278 Description of the Rotascope. 



The perfection of the workmanship on the pivots and their bear- 

 ings, will also be tested by this means, and it will be seen whether 

 both ends of the arbor be finished with the same exactness. The 

 friction on one end may, for the purpose of these experiments, be 

 increased in any moderate degree, by furnishing that end with a 

 small ring to form a bearing, through which the conical end of the 

 shaft forming the pivot, may pass, and in which it may rest instead 

 of resting and rubbing solely on the point. This ring will increase 

 the moment of friction, by carrying the rubbing surface of the shaft 

 farther from the matliematical axis of rotation. 



The following table presents the results of experiments made by 

 means of weights of various magnitudes employed to produce rota- 

 tion and shows the times in which the friction of the pivots, brought 

 the wheel to rest ; the axis in each experiment being kept vertical 

 as long as the motion continued ; but reversing the ends in the two 

 separate series. The first column indicates the weights appended 

 to the cords at each trial ; the second, the number of seconds from 

 the instant the motion commenced to that of its actual cessation. The 

 third shows the ratios of the times observed with the several weights 

 compared with the time required to bring the wheel to rest after it 

 had been set in motion by one pound weight. The fourth column 

 exhibits the square roots of the numbers of pounds. The fifth con- 

 tains the differences between these roots and the aforesaid ratios, 

 marked -\- when the root is greater than the ratio, and — when the 

 reverse is true. The sixth column contains the number of seconds 

 in which the wheel would have come to rest on the supposition that 

 the continuance of motion is precisely as the square roots of the 

 weights which produce it. 



The departure of these numbers from the observed times is great- 

 er in the higher velocities than in the lower, owing, obviously, to the 

 greater resistance of the air in those cases. But in no instance does 

 the experimental differ from the theoretical result by more than ten 

 seconds. This is shown in the last column. 



It must be observed that in all the experiments the weights de- 

 scended through precisely the same distances, and of course devel- 

 oped from the axis the same number of spirals of cord before its cen- 

 tre quitted the knob over which it had been doubled. We might re- 

 peat the operations, using all the different weights with different 

 heights. Ingenious persons will find much interest in experiments 

 of this nature. 



