ISi ANNUAL OF SCIENTIFIC DISCOVERY. 



from the reaction of the air, to the right or left, according as the vanes are 

 inclined ; and on trying this there was always observed a deviation in the 

 direction of the axis or point of the missile to the right or left accordingly, 

 relative to the experimenter. It is in fact nearly impossible to throw such 

 a body in a direction perfectly in one .plane. The true deviation is, how- 

 ever, peculiarly liable to be disguised by the general resistance of the air 

 on so light a missile, as well as by currents, &c., which it is not easy to 

 guard against. The well-known case of the boomerang exhibits effects 

 closely similar ; for it is found that, if so projected that its rotation is from 

 left to right, its deviation will be in the same direction, and vice versa ; that 

 is, supposing (as is the usual case) that its plane is inclined upwards from 

 the operator : if it be inclined downwards, the deviation is in the direc- 

 tion opposite to that of the rotation. In the former case, the reaction of 

 the air against the flat surface of the missile would tend to increase its 

 inclination upwards, in the latter downwards, with respect to the operator ; 

 and this in each case respectively would give the motion stated, as is 

 easily seen on the principle and by means of the apparatus before 

 described. Thus it would follow that this extraordinary instance of savage 

 invention, which long ago puzzled inquirers, is simply a case (like the last) 

 of "the composition of rotatory motion." It should, however, be men- 

 tioned that some experimentalists have entertained a different view of the 

 cause of deviation in this instance. From these singular applications of a 

 very simple mechanical truth, we may now turn to what is but another 

 exemplification of the same thing, however apparently remote from those 

 we have considered, and upon a far grander scale. The phenomenon of 

 the Precession of Equinoxes was known to Hipparchus ; but no explana- 

 tion of the fact was for ages imagined. Even Kepler, in the multiplicity 

 of his hypothetical resources, could not succeed in devising any thing plau- 

 sible. The axis of the earth is slowly shifting its position, so that its pole 

 points continually to a new part of the heavens, a new polestar, at the 

 rate of about 50" a year, and of course carries with it the point of inter- 

 section of the earth's equator with the ecliptic or plane of its orbit at the 

 same rate and in a direction opposite to that of its motion or the order of 

 the signs. These phenomena remained wholly without explanation till 

 Newton, led by the analogy of those disturbing forces on the orbit of a 

 planet which cause its nodes to regress, showed that the same would occur 

 in a satellite to the earth, in a ring of such satellites, in such a ring- 

 adhering to the equator, or the protuberant part of the terrestrial sphere ; 

 and thus that the equinoctial points would slowly regress. The more exact 

 determination of quantitative results was reserved for Newton's successors, 

 when a more powerful analysis had been applied by Euler, D'Alembert, 

 and others, to the full exposition of the theory, founded on general equa- 

 tions of motion. 



These higher mathematical views, though of course the most complete 

 and systematic, are not the most direct or easy mode of explaining the 

 subject to the student. Greater simplicity certainly characterizes the 



