398 NOTICES OF THE MEETINGS [March 3, 
This apparatus has been greatly improved upon by Professor 
Wheatstone, who has introduced other movements to include the 
conditions of rotation in different planes. One of these instruments 
was exhibited. 
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 con- 
sidered, and upon a far grander scale. 
The phenomenon of the Precession of Equinoxes was known to 
Hipparchus; but no explanation of the fact was for ages imagined. 
Even Kepler, in the multiplicity of his hypothetical resources, could 
not succeed in devising anything plausible. 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 pole star,—at the rate of about 50” 
a year, and of course carries with it the point of intersection 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, shewed 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. (See Principia, i. 66, Corr. 11—22.) 
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 equations of motion; as since given in 
the writings of Laplace, (Mec. Cel. liv. xiv. ch. 1.) and Pontécou- 
lant (Théorie du Systéme du Monde, liv. iv. ch. 5.), which are 
necessary for including all the minuter variations detected by 
Bradley, and subsequent observers, shewing the nutation of the axis, 
and the inequalities of precession due to the varying configurations 
of the attracting luminaries. 
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 method adopted by Mr. Airy (in the tract before 
cited) of applying directly the theorem of the composition of rotatory 
motion; as doubtless Newton would have done had it been known 
to him. But here, as in so many other instances, the first explana- 
tion presented itself mixed up with more complex considerations ; 
and as has been well observed “ simplicity is not always a fruit of 
the first growth.” 
To those not versed in the mathematical theory, of all points in 
Physical Astronomy, the ‘‘ modus operandi” of the Precession, 
perhaps, usually seems the most paradoxical, and the explanations 
