182 ANNUAL OF SCIENTIFIC DISCOVERT. 



vertical pillar. At the lower end of the first axis is a -weight which more 

 than counterpoises the upper part. If, then, there be no rotation in the 

 bar about the first axis, the effect of the weight is to produce a rotation 

 about the second alone, bringing down the first axis into a vertical position. 

 If now the first axis be held horizontally or obliquely, and a rotatory 

 motion be given to the bar about it, on letting the axis go, we compound 

 both rotations ; and the resulting effect is, that the weight will no longer 

 bring the axis down, or alter its inclination at all, but will cause it to take 

 a new position, or make the whole turn round the vertical, in a direction 

 opposite to that of the rotation. Thus, although confessedly not new in 

 principle, to make public an experimental illustration in so simple a form 

 may not be without its use for a great majority of students. Even the 

 theoretical principle is capable of being stated in a way quite intelligible 

 to those acquainted only with the very first rudiments of theoretical 

 mechanics, presenting itself in close analogy to that well-known first 

 principle, the composition of rectilinear motion. As in this last case, if a 

 body be in motion in one direction, and any cause tends to make it move in 

 another, it will move in neither, but in an intermediate direction, so we 

 have the strictly analogous case in rotatory motion ; when 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 two. Thus the result of compounding the two rotations will be that 

 the axis (carrying with it the rotating body) will simply take a new 

 position, or will move in a direction determined by the nature of the im- 

 pressed motions. Professor Magnus, in an able, but rather prolix and 

 obscurely-written memoir, speaks of the consequences 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 singular 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 rotation, the equilibrium is restored. This is nothing but a 

 simple case of the principle just stated, as shown by the author's appara- 

 tus. Besides certain other cases traceable to a different cause, Professor 

 Magnus's immediate object is to explain a curious observed anomaly in 

 the motion of projectiles of an elongated form shot from rified gun?, and 

 which consequently rotate about their axis while passing through the air 

 in the direction of that axis. He mentions the fact that artillery experi- 

 ments in different countries, with rifled cannon and missiles of a cylindrical 

 form with a conical apex, always show a deviation of the point of the 

 missile to the right, the riile-spiral being right-handed. To explain 

 the nature of this deviation was the object of special experiments on 



