256 POPULAR LECTURES AND ADDRESSES. 



infinitely accurate spirit-level or plummet far 

 enough away from the sea to be not sensibly 

 affected by the attraction of the rising and falling- 

 water would enable us to find the answer. Observe 

 1)\- level or plummet the changes of direction of 

 apparent gravity relatively to an object rigidly 

 connected with the earth, and compare these 

 changes with w r hat they would be were the earth 

 perfectly rigid, according to the known masses and 

 distances of sun and moon. The discrepance, if 

 any is found, would show distortion of the earth 

 and would afford data for determining the dimen- 

 sions of the elliptic spheroid into which a non- 

 rotating globular mass of the same dimensions and 

 elasticity as the earth would be distorted by centri- 

 fugal force if set in rotation, or by tide-generat- 

 ing influences of sun or moon. The effect on the 

 plumb-line of the lunar tide-generating influence is 

 t<> deflect it towards or from the point of the 

 horizon nearest to the moon, according as the moon 

 is above or below the hori/.on. The effect is /cm 

 when the moon is on the hori/.on or overhead, and 

 ' MI either direction uhrn tlie moon is 



