OJf PRACTICAL ASTRONOMY. SSj 



XXXV. Fig. 515, 516. Plate XXXVI. Fig. 517. Plate XXXVII» 

 Fig. 518.) 



For representing the real as well as the apparent motions of the different 

 parts of the solar system, planetariums or orreries have sometimes been era- 

 ployed, in which the comparative periods of the revolutions have been cx^ 

 pressed by various combinations of wheelwork. Of these instruments Ar- 

 chimedes was the original inventor, and Iluygens revived them, with many 

 improvements, in modern times. The construction of the large planetarium, 

 which has been made in the house of the Royal Institution, was principally 

 directed by Mr. Pearson. I suggested to him, that the instrument might be 

 placed in a vertical position, and that the eccentricities of the planetary 

 orbits might be shown by the revolution of short arms, retained in their 

 situation by weights, and their deviation from the plane of the ecliptic by 

 inclining the axes of these arms, in a proper angle, to the plane of the instru- 

 ment. The other parts of the arrangement, which have any claim to novelty, 

 were entirely of Mr. Pearson's invention, and he appears to have rendered the 

 instrument in many respects more accurate than any other planetarium that 

 has ever been constructed. 



