Royal Institution. 293 



case in rotatory motion ; ivhen a body is rotating about an axis, and 

 any cause tends to make it rotate about another axis, it will not rotate 

 about either, but about a new axis intermediate to the tioo. Thus the 

 result of compounding the two rotations will be, that the axis (car- 

 rying with it the rotating body) will simply take a new position, or will 

 move in a direction determined by the nature of the impressed motions. 



Professor Magnus, in the very able but rather prolix and obscurely 

 written memoir before referred to, speaks (p. 223) of the conse- 

 quences of such a law as evinced in the resulting rotations, but 

 ■without any distinct or explicit statement of the essential theorem of 

 the composition of rotatory motion. He gives, however, some sin- 

 gular and even paradoxical exemplifications of it. We may allude 

 to one of these, which is capable of being put into a form at once 

 more simple, and at the same time more paradoxical, than that in 

 which he describes it. It consists in this : a wheel at one end of 

 an axis and a weight at the other are suspended in equilibrio, which 

 is of course unaltered, whether the wheel be at rest or in rotation ; 

 the weight is then slid so that the balance is destroyed; now if the 

 wheel be set in rapid rotatioii, the equilibrium is restored. This is 

 nothing but a simple case of the principle just stated, as shown 

 by the author's apparatus. 



Besides certain other cases traceable to a different cause. Professor 

 Magnus's immediate object is to explan a curious observed anomaly 

 in the motion of projectiles of an elongated form shot from rifled 

 guns, and which consequently rotate about their axis while passing 

 through the air in the direction of that axis. 



He mentions the fact, that artillery experiments in diiferent coun- 

 tries with rifled cannon and missiles of a cylindrical form with a 

 conical ajoex, always shoiv a deviatioti of the popit of the missile to the 

 right, the rifle-spiral being right-handed. 



To explain the nature of this deviation was the object of special 

 experiments on the part of the Prussian Artillery Commission, in 

 which Professor Magnus assisted. The missiles were fired with low 

 charges, so as to allow the motion to be accurately observed, and 

 it was found that the axis remained sensibly in the direction of the 

 tangent to the curved path, while the deviation to the right was 

 always clearly marked. He observes that left-handed rifles have 

 never been tried. 



Professor Magnus, after some fruitless conjectures as to the cause, 

 at length sought it in the principle of the composition of rotatory 

 motion. He tried exijerimentally the effect of a current of air on a 

 projectile of the form employed, by inserting such a body instead of 

 the rotating sphere in Bohnenberger's apparatus, and observing the 

 effect on it, first at rest and then in rotation, when the strong cur- 

 rent of a blowing-machine was directed against the conical apex. 

 When at rest, the current elevated the apex ; owing to the form of 

 the missile, the resistance acting not through the centre of gravity, 

 but above it ; wlien in rotation no elevation took place, but a devia- 

 tion in the direction of the axis, in a direction opposite to that of 

 rotation. To show the application of the principle in this case, he 

 observes that the axis of the elongated projectile, which for an instant 



