Description of the Rot a scope. 2^ 



Art. III. — Description of an Apparatus called the Rotascope, for 

 exhibiting several phenomena and illustrating certain laws of ro- 

 tary motion; by Walter R. Johnson, Professor of Mechanics 

 and Natural Philosophy in the Franklin Institute, Philadelphia. 



The laws relating to rotary motion, have been frequently made 

 the subjects of profound analysis, and the transactions of some learn- 

 ed societies of Europe contain numerous papers, expressly designed 

 to elucidate this branch of mechanical science. 



Nearly all the discussions of the subject, however, which have 

 hitherto appeared, have been of the abstract and transcendental 

 character. Hence they have, in few instances, either interested or 

 instructed the cultivators of practical science, and those who have 

 long been familiar with mechanical forces and motions are frequently 

 quite unacquainted, except by casual occurrences, with any of the 

 curious laws which concern the positions and changes of position in 

 the axes of rotation, in revolving bodies. Our common books of 

 mechanics generally contain concise accounts of the doctrines of ro- 

 tary motion, limited for the most part, however, to the consideration 

 of central forces, the centre of percussion and of gyration, and the 

 centre of spontaneous rotation, to which may be added that of oscil- 

 latory motion. 



The forces tending to change the position of the axes of rotation 

 are generally either wholly omitted, or if concisely stated in an ah-' 

 stract form, are apparently regarded as incapable of experimental 

 illustration. The whirling table of Mr. Ferguson is an ingenious ap- 

 paratus for exhibiting the amount and directions of the several forces 

 exerted by a body in its own fixed plane of revolution. But that in- 

 strument makes no provision for the phenomena above referred to. 



When we consider that the extensive diffusion of a branch of 

 knowledge often depends on the facility with which its elements can 

 be made apparent to the understanding, we are at no loss in estima- 

 ting the practical value of philosophical instruments, whether intend- 

 ed for demonstration, or for research. Of this truth the machine of 

 Atwood may be taken as an illustration. This machine gives a most 

 elegant and satisfactory exhibition of the principles of uniform, accel- 

 erated and retarded motions, as dependent on the force of gravity. 

 All the motions in the machine may be so slow as to reduce the re- 

 sistance of the air to an unimportant element, and the friction and in- 



VoL. XXL— No. 2. 34 



