124 PROFESSOR AIRY ON AN INEQUALITY OF LONG PERIOD, &C. 
inents, to obtain expressions which possess these properties ; and therefore I 
have little doubt that my form will be recognised as more completely in ac- 
cordance with the principles of that method than Laplace’s. I should not in 
the present instance have raised a question on this point, but that I conceive 
the method of variation of elements, or some similar method, possessing the 
same advantages of simplicity of application and unlimited accuracy as to the 
order of the disturbing force, will ere long be adopted in the Planetary Theo- 
ries, to the total exclusion of other methods. With this expectation, it appears 
important to adhere closely to the principles of the theory in every formula 
that is derived from it. 
I believe that the paper now presented to the Royal Society contains the 
first* specific improvement in the Solar Tables made in this country since the 
establishment of the Theory of Gravitation. And I have great pleasure in 
reflecting that, after having announced a difficulty detected by observation, I 
have been able to offer an explanation on the grounds of physical theory. 
Postscript. 
In estimating the variation of the elements of the orbit of Venus, the 
change of longitude of perihelion was supposed to be the same as the sidereal 
motion of the perihelion. This is not strictly true ; as the longitude of the 
perihelion, measured as in Art. 4, depends upon the place of the node, and is 
affected therefore by the motion of the node as well as by the motion of the 
perihelion. The amount of the error is however perfectly insignificant. 
G. 13. Airy. 
Observatory , Cambridge , 
Nov. 8, 1831. 
* I am not aware that anything has been added to the theory of planetary perturbation, by an 
Englishman, from the publication of Newton’s Principia to the communication of Mr. Lubbock’s 
Researches. In Maskelyne’s tables are two for the perturbations of the Earth produced by Venus 
and Jupiter, calculated (he states) by himself; but they are utterly useless and erroneous, as they con- 
tain no terms depending on the excentricities. 
