14 HARBOUR CONSTRUCTION. 



FORECASTING GALES. 



If simultaneous readings of the barometer are available from 

 a number of stations, embracing a sufficiently large area as is 

 now the case in many places by means of the electric telegraph 

 contours, or lines of equal barometic pressure, termed " isobars," 

 may be plotted upon a chart. These will, for the most part, be 

 found to arrange themselves around high or low pressure centres 

 in an approximately circular or elliptical form (see Fig. 4, p. 6). 



Such an arrangement, when it embraces a low-pressure 

 centre, is termed a " cyclone ; " when a high-pressure one, an 

 " an ti -cyclone." 



The contours will also form what are known as " V's" of 

 low pressure, " wedges " of high pressure, etc. 



Diagrams constructed in this manner are called " synoptic " 

 or "synchronous" charts. They usually contain information 

 respecting temperature, direction and velocity of wind, clouds, 

 and weather generally. 



Upon the form and movement of these atmospheric systems, 

 or arrangements of isobars, almost the whole science of weather- 

 forecasting depends. Apart from other indications, they show 

 the force and direction of the wind which may be expected in 

 any part of their path, inasmuch as the force or velocity of the 

 wind varies in proportion to the rate of the gradient, which is 

 shown by the relative closeness of the isobars to each other. 

 The velocity is always greatest where the gradient is steepest. 

 Storms are only ordinary winds intensified by the steepness of 

 such gradients. 



The direction of the wind is generally that of the contours, 

 only it is more or less spirally incurved or inclined towards the 

 centre in cyclones, and outcurved from the centre in anti- 

 cyclones. This inclination varies considerably, making under 

 some conditions an angle of upwards of 50 with the isobars, 

 and in other cases being very small indeed. 



These gigantic eddies move about the globe without following 

 any very certain course ; but their probable direction and speed, 

 as indicated by that portion of their course already traversed, 

 may generally be surmised by a study of consecutive synoptic 

 charts, always providing that the area embraced by them is of 

 sufficiently large extent. 



It is only by a study of such charts that forecasts, with a 



