Connected with the Earth's Field of Force. 133 



For an example on a smaller scale let us take Lake 

 Michigan, which extends from latitude 41° 20' to 46° 00', 

 and let us suppose the level of its surface to be undis- 

 turbed by conditions of wind, current or temperature. We 

 should then be apt to say that such a surface would surely 

 be level, and so it would be in the proper sense of the word. 



Fig. 2. 



Fig. 2. — Showing the increase in the ellipticity of a level surface with 

 its dimensions and the convexity of the vertical towards the equator. 



Nevertheless the northern end is 8 centimeters nearer sea 

 level than the southern end.^ The elevation of a point 

 above sea level has only a geometrical significance, not a 

 dynamical one, until we know where it is located. That 

 is, suppose two points' nearly at the same elevation; we 

 cannot tell whether water would flow from the one that is 

 at the greater distance above sea level to the one that is 

 at the smaller distance above sea level or vice versa until 

 we know in what latitudes the points are. For the point 

 that seemed higher may lie on a level surface situated 

 inside the surface containing the point that seemed lower. 

 It is convenient in dynamical problems to devise some 

 system of numbering the successive level surfaces and, 

 instead of dealing with the distance of a point above sea 

 level, to deal with the number of the level surface on which 

 it lies. Meteorologists now do something of the sort,^^ 

 but the ordinary surveyor finds the idea of numbered 

 level surfaces, or dynamic heights as they have been 

 called, rather abstract, and in many cases an unnecessary 

 refinement. For his benefit the Coast and Geodetic Sur- 

 vey gives the result of its precise leveling in terms of 



^ This figure is based on an average elevation of the lake surface of 177 

 meters. If this were greater, the difference between the two ends would be 

 greater also. 



^"For example Bjerknes' Dynamic Meteorology and Hydrography (Wash- 

 ington, Carnegie Institution, 1910) p. 13. 



