moist air at the surface of the sea happens to have 

 above it a layer of cooler and drier air, and sooner or later the warm light 

 air ascends through the cooler heavier air above. Sometimes this takes 

 place over large areas; at other times it is more local, and the lower layer 

 attempts to escape through a break or opening in the upper layer as 

 through a funnel. Under favourable conditions this action becomes very 

 intense, being increased by the moisture of the ascending column becoming 

 condensed, thus liberating latent heat. As the warm air rushes up, it 

 takes a whirling motion, and a suction or partial vacuum is created at the 

 centre, which often draws water for some distance up the centre. The 

 direction of rotation is not always uniform as with Cyclones. 



When a Waterspout is forming, its upper portion is often visible first, 



