220 OBSEEVATIONS ON THE WINDS. 



West India Islands they get into another current, which carries them first 

 N.W., then North, and ultimately N.E. 



But how is the whirling cylindrical column of a Cyclone formed ? There 

 are several ways in which a whirl may be formed. (1.) If a tub be filled 

 with water, and a small orifice be made in the bottom, the water in the 

 tub as it runs out rotates. At first only the part over and near the orifice 

 rotates, but ultimately the rotation extends to more distant parts. (2.) If 

 a stream of water be made to flow through a body of water which is 

 quiescent, whirls or eddies will be formed which consist of columns more 

 or less vertical. They may be seen any day in rivers where the current 

 passes a point of rock projecting into the stream, and when that current 

 comes in contact with the body of the water sheltered by the rock. These 

 eddies are still more common where two currents, running in different 

 directions, touch one another ; along the line which separates them whirls 

 are formed, the rotation being in the direction of the stronger stream, and 

 its progressive motion being also in the same direction, (3.) If a long glass 

 cylinder be immersed a little way into a body of water, and the air be 

 suddenly exhausted or rarefied by drawing it out at the top of the cylinder, 

 the water rises, and in rising rotates. 



With regard to the cause of the air rising. There must frequently be, in 

 the upper regions of the Equatorial atmosphere, condensations of the warm 

 vapours which are carried up from the surface of the ocean by the ascend- 

 ing aerial currents. These condensations will produce great rarefaction, 

 and that rarefaction will draw towards it air from the lower regions of the 

 atmosphere, and in drawing it will probably cause rotation.* 



Low pressure, violent winds, and heavy clouds and rain, are always 

 observed in well-defined Cyclones, and are closely dependent on one 

 another. A local decrease of pressure, from whatever initial cause, re- 

 quires the inflow of the surrounding air. The winds thus aroused acquire, 

 except close to the Equator, a rotation around the low pressure centre as 

 they approach it, because they are moving on a rotating sphere. Their 

 inward spiral motion thus begun must become more rapid near the centre 

 in accordance with the well-known mechanical law, as illustrated by a 

 whirlpool. The centrifugal force produced by the rotation causes a further 

 decrease of pressure at the centre, and thereby confirms the circulation of 



the Storm. 



The low pressure at the centre, in spite of the continual spiral inflow of 

 air shows there must be a compensating escape of air by ascent and out- 

 flow above ; the slowly-ascending air, still whirling around the centre, is 

 necessarily cooled by its own expansion as it rises, and thus vapour is con- 

 densed, and heavy clouds form giving rain. The latent heat given out by 

 the condensation of so much vapour retards the cooling of the ascending 

 air and maintains it at a higher temperature than that of the surrounding 



* To cause this rotation, the ascending current would appear to require a limited 

 space to pass through, and this hole, as it were, might be formed at a point of contact 

 of the upper and under layers of three strata of air. That it is possible for patches ol 

 air of diSerent consistencies to exist side by side, has been proved by the fog-signal 

 experiments carried on by the Trinity House.— Ed. 



