150 The Realm of Nature CHAP. 



gradient of pressure. If the cyclone is of great size and 

 the diminution of pressure in the centre very slight, gentle 

 winds and light showers only will be produced. But if, 

 on the other hand, the cyclone is of small dimensions, less 

 than a hundred miles across, and the diminution of pressure 

 in the centre is great, terrific winds and deluges of rain 

 result. The centre of a cyclone is always calm, although a 

 gale may be blowing round it in every direction a few 

 miles distant. The weather of the northern hemisphere 

 is to a very great degree determined by passing cyclones of 

 large size. 



208. Hurricanes and Typhoons. Small cyclones of slow 

 motion but with steep gradients, and therefore accompanied 

 by very severe winds, are common in the tropics at certain 

 seasons. Unlike the cyclones of the temperate zone they 

 appear to originate from local heating of the air. Among 

 the West Indian Islands such storms are liable to occur 

 during the months from July to October, and their terrific 

 violence has given wide currency 'to their native name 

 of Hurricanes. In the Bay of Bengal at the changing of 

 the monsoons, and along the east coast of Africa, similar 

 storms, to which the name Cyclones was first applied, are 

 experienced. In the China Sea they are common from 

 July to November, and are known as Typhoons. These 

 tropical storms differ from the less violent cyclones of the 

 temperate zone in always having a patch of clear blue sky 

 over the central calm where the barometer is lowest ; this 

 is called the eye of the storm. Although the calm centre 

 of a cyclone is referred to poetically as " the whirlwind's 

 heart of peace," it is the part most dreaded by sailors. 

 There is no wind to move a sailing ship, and a terrible 

 chaos of heavy waves is driven in by the winds raging on 

 every side. A ship -captain in the season when these 

 storms are prevalent is always on the watch for them, 

 and as their approach is heralded by a fall in the barometer 

 and the way in which the wind changes, he can find the 

 direction of the centre. A steamer in many cases can then 

 by changing its course let the storm pass harmlessly. 



The commonest Cyclone tracks of the tropics and the 



