4 HARBOUR CONSTRUCTION. 



exposure of any site which may be under consideration, the 

 direction of the heaviest waves, and so forth. 



If it should be desired to show the winds separately for each 

 month, it will be found convenient to distinguish the several 

 months by means of colours instead of zones. Thus, for instance, 

 the observations for January, February, and March may be shown 

 by three shades of green ; April, May, and June, by three shades 

 of red ; July, August, and September, by three shades of brown ; 

 and October, November, and December, by three shades of grey. 



In order to show to how great an extent the relative 

 frequency of certain winds may vary from year to year, and the 

 consequent necessity for obtaining information which embraces 

 as long a period as practicable, I have compiled the foregoing- 

 diagrams from observations taken at Peterhead during two 

 different years. 



The generally accepted theory of wind is so well known that 

 I shall not occupy the time of the reader by describing it here. 1 

 Suffice it to say that the never-ceasing effort of the atmosphere 

 to equalize temperature and restore lost equilibrium all over 

 the globe, and the rotation of the earth, are the two principal 

 forces which affect the direction and velocity of the main atmo- 

 spheric currents. 



There are, however, many minor forces or influences at work, 

 which produce changes in the direction and velocity of the wind ; 

 such, for example, as the variation of day and night temperature, 

 the distribution and trend of land, mountain chains, clouds, ocean 

 currents, drift ice, and even the contact of one aerial system with 

 another. 



Ocean currents, being slow to part with heat, and being often 

 thrown far out of the courses they would naturally pursue, by 

 the irregular distribution of land and other physical conditions, 

 affect, to a very great extent, the temperature of the atmosphere 

 along their course. 



A change in the* temperature of the air is synonymous with 

 a change in its pressure ; consequently, if the air in any locality 

 should be warmed, and thus rarefied by contact with a warm 

 ocean-current, an upcast aerial current will at once be formed, 

 and the colder air from surrounding districts will flow in 

 to re-establish equilibrium. Thus variations in temperature, 



1 The subject is very fully treated in a book to which I shall hereafter refer, viz. 

 "Weather," by Hon. Ralph Abercromby (Kegan Paul & Co.). 



