FOR GREAT BRITAIN AND IRELAND. 45 



interior, the depth of water, the form of the entrance and the 

 relation between the direction of the entrance and that of 

 the line of maximum exposure'' 



ON THE DIFFERENT CLASSES OF HARBOURS. 



In this division of the subject Mr. T. Stevenson gives us 

 the following description, to which the writer will make a 

 few additions en passant : 



ist " Harbours of refuge and anchorage breakwaters^ 

 consisting of one or more breakwaters so arranged as to 

 form a safe roadstead, which shall be easily accessible to 

 the largest vessels in all states of the weather and tide." 



" A breakwater forms a barrier either complete or partial to 

 the progress of the waves, and is intended to shelter anchor- 

 age ground under its lee. It is not used directly for com- 

 mercial traffic as piers or quays are, and therefore a parapet 

 is not necessarily required for preventing the waves from 

 breaking over the top." Breakwaters are either insu- 

 lated from or connected with the land, or combine both 

 properties ; of the first kind we have instances at 

 Plymouth and Cherbourg, of the second at Holyhead, and 

 of the third at Portland, which has a passage through it 

 near the land. 



2nd. " Deep-water and tidal harbours for commercial pur- 

 poses (Fig. I, a and b, facing p. 80). A harbour for com- 

 mercial purposes is any arrangement of piers or breakwaters, 

 or of both, which incloses and so tranquillizes a sheet of water 

 that vessels may be moored at the quay walls or wharfs 

 which form the inner sides of the piers. Where the coast- 

 line lies open to a heavy sea it is often found necessary to 

 make a double harbour (Fig i, b, facing p. 80). In such 



