the period of revolution had happened to be an hour or two different from what it actually is, then there would have been no such difficulty. A few nights after one elongation had been observed the other would have been presented in a convenient phase for measurement. As it is, however, Professor Barnard must have his observations supplemented by those at some other observatory in a considerably different longitude. Here is the difficulty. Telescopes of sufficient power to show the new object there may be, but the exceptionally favourable conditions for observation presented on the top of Mount Hamilton are not easily to be paralleled elsewhere. It is, however, generally found that once an object has been discovered it is frequently possible to observe it again with telescopic advantages greatly inferior to those with which the discovery was made.
As to the physical character of the new satellite it seems difficult to offer any surmise. It seems probable that so small an object must be in the solid state. We are in the habit of accounting for the obviously non-solid condition of Jupiter himself by the excessive heat which he still contains. But the new satellite bears to Jupiter a proportion, let us say, of a grain of mustard-seed to a cocoa-nut. It therefore appears that though such a vast bulk as Jupiter may not yet have had time to cool down into the solid form, the same can hardly be averred of its tiny companion. If there should be any fluid materials on the small satellite they must be distracted by the most terrific tides. It is certain that seas on its surface would be submitted to tidal forces at least thirty times as potent as those on the earth.
There cannot be a doubt that Barnard's discovery will
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