Item 17 - Letter from George Airy

Observatory - Gives a note on perturbations intended for John W. Lubbock: 'If perturbations are applied to x y & z, there is no practicability of dividing the time of an apposition into different parts, as the calculation does not give the means of correcting the elements for the beginning of each part. Consequently the series used must be such as will apply from the beginning of an apposition to the end. It seems to me very probable that 5th or higher powers may be wanted'.