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determined. It was again seen in 1795 by Miss Caroline Herschel. It was found again in 1805 and 1818. Not until the latter date was the accurate orbit determined, and then the periodic character of the comet and its identity with the comet observed in previous years was established.

Encke now found the period to be about three years and one hundred and ten days, varying a little according to the attraction of the planets, especially of Jupiter. In recent times it has been observed somewhere at almost every return. Its last return was in September, 1901.

What has given this comet its celebrity is the theory of Encke that its orbit was continually becoming smaller, probably through its motion being resisted by some medium surrounding the sun. A number of able mathematicians have investigated this subject on the various returns of the comet. Sometimes there appears to be evidence of a retardation, like that found by Encke, and sometimes not. The question is, therefore, still in an unsettled condition. The computations are so intricate and difficult, and, indeed, the whole problem of the motion of a comet under the influence of the planets is so complicated, that it is almost impossible to secure a solution which can be guaranteed as absolutely correct.

A remarkable case, in which a new comet was made a member of the solar system, occurred in the years 1886-1889. In the latter year a comet was observed by Brooks of Geneva, New York, which proved to be revolving in