le 
| 
' 
Cuap. II., § 4.] 
eclipse (that of p.c. 720), the rotatory velocity has 
not altered by one ten-millionth part. 
Aneminent contemporary and rival of Poisson, M. 
Poinsot, has added many elegant propositions to the 
Theory of Rotation. M., Poinsot is also the author of 
the Theory of Statical Couples, which now forms part 
of all elementary treatises on mechanics, 
Poisson wrote many other papers on subjects of 
physical astronomy, such, for example, as the Lunar 
Theory, but they did not lead, on the whole, to strik- 
ing conclusions. In fact, he allowed himself to 
be diverted from this his most natural calling, by 
the ambition of constructing a system of Physics 
mainly founded on the applications of analysis. 
Some bulky volumes of this series appeared, espe- 
cially those on Capillary Attraction, and on the 
Theory of Heat. The author here shows him- 
self as a profound analyst, but adds little to our 
knowledge either of principles or of important re- 
sults. A similar criticism may be applied to his 
theory of Elastic Substances, and to his doctrine of 
Waves. His papers on Magnetism and Electricity will 
be mentioned elsewhere, but their character is some- 
what similar. In Optics he was attached to the 
Newtonian theory. In Mechanics not requiring as- 
sumptions as to the properties of matter, he was very 
successful. He was eminently a solver of hard pro- 
blems : his investigation of the whole circumstances 
of motion of a projectile in air deserves notice. 
Indeed every branch of mechanics received his atten- 
tion, and the number of his printed papers is said to 
exceed three hundred. On the whole, he will, per- 
haps, be most generally and favourably known by the 
excellent Treatise on Mechanics which he wrote for the 
use of advanced students. He was an eminent and 
diligent Professor, and his whole life was one of almost 
unremitting study. ‘ La vie c’est le travail” was his 
reply, when urged to consult his health by reposing 
from his labours: and he actually died in the dis- 
charge of his duty as examinator of the Polytechnic 
School. This occurred on the 25th April 1840, in 
the fifty-ninth year of his age. 
(113.) 
M. Poinsot 
on Rota- 
tion, 
(114.) 
Other 
writings of 
Poisson. 
His Trea- 
tise on Me- 
chanics, 
Be Ais on Mr Arry on the perturbation by Venus of the 
the pertur- Larth’s motion.—We here detach from what we shall 
bation of elsewhere have to record of the eminent services ren- 
hae dered to the cause of astronomy by the present As- 
by Venus. te onomer Royal of England, a notice of his chief dis- 
covery in physical astronomy, the more remarkable 
_ from being almost the only improvement in the theory 
of the planetary motions, as applicable to the tables, 
which had proceeded from an English mathematician 
for a very long period. Sir James South called at- 
tention, in 1826, to a small but well marked devia- 
tion of the Sun’s place from that given by Delambre’s 
solar theory (the Sun’s place or the Harth’s are of 
course convertible terms). Mr Airy, then Lucasian 
professor at Cambridge, instituted, in 1827, a more 
extensive comparison with the Greenwich Observa- 
VOL. I. 
PHYSICAL ASTRONOMY—MM. AIRY, PLANA, AND HANSEN. 
825 
tions, and attributed the error chiefly to the assump- 
tion of erroneous masses for Venus and Mars, But 
prolonged study satisfied him that it arose from a “long 
inequality,” arising from the mutual action of the 
Earth and Venus, similar in its nature to that men- 
tioned in Art. (66), as detected by Laplace in the 
ease of Jupiter and Saturn. It arises from the cir- 
cumstance that eight times the orbital period of the 
Earth is almost equivalent to thirteen times that of 
Venus. Terms of the series expressive of the mu- 
tual action of the planets, which are divided by thir- 
teen times the mean motion of the former minus eight 
times the mean motion of the latter (which is a very 
small quantity), may consequently become consider- 
able ; still more, those involving the square of this 
quantity. Such terms belong to the fifth and higher 
orders of the excentricity, and would consequently be 
very minute indeed, but for the casual magnitude of 
their coefficient. The labour of tracing out and cal- 
culating the effects of these important terms from the 
vast mass of algebraic developments is enormously 
great,—much greater than in the corresponding case 
of Jupiter and Saturn. As the calculations are not 
very likely to be repeated, Mr Airy took extraordi- 
nary precautions in their verification. The period 
of the inequality depends, first, as has been explained 
in Art. (67), upon the period required to carry the 
point of conjunction of the two planets completely 
round the circumference. This period is no less than 
270 years. The perturbation is of course mutual, 
and affects the place of Venus as well as of the Earth. 
Mr Airy is also the author of investigations con- 
nected with the Figure of the Earth, and the Theory 
of the Tides [see Art. (82)]; and he has published va- 
luable Tracts on Physical Astronomy for the use of 
students, 
Sir John Lubbock’s name deserves mention here (117,} 
as having devoted his energies, about the same time Sir John 
with Mr Airy, to intricate and laborious researches !™” 
connected with the least inviting parts of physical 
astronomy, and striven to redeem England from the 
reproach of indifference or incapacity in respect of 
such inquiries. The chief object of his numerous 
memoirs (in the Philosophical Transactions for 1830, 
and following years) is to express, in a more conve- 
nient and exact manner than had been in use, the 
complicated serieses indicative of the perturbations as 
well in the Lunar as in the Planetary Theory. 
(116.) 
The Lunar Theory: MM. Prana and Hansen.— ps i 
M. Plana, an astronomer and analyst of the greatest and Han- 
merit, who fortunately still does honour to the native sen on the 
city of Lagrange (Turin), though the author of a great 
number of Memoirs in the Transactions of the Turin 
Academy, on different points of Applied Mathema- 
tics, is, and will be, best known by his elaborate and 
learned treatise on the Theory of the Moon. This 
extends to three very bulky quarto volumes, which 
are, so to speak, one mass of symbols, Nothing can 
5M 
Theory. 
