OF TIDES. 31Q 



A detached portion of a fluid would .naturally assume, by its 

 mutual gravitation, a spherical form, but if it gravitate towards 

 another body at a distance, it will become an oblong spheriod of 

 which the axis will point to the attracting body : for the difference 

 of the attraction of this body on its different parts will tend to se- 

 parate them from each other in the greatest part of the sphere, that 

 is, at all places within the angular distance of 79* from the line pass- 

 ing through the attracting body, either in the nearer or in the remoter 

 hemisphere; but to urge them towards the centre, although with a 

 smaller force in the remaining part. Hence, in order that there 

 may be an equilibrium, the depth of the fluid must be greatest where 

 its gravitation, thus composed, is least ; that is, in the line directed 

 towards the attracting body, and it may be shown that it must as- 

 sume the form of an oblong elliptic spheroid. 



If the earth were wholly fluid, and the same part of its surface 

 were always turned towards the moon, the pole of the spheroid 

 being immediately under the moon, the lunar tide would remain 

 stationary, the greatest elevation being at the points nearest to the 

 moon and furthest from her, and the greatest depression in the 

 circle equally distant from these points; the elevation b?ing, how- 

 ever, on account of the smaller surface to which it is confined twice 

 as great as the depression. The actual height of thi selevation 

 would probably be about forty inches, and the depression twenty, 

 making together a tide of five feet. If also the waters were capa- 

 ble of assuming instantly such a form as the equilibrium would re- 

 quire, the summit of a spheroid equally elevated would still be 

 directed towards the moon, notwithstanding the earth's rotation. 

 This may be called the primitive tide of the ocean : but on account 

 of the perpetual change of place which is required for the accom- 

 modation of the surface to a similar position with respect to the 

 moon, as the earth revolves, the form must be materially different 

 from that of such a spheroid of equilibrium. The force employed 

 in producing this accommodation may be estimated by considering 

 the actual surface of the sea as that of a wave moving on the sphe- 

 roid of equilibrium, and producing in the water a sufficient velocity 

 to preserve the actual form. We may deduce, from this mode of 

 considering the subject, a theory of the tides which appears to be 

 wore simple and satisfactory than any which has yet been publish- 



