12 



HARBOUR CONSTRUCTION. 



At 9 p.m. it had fallen to 39 - 0, and at 9 a.m., 18th, to 3O5 a varia- 

 tion in temperature of 18 degrees in 24 hours. The annexed table 

 shows the force and direction of wind as recorded by Robertson's 

 anemograph, with estimated wind pressure in pounds per square foot 

 as calculated by Sir Henry James, Director of Ordnance Survey. 



From the above it will be noticed that, taking the velocity for 15 

 hours from 6 p.m., 17th, to 9 a.m., 18th, the average rate is 82 

 miles, which, for duration and high velocity, has probably not been 

 surpassed in Great Britain. The following are dates of gales with 

 velocity over 80 miles experienced in Orkney since 1863 : 



Date. 



1869 February 27 

 1877 November 16 



1882 January 6 



1884 January 20 

 1885 February 21 

 1893 November 17 



Attempts bave been made to construct empirical formulae, 

 based upon the average of a great number of observations, by 

 which the height of waves may be deduced from the velocity of 

 the wind. It is, however, impossible to establish any reliable 

 comparison between them. 



