252 ME. CLERK MAXWELL ON THE VISCOSITY 



proper places, and the fixed disks are brought to their proper distances from them by 

 means of the adjusting nuts. 



« s is a small piece of magnetized steel wire attached to the axis. 



When it is desired to set the disks in motion, a battery of magnets is placed under N, 

 and so moved as to bring the initial arc of vibration to the proper value. 



Fig. 4 is a brass ring whose moment of inertia is known. It is placed centrically on 

 the vibrating disk by means of three radial wires, which keep it exactly in its place. 



Fig. 7 is a tube containing two nearly equal weights, which slide inside it, and whose 

 position can be read off by verniers. 



The ring and the tube are used in finding the moment of inertia of the vibrating 

 apparatus. 



The extent and duration of the vibrations are observed in the ordinary way by means 

 of a telescope, which shows the reflexion of a scale in the mirror d. The scale is on a 

 circular arc of six feet radius, concentric with the axis of the instrument. The extre- 

 mities of the scale correspond to an arc of vibration of 19° 36', and the divisions on the 

 scale to 1''7. The readings are usually taken to tenths of a division. 



Method of Observation. 



When the instrument was properly adjusted, a battery of magnets was placed on 

 a board below N, and reversed at proper intervals till the arc of vibration extended 

 slightly beyond the limits of the scale. The magnets were then removed, and any acci- 

 dental pendulous oscillations of the suspended disks were checked by applying the hand 

 to the suspension-tube. The barometer and thermometer were then read off", and the 

 observer took his seat at the telescope and wrote down the extreme limits of each vibra- 

 tion as shown by the numbers on the scale. At intervals of five complete vibrations, 

 the time of the transits of the middle point of the scale was observed (see Table I.). 

 When the amplitude decreased rapidly, the observations were continued throughout the 

 experiment ; but when the decrement was small, the observer generally left the room 

 for an hour, or till the amplitude was so far reduced as to furnish the most accurate 

 results. 



In observing a quantity which decreases in a geometrical ratio in equal times, the 

 most accurate value of the rate of decrement will be deduced from a comparison of the 

 initial values with values which are to these in the ratio of e to 1, where e = 2-71828, 

 the base of the Napierian system of logarithms. In practice, however, it is best to 

 stop the experiment somewhat before the vibrations are so much reduced, as the time 

 required would be better spent in beginning a new experiment. 



In reducing the observations, the sum of every five maxima and of the consecutive 

 five minima was taken, and the differences of these were written as the terms of the 

 series the decrement of which was to be found. 



In experiments where the law of decrement is uncertain, this rough method is inap- 

 plicable, and Gauss's method must be applied ; but the series of amplitudes in these 



