82 Review of the Results of the U. 8. Coast Survey. 
great measure sufficiently free from the offsets of local causes to — 
enable us to obtain from them results of definite value for the — 
general theory. On the other hand the Gulf of Mexico and — 
particular portions of the Atlantic Coast exhibit peculiarities of | 
much interest, as yet imperfectly investigated, but seeming to — 
show the importance of a careful study. While the tidal obser- — 
vations hitherto discussed have been for the most part isolated, 
made at different points upon the earth’s = by individuals, : 
during a period of about 200 years, those of the Coast Survey 
have been made systematically, at numerous carefully selected 
stations, upon the coasts of a continent lying between two great 
oceans, ‘and under the direction of a single person ; 
An elaborate discussion of these observations has led to the 
construction of maps of the cotidal lines of the Atlantic, Gulf 
and Pacitic coasts, which are of especial interest not merely from — 
their connection with our own shores, but from the fact that they 
F . lig 
line” it will be remembered was first introduced by Mr. Whewell 
to denote a line passing. through all those points which have 
high water at the same hour of the day. It is convenient to 
anne twenty-four aaah lines, and they may obviously be re 
ed as forming the crests of successive advancing ed gent 
thie shape, velocity, and mes of motion, will de 
configuration of the coast, the depth of ‘the ocean ~e the | 
si local causes which disturb the uniformity of their pro- — 
gress and cause divisions and interferences of divided waves. 
Were no disturbing causes present, the cotidal lines would cor 
respond with the meridians, each line at a certain distance be 
hind the meridian of the moon at its culmination. It is easy to 
see too that the cotidal lines must differ upon the eastern and — 
western shores of a continent like that of North America, since 
_ the tide wave moves from east to west and is therefore upon the — 
eastern coast an incident and upon the Mees a receding wave, - 
the character of which is determined by the flow of water and 
its pressure from north, south and west. The cotidal lines of the 
Atlantic coast follow the general outline of the coast itself in@ - 
remarkable manner, the velocities measured 1 in a direetion pe 
ar semi-diurnal class; the diurnal inequality is not large @ 
generally difficult to trace, though easily recognized at particu 
periods. On the Gulf coast, on the contrary, the tides are small, — 
the semi-diurnal being masked by the diurnal waves. The tides _ 
of the Pacific coast are remarkably regular, both in the diurnal 
and semi-diurnal waves, and moreover rise to such pt am 
render observation easy. eens the extent of coas 
