CHAP. II., 4.] PHYSICAL ASTRONOMY MM. AIRY, PLANA, AND HANSEN. 



27 



eclipse (that of B.C. 720), the rotatory velocity has 

 not altered by one ten-millionth part. 



An eminent contemporary and rival of Poisson, M. 

 '* Poinsot, has added many elegant propositions to the 



tion. 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. 



(114.) Poisson wrote many other papers on subjects of 



inTs of physical astronomy, such, for example, as the Lunar 



PoUson, 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 



re , 8 t" excellent Treatise on Mechanics which he wrote for the 

 tise on Me- . 



chanics. u se 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. 



Mr Air '^ n ^ r -^ IRY on ^ e perturbation by Venus of the 

 the pertur- Earth's motion. We here detach from what we shall 

 bation of elsewhere have to record of the eminent services ren- 

 ;he Ear i <j ere( j to the cause of astronomy by the present As- 

 tronomer 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 Earth's are of 

 course convertible terms). Mr Airy, then Lucasian 

 professor at Cambridge, instituted, in 1827, a more 

 extensive comparison with the Greenwich Observa- 





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 

 case 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- (116.) 

 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 u 

 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. 



The Lunar Theory : MM. PLANA and HANSEN. 

 M. Plana, an astronomer and analyst of the greatest and Han- 

 merit, who fortunately still does honour to the native sen on 

 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 



