614 



SCIENTIFIC RECREATIONS. 



fir 



Fig. 700. Tidal Attraction. 



These tidal waves are also experienced in the Ganges, the Amazon, and 

 at Bordeaux, as well as in China and elsewhere. 



It may well be imagined that the tides also affect 

 the land, and the theory of these ocean movements is 

 a very interesting study. We have already referred to 

 it under ASTRONOMY, for the Sun's and Moon's attrac- 

 tion is the main cause of the phenomenon, which is so 

 familiar and yet so strange. But the consideration of 

 the tides must be again entered upon here ere we pro- 

 ceed to view the effects of the sea upon the land, and 

 how the physical geographical features alter. 



Isaac Newton rightly attributed the cause of the 

 tides to the attraction of the moon and sun. Spring 

 tides occur when both luminaries are above the meridian, 

 and the neap, or low tides, happen when the sun and 

 moon are farthest apart. The highest tides are perceived 

 after a new or full moon ; the lowest, after she has passed 

 the first or third quarter. In January the spring tide is 

 highest of all, because the earth is nearest to the sun 

 then, and his force of attraction, added to that of the moon, causes a very 

 high tide. With the assistance of the accompanying dia- 

 grams we shall be able to make the tidal phenomena clear. y 



Suppose the moon to be at M, the point J (the sea) 

 will be nearest to the moon and will be attracted, while 

 the earth will exercise a retarding power to a certain extent. 

 This attraction of the water from its usual level causes a 

 kind of vacuum, into which the surrounding water flows 

 and causes a high tide at H. At the opposite side the 

 earth, not the water, is most attracted, and then the water 

 rushes in to a certain extent to fill the vacancy left by the 

 earth's movement towards the moon. Another high tide 

 is therefore caused at L, but not so high as the tide upon 

 the opposite side, as the Moon is so much nearer the latter. 

 The tide, then, is only the natural movement of the sea 

 water to fill up the space the earth and other portions of 

 the watery mass have vacated in obedience to lunar and 

 solar attraction, which is, to a certain extent, counterbalanced 

 by the attraction and resistance of the earth. 



The neap tides are caused by the opposing forces of 

 attraction of the sun and moon. The sun, as it were, pulls 

 one way, the moon the other. The latter (being nearer) 

 having twice the power of the former, causes a tide indeed, 



Fig. 701. Tidal Attraction, 



but it is a low one. The spring tide occurs when sun and 

 moon together attract the water. 



The effects of the rise of the tide are sometimes very disastrous, 



