119 
the particles of magnetic bodies , &c. 
the expression for the force will become equal to o ; and 
with a still farther increase of the velocity, the force will 
become negative, and the motion of the ring consequently 
retrograde. The velocity that would be required to produce 
this effect with copper, may be much beyond what could be 
produced with the requisite apparatus ; but as the value of %[/, 
the magnets revolving with the same velocity, must vary 
considerably with different substances, it is by no means 
improbable that with steel slightly hardened, or perhaps even 
with hammered iron, a retrograde motion might be produced 
by an angular velocity more within our command. The 
success of this experiment would afford a very striking illus- 
tration of the principle which is the basis of the preceding 
calculation. 
With regard to the value of «, all that we can infer from 
the experiments is, that it must be extremely small. If 
c'=' 6 , then u ='*00005 would reduce the expression for 
the force to its first term ; and the same would be the case 
when f= 7, or d = 2-646, if u = *038. Taking a to be -oi, 
the error that would arise by neglecting the second and 
following terms in the expression for the force, considering 
the first term as 1, would be — '0147, when d = -6 ; and it 
would be + -0084 when d = 2-646, the first term being also 
considered as 1 in this case. This probably is not far from 
the real value of a in the present instance, since if it were 
much greater than *oi, the error that would arise from 
neglecting it, when d is small, would be considerable, and 
would be minus ; and although this is the case in the results in 
Tables VII. and X. it is not so in that in Table III. The errors 
however by which the observations would be affected from a 
