THE PLANET JUPITER.

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minations yet made being that by Sir G. B. Airy , in the year1835, when Plumian Professor of Astronomy at Cambridge,which gave for its result 9 h 55 m 2P3 S .

But it seems that any attempt to determine the rotation-period with extreme accuracy must now be altogether given up.We can adopt no other method for the purpose than thatwhich was used in the year 1664, by the very first observerwho detected the rotation of Jupiter, the celebrated Dr. Hooke ,then Professor of Geometry in Gresham College . He watchedthe movement of a spot upon its surface, as the rotation of theplanet carried it round; and we must do the same. Our difficultyarises from the fact, that the more carefully we observe variousspots, the less are we able to obtain a decisive result, owing tocertain irregularities in their movements, and to a very appre-ciable, although moderate, difference in their speed of rotation,which seems to depend upon their distance from the planet’sequator.

It was in pursuance of the above method that Cassini, in1665, observed a spot which he followed during 29 consecutiverevolutions, from which he obtained for the duration of onerevolution a period of 9 1 ' 56 m . It was by the same method thatSir George (then Professor) Airy deduced his more accurateresult of 9 h 55 m 21 -3 s , from a dark spot which performed 225consecutive revolutions, and by the long interval during whichit was thus visible diminished any probable errors of observar-tion. We must, however, consider all such determinations,whether of earlier or of more recent date, whether more or lessrefined and precise, simply as the values which correspond tothe individual spots to which they belong, and not as giving usany certain and undoubted information as to the exact durationof the planet’s own rotation.

And here we may remark, that it is not only found that, asa rule, spots upon Jupiter go round more rapidly the nearerthey are situated to its equator, in which they follow the samelaw that Carrington noticed in the case of the spots upon theSun, but individual spots have very decided individual pecu-liarities of speed, and the speed of the same spot is often fromtime to time somewhat irregular. So long ago as 1779 Sir