20 Wilde, Evolution of the Mental Faculties. 



only disturbing element to be accounted for is the small 

 amount of friction at the centres of rotation. 



The method of experimenting with the instrument 

 was as follows : — The ring in which the disc rotates was 

 firmly supported in a horizontal position, and at a 

 sufficient height to permit two weights of 0'5 lb. and 2 lbs. 

 each to fall through spaces of 30 inches and 120 inches 

 respectively. The diameter of the spindle on which the 

 disc revolved could be doubled, as required, by means of 

 a loose cylinder secured thereon and carried round with 

 the spindle. Motion was given to the disc by means of a 

 fine thread closely wound on the spindle without the turns 

 overlapping each other. The weights were attached 

 directly to one end of the thread, while the other end 

 terminated in a loop to slip over a small pin projecting 

 from the spindle in the usual manner. The time in which 

 the thread was unwound from the spindle at the com- 

 mencement of the fall of the weight, to the time when 

 the last turn of the thread left the spindle, was determined 

 with precision by means of a stop watch. 



With the smaller weight and a fall of 30 inches, it was 

 found that the friction of the centres of rotation increased 

 the time of descent by nearly half a second, as in Smeaton's 

 experiments, but as this retardation was not observable in 

 experiments with the larger diameter of the spindle and 

 with the greater weight, the discrepancy has been elimin- 

 ated from the table of results. The number of revolutions 

 of the disc per minute was calculated from the observed 

 times of revolution at a low velocity, and also from the 

 time of descent into the number of turns of the thread on 

 the spindle and twice the uniform rate of revolution for 

 the final velocity, in accordance with the law of acceleration 

 of movine bodies from a state of rest. 



