Rotatory Motion, the Gyroscope, ^c. 149 



up to the same level again ? Any dynamical power its descent may give 

 it is entirely reabsorbed by its reasceut. Again, the mere pendulous 

 action or oscillation would not account altogether for the pressure of 

 the weight. What, according to Major Barnard's theory, becomes of 

 the pressure corresponding to that of an ordinary pendulum upon its 

 point of support— and which, in the gyroscope, acts on a point at some 

 distance from the point of support ? 



Major Barnard makes some remarks on what he terms " the popular 

 idea that a rotating body offers direct resistance to a change of its plane." 

 As the supposed resistance is a phenomena of rotatory motion, it will 

 not be out of place to 'introduce the subject here, particularly as very 

 incomplete, if not erroneous notions about it appear to be prevalent. 

 Major Barnard says, " If the extremity of the axis of rotation were 

 confined in a vertical circular groove, in which it could move without 

 friction, the rotating disc would vibrate in the vertical plane as if no 

 rotation existed. What, then," he adds, "is the resistance to a change 

 of plane of rotation so often alluded to and described ? A misnomer 

 entirely." Now, the application of the groove does not quite reduce 

 the case of rotation to that of non-rotation ; for in the former case, 

 since the weight tends to produce a horizontal movement of the axis, it 

 must exert a horizontal pressure against the side of the groove ; whilst 

 in the case of non-rotation there is of course no such horizontal pressure. 

 Indeed, the pressure against the side of the groove, and consequently 

 on the pivots of the fly-wheel, is so great as to considerably reduce the 

 rotatory velocity of the wheel. My gyroscope will spin twelve minutes 

 when no force is apphed to change its plane ; but with an equal velocity 

 of starting it will not spin longer than three or three and a-half minutes, 

 if a force acts on it tending to produce a precessional motion, which is 

 prevented. However, notwithstanding that Major Barnard's remarks 

 are unsatisfactory, the popular idea on the subject requires correction. 



There is, in reality, no greater resistance connected with rotatory 

 motion than with rectilineal motion. It is inaccurate to say, that a 

 rotating body offers a greater resistance to a change of plane than a 

 non-rotating body ; but it is correct to say that a rotating body offers a 

 (jreater resistance than a non-rotating body to an attempt to make it rotate 

 in a particular new plane, by a force directed parallel to, or in that tiew 

 plane. We should feel precisely the same resistance, under the same 

 circumstances, in attempting to change the direction of rectilinear 

 motion. If a body is moving rectilineally in one direction, and we wish 

 to make it move in some other particular direction, we must apply an 

 impulse to it, of such an intensity, and in such a direction, as when 

 compounded with the original motion will give the desired result, on 



