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 j-f-g-g 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 



