50 HARBOUR CONSTRUCTION. 



that the range of mid- winter tides is not so great as that of 

 those occurring at the equinoxes, when the sun, although slightly 

 more distant from the earth, is vertical over the equator, in 

 which position its effect upon the tides is greatest. 



At mid-winter, the increase of tidal range due to the nearness 

 of the sun to the earth, is reduced by the sun's declination ; but 

 at mid-summer both the sun's distance and declination tend to 

 reduce the tides, which are thus the worst in the year. 



Progress of the Tidal Wave. In consequence of the free 

 motion of the tidal wave being obstructed by the irregular 

 contour and distribution of land, and also by the retarding effect 

 upon its progress produced by decrease of depth and other 

 disturbing causes, the time of high water at any given place 

 will not be found to coincide with the time of the moon's 

 passage of the meridian, unless, indeed, the retardation has been 

 such as to cause the crest of some previous tidal wave to arrive 

 at that time. 



A chart with cotidal lines marked thereon,' shows at a glance 

 to how great an extent the tidal wave is thus interfered with, 

 and how it is made to wheel and expand in all directions in 

 order to fill the various more or less land-locked seas, gulfs, and 

 inlets, so that in many places the tides on any given day are 

 really those due to the position of the sun and moon, it may be, 

 two or three days previous. 



Inasmuch as the tidal wave traverses the globe once in about 

 every 24 hours 49 min., its velocity at the equator, where 

 unimpeded, is about 1000 miles per hour. In the preceding 

 chapter it was stated that the velocity of long waves depends 

 on the depth of the water which they traverse, and that, as they 

 advance into shoaler water, their length, and consequently their 

 velocity, decreases, while their height increases. This applies 

 equally to the tidal wave ; and we find that as it advances into 

 shallow seas and estuaries its velocity is diminished and its 

 height increased, to the extent which local physical conditions 

 demand. 



The tidal wave in its normal state is one of oscillation. 

 When, however, its progress is interfered with as, for instance, 

 when it has to find its way through shallow and contracted 

 channels, or by circuitous routes, into seas or estuaries its 

 character is changed, and where the obstruction is great, it seems 

 to lose its wavelike character, and to assume that of a stream, 



