ESTUARY HARBOURS, ETC. 161 



In fishery harbours, with areas varying from, say, 6 to 10 

 or 15 acres, entrances of from 100 to 200 feet in width are 

 commonly adopted. Thus Lowestoft outer harbour, with an 

 area of about 15 acres, has an entrance of 150 feet in width; 1 

 while the north and south old harbours at Peterhead, each with 

 an area of about 6 acres, have entrances of 100 feet in width. 



Reduction of Waves. In the case of a harbour which opens 

 out quickly within the pier-heads, waves running straight in do 

 not disturb the tranquillity of the water to such an extent as 

 might be supposed, because immediately they pass the entrance 

 they rapidly expand, and their height is thereby at once reduced. 



The late Mr. Thomas Stevenson, C.E., has given us a formula 

 for calculating the reductive power of harbours, by which is 

 meant their power to reduce the height of waves after passing 

 within the entrance. 2 



It is based upon the expansion of the wave, which is in- 

 versely proportional to the square roots of the breadth of entrance 

 and the greater breadth within the piers upon any line measured 

 at a given radius from the entrance. 



The formula is as follows : 







71" "50" 



Where H = height of wave at entrance, in feet. 

 b = breadth of entrance, in feet. 



B = breadth of harbour at place of observation, in feet. 

 D = distance from month of harbour to place of observa- 

 tion, in feet. 

 x = reduced height of wave at place of observation, in 



feet. 



When H is assumed as unity, the formula assumes its simplest 

 form 



X = 7S 



50 



and x will come out a fraction, which will represent what may be 

 called the reductive power of the harbour at the point for which the 

 calculation has been made ; and by multiplying the height of any 

 wave at the entrance by the fraction x, its reduced height is found at 

 once. 



1 The inner harbour, 10 acres in area, has an entrance 100 feet wide. 



2 " Report of Royal Commission on Harbours of Refuge, 1859," yol. i. p. 240. 



M 



