THE TIDES. 107 



LESSON 48. 



i 



The Tides. 



'The sea is observed to flow for certain hours from the 

 south towards the north. In this motion, which lasts about 

 six hours, the sea gradually swells ; so that entering the 

 mouths of rivers, it drives back the waters towards their 

 heads. After a continual flow of six hours, the sea seems 

 to rest for about a quarter of an hour ; it then begins to ebb, 

 or retire back again from ^orth to south for six hours more ; 

 and the rivers resume their natural course. Then^ after a 

 seeming pause of a quarter of an hour, the sea again begins 

 to flow, as before, and thus alternately. This regular and 

 alternate motion of the sea constitutes the tides. They are 

 chiefly occasioned by the attraction of the moon, but are af- 

 fected by that of the sun. There are two tides in about 

 twenty-five hours ; and the time of high or low water is eve- 

 ry day fifty minutes later than on the preceding day. The 

 moon is supposed to draw the earth towards itself, and to 

 act upon the solid parts of it, in the same manner as if its 

 whole weight were in a single point in or near the centre. 

 Now the waters al any place over which the moon is passing, 

 will be more attracted than the earth ; and therefore will be 

 heaped up under the moon. But the waters on the opposite 

 side of the globe will be less attracted than the earth ; con- 

 sequently the earth is drawn away from them ; and they are 

 heaped up, or, in other words, it is high water there. When 

 the waters are elevated at the side of the earth under the 

 moon, and at the opposite side also, it is evident they must 

 recede from the intermediate points, and thus the attraction 

 pf the moon will produce high water at two places and low 

 water at two places on the earth at the same time. The 

 tide is fifty minutes later every day, because it is twenty-four 

 hours and fifty minutes before the same meridian on our 

 globe returns beneath the moon. The earth revolves on its 

 axis in about twenty-four hours ; if the moon, therefore, 

 were stationary, the same part oC our globe would return 

 beneath it, every twenty-four hours ; but as during our daily 

 revolution the moon advances in her orbit, the earth must 

 jnake more than a complete revolution in order to bring the 



