90 THE POPULAR SCIENCE MONTHLY. 



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 sppt gained on the other 

 about 51 seconds, or, roughly, about tj-^-jj- 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 

 amono- cloud-masses. One of these cases I proceed to consider at 

 leno;th : 



In June, 1870, two spots were visible upon the disk of Jupiter, so 

 distinct and isolated as to be well adapted for measurement to deter- 

 mine the rate of the planet's rotation. Mr. Brett, observing them 

 first as illustrative of the phenomenon to which he had called atten- 

 tion 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 wore several peculiai'ities about 

 these two spots," he says, " which seemed to me to give them an emi- 

 nent claim to attention. They occurred very near to the equator, 

 and were very Avell defined, and free from entanglement with other 

 markings an advantage which they have maintained with singular 

 uniformity throughout the period mentioned ; bat the special pecu- 

 liarity to which attention is asked is, that during an interval of five 

 days they remained in the same relative position without any varia- 

 tion wliatever. 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 occu- 

 pied 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 be- 

 tween these first two observations "was 119 hours, that is to say^ 

 twelve rotations of the planet according to Airy's determination, dur- 

 ing which time their distance apart and their latitude remained con- 

 stant." 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 in- 

 terval of seven days, or seventeen rotations of the planet ; and the 



