THE SMITHSONIAN INSTITUTION. 179 



axis parallel to itself, which explains this apparently singular phe- 

 nomenon. Observe that as the booraereng ascends, it is whirling on. 

 an axis perpeiidicuiar to the plane of ascent. Should it go onward in 

 its descent, and cut the air edgewise, it must necessarily change its 

 plane of rotation ; it will not, therefore, do this. If it goes on, keep- 

 ing its axis parallel to itself, it must strike broadside through the air, 

 and the resistance is too great to allow of this. The only way in 

 which it can maintain a parallelism of rotation, and yet cut the air 

 edgewise, and also descend wnth the largest angle of inclination, is to 

 come back to its place of projection, as you have seen it do. It does, 

 in fact, as the foregoing explanation requires, ascend and descend on 

 an inclined plmie, instead of pursuing the parabolic or atmospheric 

 curve at alL 



But I have already intimated that, in the solar system, this paral- 

 lelism, is rarely, if ever, perfectly maintained. The earth's equator 

 deviates at a very slov/ rate, (about lifty seconds in a year,) so that 

 for many years it was not perceived by the rude means of measure- 

 ment which ancient astronom.ers possessed. But its deviation has 

 been going steadily on in the same direction, until the signs of the 

 zodiac and the signs of the ecliptic are now separated by the extent 

 of an entire sign, or thirty degrees. The plane of the moon's orbit 

 deviates from parallelism much faster, so that in about eighteen years 

 it inclines in every direction at its given angle with the orbit, and 

 comes round again into its former position. Groing back to our first 

 illustration, in which the small globe represents the earth, and the 

 wooden ring the ecliptic, I carry the globe round the ring, from the 

 west side, through the south, to the east, and onward, at the same^ 

 time inclining the north pole towards me, so that the planes of the 

 equator and the ecliptic intersect in an east and west line. But, after 

 I liave carried it round a number of times, please to observe that I 

 shift the position of the axis, by which I hold the globe, in such a 

 manner that the line of intersection lies a little to the south of east and 

 north of west. The ends of that line, representing the equinoxes, 

 have moved a little from the east (through the south) to the west ; 

 that is, in a direction contrary to that in which the earth revolves. 

 At length, as the revolutions proceed, the line of equinoxes is found 

 lying north and south ; and thus it perpetually retrogrades. This is 

 called the " Precession of the equinoxes." It is so exceedingly slow, 

 that in order to describe ninety degrees, as just represented, it will 

 require between 6,000 and 7,000 years, and, therefore, about 20,000' 

 years to complete the circuit of the heavens. Again, if I carry 

 this two-inch brass ball round from west to east, but oblique to the 

 wooden ring, passing above it through the southern half, and be- 

 low it through the northern, we shall have a representation of the 

 moon's path around the earth, oblique to the ecliptic. The intei'sect- 

 ing points, called the nodes, now lie in an east and west line ; but as 

 I carry it round repeatedly, I make the ball descend below the ecliptic, 

 at a point a little lurther to the west, every time, and thus cause the 

 line of nodes to move backward, while the moon itself goes forward. 

 TJiis is called the ' ' Retrogradation of the moon's nodes. ' ' It is vastly 

 more rapid than the precession just described, since the line of the 

 nodes passes quite round the sky in eighteen or nineteen years. 



