526 ADDITIONS. 



turns away from its first position. Now this is what really happens, 

 and thus the revolution of the Earth in absolute space is experimentally 

 proved. 



In subsequent experiments, M. Foucault has used the rotation of a 

 body to prove the same thing. For when a body rotates freely, acted 

 upon by no power, there is nothing to change the position of the axis 

 of rotation in absolute space. But if the position of the axis remain 

 the same in absolute space, it will, in virtue of its relative motion, 

 change as seen by a spectator at any spot on the rotating Earth. By 

 taking a heavy disk or globe and making it rotate on its axis rapidly, 

 the force of absolute permanence (as compared with the inevitable 

 casual disturbances arising from the machinery which supports the 

 revolving disk) becomes considerable ; and hence the relative motion 

 can, in this way also, be made visible. 



Mr. De Morgan has said (Comp. to Brit. Aim. 1836, p. 18) that 

 astronomy does not supply any argument for the earth's motion which 

 is absolutely and demonstrably conclusive, till we come to the Aberra- 

 tion of Light. But we may now venture to say that the experiments 

 of M. Foucault prove the diurnal motion of the Earth in the most 

 conclusive manner, by palpable and broad effects, if we accept the 

 doctrines of the science of Mechanics : while Aberration proves the 

 annual motion, if we suppose that we can observe the places of the 

 fixed stars to the accuracy of a few seconds ; and if we accept, in addi- 

 tion to the doctrines of Mechanics, the doctrine of the motion of light 

 with a certain great velocity. 



CHAPTER III. 



SEQUEL TO COPERNICUS. 



English Copernicans. 



"HROFESSOR DE MORGAN has made numerous and interesting 

 1- contributions to the history of the progress and reception of the 

 Copernican System. These are given mainly in the Companion to 

 the British Almanac; especially in his papers entitled "Old Argu- 

 ments against the Motion of the Earth" (1836) ; " English Mathemat- 

 ical and Astronomical Writers" (1837) ; " On the Difficulty of Correct 



