40 OUTLINES OF NATURAL PHILOSOPHY. 



dicular to the direction of gravity, or to the di- 

 rection of the plumb-line, ( 24. .). 



This surface is the same that the sea would have if it 

 were continued all round the earth ; or, if we were 

 to trace curve lines, by levelling from a given point 

 round the earth, in every direction, till they re- 

 turned into themselves, the superficies in which all 

 these lines would lie, is that which we consider as 

 the superficies of the earth. The given point may 

 be supposed any one, on the level of the sea. 



The figure bounded by this superficies, is that which 

 is really measured by the combined methods of 

 astronomy and practical geometry, and is to be 

 carefully distinguished from the actual figure of 

 the earth, including all its inequalities ; or from an 

 average figure that should leave out as much of 

 solid matter above it, as is included of empty space 

 under it. 



54. The length of an arch of the meridian, 

 traced on the superficies above defined, may be 

 measured by observing the latitude of the two 

 extremities of the arc, and then measuring the 

 distance between these points in fathoms, toises, 

 pr any other known measure. 



The distance, as measured on the surface, divided by 

 $he degrees, and parts of a degree contained in the 



difference 



