NEWTON S PRINCIPIA. 337 



inferior limit to the thickness of the crust. We learn 

 that it must exceed 1000 miles. Mr. Hopkins has lately 

 undertaken a series of experiments with a view to ascer 

 taining the temperature at which bodies liquefy under 

 great pressures. An account of these was given at the 

 meeting of the British Association. When finally com 

 pleted they will throw great light on the thickness of the 

 earth s crust. 



8. It is not to be supposed that no measures were under 

 taken to discover by actual observation the ellipticity of 

 the globe : such measures would be an excellent test of the 

 truth of the theory. There are three methods by which 

 this may be effected. The earliest of these is by measure 

 ment of the length of a degree in different latitudes. ^The 

 length of a degree varies as the radius of curvature of the 

 elliptic meridian, and therefore increases from the equator 

 to the poles. Now when the ellipticity is given, we can 

 express in terms of it the latitude and equatorial radius, 

 the length of a degree at that place. By two measure 

 ments of degrees we get two equations, and therefore can 

 eliminate the equatorial radius and find the ellipticity. 

 Some of these measures were effected before Newton s 

 time, and their results, as we have seen, were used by him 

 in determining the ellipticity. In 1684 Cassini measured 

 an arc of eight degrees,* and found, to the astonishment of 

 every one, that the length of the degree shortened as he 

 approached the poles. This was in direct opposition to 

 Newton s theory. It was objected that the difference be 

 tween the measures of two consecutive degrees was so 

 small, that it was possible that the errors of observation 

 might make that which was really the lesser appear to be 

 the greater. To settle this doubt it was determined that 

 three arcs should be measured. Godin, Bouguer and 

 La Condamine were sent to Peru in 1735 to measure an 



* Encyc. Met., &quot;Figure of the Earth.&quot; 

 Z 



