THE TIDES. 



229 



very great, and therefore not much deflected from a straight line, the 

 waters are driven very little above the usual surface, no matter how 

 rapidly the earth herself may move in this curve. The centrifugal 

 force or original impulse felt by the whole earth is very great, but 

 that felt by her waters is hardly visible or sensible in mid-ocean. For 

 the tide-waves cannot get above the line tangent to the curve of the 

 earth's orbit. The following illustration will show this: 



Let ABC (Fig. 2) represent a part of the curve of the earth's 

 orbit, in its motion around the central sun, and B D a line tangent 



_o a 



~"~" ~ 



A c 



Fig. 2. 



to the curve at the point B. Now it is very evident that no tide- 

 wave produced by centrifugal force can get higher above the curve of 

 the orbit than this tangent line, and the distance between the curve 

 and the tangent, as at E, is very small. The part of the earth's sur- 

 face most remote from the sun has indeed a greater tendency to con- 

 tinue moving on in the straight line of the original impulse than any 

 other part. The particles of water have a small degree of cohesion, 

 and they will therefore continue to move a short distance along this 

 tangent, but only a little above the usual surface of the earth. 



The curve in which the surface of the earth moves around the cen- 

 tre of gravity between herself and the moon is much more deflected 

 from a straight line. Here also the tide-wave can rise no higher than 

 to the line tangent to this curve. The distance of the point G (Fig. 3) 

 from the curve is, however, much greater than the point E'm Fig. 2 

 from its curve. The motion of the surface of the earth at H around 



