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72 owing to its being close to the Sun, coupled with its intrinsic faintness. It again returned to perihelion in 1906, being discovered by Kopff, and passing perihelion early in September. Owing to its being missed in 1900 some uncertainty existed as to its probable path in 1906, and its discovery in that year was very fortunate, for in 1910 it will approach so close to Jupiter as to be seriously affected by that planet. To this it may be added, that it is thought that its proximity to Jupiter in the year named will afford an opportunity for obtaining a new value for the mass of that planet.

On June 27, 1851, D' Arrest, at Leipzig, discovered a faint telescopic comet in Pisces. Within a fortnight of its discovery the observations of its path through the heavens were found to be irreconcilable with a parabolic orbit; and the ellipticity of the orbit was soon placed beyond a doubt. Though the comet was visible for more than 3 months, the calculations of the orbit yielded very discordant results, and the successful prediction of the comet's return in the winter of 1857-8 must be regarded as something in the nature of a successful guess. Sir T. Maclear, at the Royal Observatory, Cape of Good Hope, was the only observer of the comet at this apparition.

Villarceau communicated to the Academy of Sciences at Paris on July 22, 1861, a memoir on the orbit of this comet which may be usefully placed on record here (in an epitomised form) as it will give some insight into the nature of the mathematical investigations which the calculators of cometary orbits are called upon to conduct.

From a table of the elements of the perturbations produced by Jupiter, Saturn, and Mars, in the interval between the appearance of the comet in