44, he has changed his period to 9h. 50m. 20s., this being 

 the rate of rotation of a certain white spot in the equa- 

 torial regions. This seems to be a great mistake, for if 

 it has always been the same white spot observed, it is far 

 more reasonable to suppose that this spot has an enormous 

 proper motion (something like 2,000 miles per day) westward, 

 than to imagine that this spot has remained in the same place 

 while all the other markings have been moving simultaneously 

 eastward at this pace. The Director of the Dearborn Obser- 

 vatory seems to have devoted a considerable amount of atten- 

 tion to the rotation period of the red spot, and he obtains a 

 mean for 1883-4 of 9h. 55m. 38'5s., or a mean for five years 

 (1879-84) of 9h. 55m. 3701s. There is one great disadvantage 

 in taking "white spots," viz., that on the equatorial belt there 

 are such a number of them, often coming and going on the 

 same evening with the variability of definition. But the red 

 spot cannot be mistaken, and it has always kept its position 

 immediately south of the great hollow in the southern dark 

 belt. Last year there were three beautiful little dark spots on 

 the south edge of the southern dark belt, and these kept their 

 positions relative to the red spot approximately ever since they 

 were first observed here on February 19, 1885. They were 

 situated a little east of the red spot, and could all be seen on 

 the planet at the same time. The most eastern was just like the 

 shadow of one of the satellites, quite black and round ; the 

 centre one not nearly so dark as either of the others, and elon- 

 gated ; the most westerly was of an elongated oval shape, and 

 of a very dark red colour. No very accurate measures were 

 taken, but a sufficient number of sketches were made to give a 

 tolerably exact period of revolution, and the central spot gives 

 for 77 revolutions 9h. 56m. 7s. In a similar manner the red 

 spot from 70 revolutions gives 9h. 55m. 34s. On March 18, 

 18S5, we discovered a round dusky spot on the equatorial light 

 belt, just below the northern dark belt ; this passed the central 

 meridian at 11.41 a.m. It was observed on several occasions 

 since, and again, after twelve Jovian days, passed the central 

 meridian on March 23 at 9.47 a.m. This gives a rotation period 

 of 9h. 50m. 30s., almost identical with that adopted by Mr. 

 Marth in his " Ephemeris." I think there can be no doubt, 

 however, that this is not a fair rotation period to assume, for by 

 far the majority of the markings rotate in periods varying from 

 9h. 55m. to 9h. 56m., keeping their positions relative to one 

 another at all events approximately. It is certainly curious 

 that the white spot of Mr. Marth and our own little dusky 

 spot should give so nearly equal a result, but from an inspec- 

 tion of our sketches it is easily seen that the dusky spot has a 

 large proper motion westward relative to its environment. 



