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 knowledge of Jupiter's satellites. There is much work yet to be done before the movements of this system can be reduced to satisfactory order.

The object in now mentioning these matters is not assuredly to attempt, in these pages, any contribution towards the task of improving the tables of Jupiter's satellites. My purpose is to show how much attention has been paid to the system by astronomers of every class. It would be utterly impossible to obtain any accurate notion of how often Jupiter has been carefully observed, let us say, within the last hundred years. We can, however, obtain some sort of estimate which will help to explain the profound impression that the announcement of a discovery of a new satellite is calculated to make. The great planet is visible for some months every year. We shall certainly be well within the mark if we say that it must have been scrutinised carefully by skilful observers at least a thousand times each year. For, remember how many observatories there are in the world, where special attention is given to such work, and also of late years how many excellent telescopes there are in private hands. Considering, too, that the Jovian system is one of such intense interest to all observers, and that, except the moon, there is no object in the sky more frequently and more carefully studied, it is not at all an undue estimate to assume that Jupiter and his moons must have been carefully examined, I do not mean merely looked at, at least one hundred thousand times during the last hundred years.

Now we shall be able to understand the extraordinary interest which the announcement of the detection of a fifth satellite has created. Here was this system which everyone knew, which had occupied so much attention, and