550 ADDITIONS. 



Saturn. And in the same way, if the attraction of the Earth had anv 

 specific relation to different kinds of matter, the time of oscillation ot 

 a pendulum of equal length composed wholly or iu part of the two 

 substances would be different. If, for instance, it were more intense 

 for magnetized iron than for stone, the iron pendulum would oscillate 

 more quickly. Bessel showed 1 that it was possible to assume hypothet- 

 ically a constitution of the sun, planets, and their appendages, such 

 that the attraction of the Sun on the Planets and Satellites should be 

 proportional to the quantity of matter in each ; but that the attraction 

 of the Planets on one another would not be on the same scale. 



Newton had made experiments (described in the Printiipia, Book 

 iii., Prop, vi.) by which it was shown that there could be no consider- 

 able or palpable amount of such specific difference among terrestrial 

 bodies, but his experiments could not be regarded as exact enough for 

 the requirements of modern science. Bessel instituted a laborious seriec 

 of experiments (presented to the Berlin Academy in 1832) which com- 

 pletely disproved the conjecture of such a difference ; every substance 

 examined having given exactly the same coefficient of gravitating in- 

 tensity as compared with inertia. Among the substances examined 

 were metallic and stony masses of meteoric origin, which might be 

 supposed, if any bodies could, to come from other parts of the solai 

 system. 



CHAPTER IV. 



VERIFICATION AND COMPLETION OF THE NEWTONIAN THEOF.V 



Taltles of the Moon and Planets. 



fTUIE Newtonian discovery of Universal Gravitation, so remarkable 

 -L in other respects, is also remarkable as exemplifying the immense 

 extent to which the verification of a great truth may be carried, the 

 amount of human labor which may be requisite to do it justice, and 

 the striking extension of human knowledge to which it may lead. I 

 have said that it is remarked as a beauty in the first fixation of a the- 

 ory that its measures or elements are established by means of a few 



' Berlin Man. 1S24. 



