26 



When the most powerful ascending current, and consequent 

 indraft, are set up at the equator, no storm is produced ; but 

 when the same occurs far enough from the equator to enable 

 the earth's rotation to have effect, every current of air as it flows 

 in towards the centre is deflected to the right (or, in the southern 

 hemisphere, to the left), and thus a vortex, or cyclone, is formed, 

 with a rotatory velocity which may be very much greater than 

 the original velocity of indraft. This may be illustrated by 

 filling a wash-hand basin with water, removing the plug at the 

 bottom, and then giving the water a slight rotatory impulse 

 with the hand, when the water will begin to rotate with an 

 impulse very much greater than the force with which it was set 

 in motion. The mechanics of such a water vortex, or whirlpool, 

 closely resemble those of a cyclone or revolving storm ; — the 

 ascending current at the centre of the storm corresponds to the 

 current out through the hole in the bottom of the water basin. 

 Although the earth's rotation in relation to an axis vertical to the 

 horizon is less in tropical than in European latitudes, yet the 

 storms of the tropics are more violent, in consequence of the 

 greater steam power of the atmosphere, due to the hotter 

 climate. On the equator, however, cyclones are not found, 

 because there the earth's rotation does not deflect the wind, 

 either to right or to left. 



Dr. Hann has made a mathematical examination of the 

 observed data of some European storms, which shows that the 

 barometric gradient — that is to say the ratio of the difference 

 between the height of the barometer at different places to the 

 distances between those places — is greater than is due to the 

 centrifugal force generated by the rotation of the storm, and he 

 infers that the excess is due to the deflecting force of the earth's 

 rotation. 



Mr. Murphy concluded by expressing the opinion that the 

 origin of those fluctuations of the barometer, or barometric 

 waves, which accompany and bring storms, is to be found in 

 the inter-action of currents of air flowing side by side in opposite 

 directions, modified by the earth's rotation. 



