210 PROCEEDINGS OF THE AMERICAN ACADEMY 



its horizontal axis. But if the machine is prevented from moving 

 round a vertical axis, there is no difficulty in disturbing it around its 

 horizontal axis. 



" Thus it appears in this experiment, as well as in those made with 

 the Bohnenberger and Fessel machines, that a force acting upon a free 

 body is prevented from producing motion in its oivn direction by the 

 conical motion which exists around a rectangular axis. The same 

 experiments can be made with the Bohnenberger apparatus, by hold- 

 ing or releasing the middle ring. In mechanics, a body has lost its 

 stability of rotation when it has lost its freedom : and the most complete 

 stability is consistent with perfect freedom. Astronomy hangs up for 

 ever in the sky a splendid illustration of this principle. It cannot be 

 that a less noble law prevails in the kingdom of mind than in that of 

 matter. When the two discs are made to rotate in opposite directions 

 with the same velocity, there is no stability, even when the apparatus 

 is most free. For the tendency of the two rotations when combined 

 with a foreign disturbance being to produce equal and opposite conical 

 motions, the result is the same want of stability as if there was no 

 conical motion in either direction." * 



Professor Felton made a short communication on the Pnyx 

 and Bema, at Athens. He remarked that Greek topography 

 is to a great extent a restored science, indebted for its present 

 form and precision to the labors of modern scholars, and to 

 none so much as to Colonel William Martin Leake. A map 

 was exhibited, on which the physical features of Athens were 

 delineated, and the sites of the principal antiquities indicated. 

 Another map was also shown, exhibiting the hill of the Pnyx, 

 with the Bema, carefully drawn according to their exact meas- 

 urements. The meaning of the names was explained, and 

 it was remarked, that, if these objects are what they are 

 now generally supposed to be, the spot is not only one of the 

 most interesting in Athens, but in the world. The references 

 in the ancient writers were then reviewed in the following 

 order : — 1. Thucydides, B. C. 471. 2. Aristophanes, 444, 

 in several plays, — the Acharnians, Ecclesiazousae, Knights. 



* Ann. Pogg., LXXXVIII. 



