76 PROCEEDINGS OF THE AMERICAN ACADEMY. 



mounted on a square vertical mica vane, of about 3 square centimeters 

 area, which was fastened symmetrically on a slender but stiff bit of 

 glass filament. The filament was stretched between two pieces of 

 No. 36 B. & S. steel wire about 2 centimeters long. The righting 

 moment was due partly to the torsional forces in the wire and partly 

 to a strong electromagnetic field about the needles. When the circuit 

 of the magnetic field used to deflect the needle through the initial 

 angle 6q was suddenly broken, the vane and its belongings moved 

 quickly (in perhaps 1 /250th of a second) through the position of equi- 

 librium and out on the other side to a turning point corresponding to 

 a deviation of about three fourths of 0q. After this the amplitudes 

 decreased slowly and continuously. The curve drawn through the 

 crests of the oscillations consists at the start of a vertical line, as it 

 would if, for instance, the resistance followed the law of the square 

 of the angular velocity. After a short time, however, the curve, like 

 most of those which I have obtained, follows more nearly a course 

 which corresponds to the equation y = A • e~ at . The numbers in the 

 next table show well enough what the character of the agreement is. 

 The first column gives ordinates of the photographic record taken at 

 equal time intervals. The second column gives corresponding ordi- 

 nates of a curve of the family y = A • e~ at which falls in very nearly with 

 the first curve for a portion of the middle of its course. 



V. Figure C represents curves taken with this apparatus when the 

 filament was made of a piece of manganine wire. One curve is here 

 displaced an arbitrary amount with respect to the other, for purposes 

 of comparison. The sudden drop (ST) from the original deflected 

 position to one of much smaller displacement, after which the de- 



