6 HARBOUR CONSTRUCTION. 



being less than 600 miles. In such systems, however, the inner 

 or more dangerous portion is of much smaller proportions, 

 probably not exceeding 200 to 250 miles in diameter. 



A typical cyclone, having its centre over England, is repre- 

 sented in Fig 4. 



The rate of the progress of storms, i.e. of the cyclonic areas, 

 varies considerably. At times their forward movement does not 



FIG. 4. Diagram of typical cyclone. The arrows indicate the direction of the wind. The curved 

 lines are isobars, or contours of equal barometric pressures, varying, in this case, from 28-8 inches 

 at the centre of the storm to 30 inches at its outer limit. 



amount to more than 2 or 3 miles per hour, while at other times 

 they travel at from 40 to 50 miles per hour; 17 to 25 miles per 

 hour seems to be a common rate for European storms. 



Tropical cyclonic hurricanes generally travel at a slower 

 speed, but rotate at a much greater velocity than extra- 

 tropical ones. 



The progressive motion does not indicate either the direction 



