DESCRIPTION OF TIDES. 33 
aspect must increase, for we cannot walk upon the beach with- 
out being constantly reminded that all the shining worlds that 
stud the heavens are linked together by one Almighty power, 
and that our spirit, which has been made capable of unveiling 
and comprehending so many of the secrets of creation, must 
surely possess something of a divine nature! 
On all maritime coasts, except such as belong to mediterra- 
nean seas not communicating freely with the ocean, the waters 
are observed to be constantly changing their level. They regu- 
larly rise during about six hours, remain stationary for a few 
minutes, and then again descend during an equal period of time, 
when after having fallen to the lowest ebb, they are shortly 
after seen to rise again, and so on in regular and endless succes- 
sion. In this manner twelve hours twenty-four minutes elapse 
on an average from one flood to another, so that the sea twice 
rises and falls in the course of a day, or rather twice during the 
time from one passage of the moon through the meridian to the 
next, a period equivalent on an average to 1,3, day, or nearly 
twenty-five hours. Thus the tides retard from one day to 
another; least at new and full moon, when our more active satel- 
lite accomplishes her apparent diurnal motion round the earth 
in twenty-four hours, thirty-seven minutes; and most at half- 
moon, when, sailing more leisurely through the skies, she takes 
full twenty-five hours and twenty seven minutes to perform her 
daily journey. 
As the retarding of the tides regularly corresponds with the 
retarding of the moon, they always return at the same hour 
after the lapse of fourteen days, so that at the end of each of 
her monthly revolutions, the moon always finds them in the 
same position. The knowledge of this fact is extremely useful 
to navigators, as it is easy to calculate the time of any tide in a 
port by knowing when it is high-water on the days of new and 
full moon. 
The height of the tides in the same place is as unequal and 
changing as the period of their intervals, and is equally depen- 
dent on the phases of the moon, increasing with her growth, and 
diminishing with her decrease. New and full moon always 
cause a higher rising of the flood (spring-tide), followed by a 
deeper ebb, while at half-moon the change of level is much less 
considerable (neap-tide). Thus in Plymouth, for instance, the 
D 
