TIDES. 57 



the North Sea through the Pentland Firth and other northern 

 channels. A northerly or north-easterly wind checks the flow 

 of the tide along the west coast, and thus lowers the tide in the 

 northern compartment of the North Sea. In the southern com- 

 partment, and along the west coast of Holland, the tides are 

 raised, or banked up, by northerly and north-westerly winds. 



The normal rise of spring tides on the west coast of Holland 

 is about 5 feet 9 inches, but strong north-west gales sometimes 

 cause a rise of from 9 to 10 feet. 



In the English Channel south-westerly winds raise the water- 

 level, whereas north-easterly winds depress it. 



The Black Sea, being practically tideless, offers facilities for 

 noting the effect of various winds upon the water-level ; and it 

 is found that along the west shore say the coast of Bulgaria 

 during a strong off-shore or westerly wind, the surface of the 

 water is lowered to the extent of 18 inches, whereas during a 

 strong easterly wind which crosses a reach of water nearly 600 

 miles in length the surface is raised about two feet above the 

 mean water-level ; thus giving a total range of 3 feet 6 inches, 

 due entirely to the action of the wind. 



At Venice, a strong south-east gale raises the tides to the 

 extent of about 18 inches, and a strong northerly gale lowers 

 them to a corresponding extent. 



What has been said on this point will suffice to show how 

 important it is to ascertain the effect of gales upon the water- 

 level, because the height of quays, stagings, or even of break- 

 waters themselves, may be greatly affected by it, and the 

 injurious effect of waves be much increased. 



Effect of Atmospheric Pressure. With regard to the effect of 

 atmospheric pressure on the tides, it may be remarked that the 

 weight of the atmosphere at the earth's surface is represented 

 by a pressure of about 15 Ibs. per square inch. This is equiva- 

 lent to nearly 8| tons per square yard, or to a layer of iron a 

 yard and a half thick, spread evenly over the whole surface 

 of the globe. 



Seeing that the entire surface of the ocean is thus loaded, it 

 is evident that if a portion of the load be removed locally, the 

 water will rise in proportion to the amount of relief so given, 

 and conversely, if the load be increased the water-surface will 

 be depressed. 



The barometers in common use record atmospheric pressure by 



