parallel of latitude passing midway between the spots (only a very rough calculation need be made), we should find that in a period of one rotation, or, roughly, of ten hours, one spot gained on the other about 51 seconds, or, roughly, about 1700 part of a rotation—that is, in distance (dividing 200,000 by 700) about 286 miles in ten hours, or nearly 29 miles an hour.
We have, however, instances of yet greater relative proper motion among cloud-masses. One of these cases I proceed to consider at length:
In June, 1876, two spots were visible upon the disk of Jupiter, so distinct and isolated as to be well adapted for measurement to determine the rate of the planet's rotation. Mr. Brett, observing them first as illustrative of the phenomenon to which he had called attention in 1874, turned his attention afterward to their rate of motion. He would seem not to have been aware of the fact that the proper motion of bright spots and other markings on Jupiter was already a recognized phenomenon; for he asks whether his "observations of these spots, forming a series extending over a period of 280 hours 20 minutes, afford evidence of proper motion, or whether, on the other hand, they tend to cast any doubt on the accepted rotation of the planet." However, his observations are all the freer from the bias of preconceived opinions. "There were several peculiarities about these two spots," he says, "which seemed to me to give them an eminent claim to attention. They occurred very near to the equator, and were very well defined, and free from entanglement with other markings—an advantage which they have maintained with singular uniformity throughout the period mentioned; but the special peculiarity to which attention is asked is, that during an interval of five days they remained in the same relative position without any variation whatever. Their stability in respect of latitude during those five days is undoubted; but the question is, whether or not they were equally stable in longitude. This remark only applies to the first five days of the series, because at the end of twelve days a certain deviation was obvious. The distance between the two spots occupied about 42° of Jovian longitude, or about 33,000 miles. Their diameter is nearly equal, being estimated at about one-fourteenth of the planet's diameter, or 6,310 miles." The interval of time between these first two observations "was 119 hours, that is to say, twelve rotations of the planet according to Airy's determination, during which time their distance apart and their latitude remained constant." Between the first and second observations the two spots had gained "44 m. 6 s. in time. Assuming Airy's rotation, viz., 9 h. 55 m. 21 s., the spots have gained on the planet's surface at the rate of 4 m. 2 s. in each revolution."
Between the second observation and the third "there was an interval of seven days, or seventeen rotations of the planet; and the
