NATURAL PHILOSOPHY. 149 



will be to refer to mere hydrostatic equilibrium, in which any portion 

 of these waters mav be regarded as exactly b alanced by any other con- 

 tiguous portion, eaeh being maintained at its level by the pressure of the 

 other w/iich supports it ; consequently, if this pressure, be in the case of 

 either portion, lessened or increased, in the least degree, that is to say, 

 if the force of gravity, to which this pressure is du-, be in any degree 

 counteracted or added to by any other force in one of these portions and 

 n >t in the other, the lighter portion will immediately give way and be 

 buoyed up by the heavier, which will of course simultaneously sink: and 

 this would be an explanation sufficient for the purpose. But as this 

 stiiemeat, though true, is not the whole truth, it may be well to go a 

 step fa iher. The waters of the ocean do not maintain their general 

 figure and outline under the influence of gravity alone. On the contrary, 

 it is well known that, by the centrifugal force generated by the earth's 

 rotation on its axis, they are kept at a higher level or greater distance 

 from the center on other parts of the globe than at the poles, this eleva- 

 tion amounting at the equator where it is greatest to about 13 miles. 

 They are therefore exactly suspended or poised between these two forces, 

 namely, the force of gravity and the centrifugal force just mentioned, 

 and any other force that should in the least degree add to or counteract 

 the influence of either of these forces would at once cause a change in 

 the figure of these waters. While therefore it is properly the hydrostatic 

 equilibrium existing between the different portions of the waters them- 

 selves that is disturbed by the action of the forces that produce the tides, 

 the statement just made may serve to show more clearly how far these 

 waters are, in their normal condition, from lying as a dead weight in 

 the depressions of the earth's surface that contain them. 



Again, it is stated in a familiar way, that the tide on the side of the 

 earth towards the moon is owing to the waters there being attracted by 

 it more than the mass of the earth because they are nearer, while the 

 tide raised at the same time on the opposite side of the earth results 

 from the earth being drawn away from the waters there because they 

 are more remote than the mass of the earth and are thus " left behind," 

 or " left heaped up ;" and then we are told that, at full moon, when the 

 attractions of the sun and moon are opposite in direction, they conspire 

 to pro luce spring tides in the same manner as at new moon when their 

 attractions coincide in direction. Now, as it is not easy to see how a 

 body can be drawn away so as to leave any thing behind in two opposite 

 directions at the same time, these statements appear quite inconsistent 

 and are well calculated to confuse and perplex. It is therefore important 

 and indeed indispensable to the c nnmunication of an intelligible view 

 of this phenomenon to explain, as be ore remarked, the conditions and 

 circumstances, or, to express it more definitely, the relations and de- 

 pendences existing among the bodies concerned in it : a course at once 

 so natural and so needful that it seems remarkable that it should not 

 have been more generally and more fully adopted. 



As the earth is held to its curved path around the sun by the attrac- 

 tion of that body acting in opposition to the centrifugal force generated 

 by its rapid motion, in the same manner that a heavy ball or weight 

 attached to the end of a cord and whirled around the head is held or 

 restrained by the cord, we may regard it as suspended between these 

 two forces, and if a ball bj merely suspended by a cord it will be a fair 

 13* 



