10 



SCIENCE. 



Assuming that the red spots period of rotation is 9> 

 55 m 37 8 .o65 — which is probably very near the truth — we 

 find that in one minute of time o°.6o4 of the surface will 

 pass a given meridian. Multiplying the minutes in the 

 last, or column 7, by .604, we get the following table of 

 lengths in longitude on the surface of Jupiter. The first 

 nine are taken from a table of eleven transits observed by 

 me previous to August 30, and published in English Me- 

 chanic, No. 809 : 



Julv 



.400 



■45 



Oct. 



6, 1880 



26 



58 





7. " 



.27 



.78 





10, " 



•32 



.62 





13. " 



• 30 



20 





20, " 



■33 



.82 



Nov 



1, " 



•23 



.86 





8, " 



.27 



78 





10, " 



.27 



78 





11, 



.29 



.90 





18, " 



28 



•75 

 41 





20, ' ' 



■31 





22, " 



.29 



60 





23. " 



28 



•57 



Dec. 



2, " 



.29 



23 





S. " • 



3 1 



71 





9. *' • 



33 



04 





14. " 



.31 .29 

 .28 .87 



■31 -17 

 .30 .86 

 .29 .72 

 .30 .86 



•31 -OS 

 .30 .08 

 .29 .96 

 .28 99 

 .27 .18 

 .26 .76 

 .28 .87 

 .29 .11 



10, 18H0 



17. " 



" 24, " 



" 29. " 



" 31. " 



Aug. 13, '• 



" 16, " 



" 17. " 



" 23. " 



" 3°. " 



Sept. q, " 



" 16, •' 



" 18, " 



" 25. " 



" 28, '• 



" 3°. " 



Oct. 1, " 



On July 10 the spot had a narrow strip running from 

 its preceding end. To this is due the great length of the 

 spot on that date. This does not indicate the true length 

 of the spot proper, but as it was a portion of the spot, 

 or continuation, I give the length on that date. 



It must not be supposed that, because I have carried 

 the lengths to two places in the decimals, I consider the 

 length accurate to that degree, for the observations have 

 been entirely eye estimations, yet they were very care- 

 fully made. I think a variation of one degree in the 

 length of the spot would be easily detected, and probably 

 a less amount, as the agreement between most of the 

 figures is too close and regular to attribute to chance. 

 As my method of observing may be of interest, I will 

 give an example from my note book. First : I watch 

 closely the first end of the spot, and imagine a line drop- 

 ped from it to the equatorial belt and observe when this 

 is central, for it is much easier to halve the straight edge- 

 like line of the equatorial belt than to halve the disk on 

 a parallel with the spots centre, because the spot itself 

 being on one side of the meridian biases our judgment 

 to a certain extent, while the clean edge of the equatorial 

 belt is free of any obstacle to interfere with our judg- 

 ment. Second: I compare the spaces between the limbs 

 of the planet and the ends of the spot, when these are 

 seen to be equal, of course the spots centre is in transit. 

 For determining the transit of the following end, the 

 same method as that in determining the preceding end is 

 followed. At the observation of each part of the spot 

 there exists for a short while a period of uncertainty. 

 The beginning of this uncertainty I indicate by u, noting 

 the time. In a minute or so I feel sure the time of true 

 phase has arrived, this is noted by /, with its time. 

 Shortly, I am certain the phase has passed, this I ncte as 

 c, with its time. The mean of the three is taken for the 

 true phase. 



The following is an observation of the transit of the 

 red spot on October 13, 1880, Nashville, mt, taken from 

 my note book. 



1 



h 



" 7 40 r 



P t 7 44 I 7 44 



c 7 48 ) 



,}, 



" 8 07 ) h 

 t 8 10^ 



eS 13) 8 



ft 



A m. 

 8 0!).l) 



u 8 32. s 

 ./ 8 36 

 c 8 38 



35-8 



The mean of the nine observations agrees with the 



observed middle transit to .1 m. This close agree- 

 ment cannot, of course, be expected often. How- 

 ever, they generally agree to within a few fractions of a 

 minute. In no case have I allowed myself to know be- 

 forehand what time any phase should occur, as this might 

 influence the observations. 



The variations in length of the spot are not only 

 shown by the duration ot transit, but are sensible to an 

 observing eye. At each observation I estimate its 

 length, comparing it with the breadth of the disk on the 

 same parallel of latitude. These comparisons show changes 

 in its length, as they vary from 1-3.5 10 l A tr >e breadth 

 of the disk, but it is generally the slightest bit less than 

 one-third. 



The variations in breadth are compared with the great 

 equatorial band, but unfortunately this is a standard that 

 probably varies itself. The spot's breadth is generally 

 slightly less than y z the width of the equatorial belt, 

 sometimes it is probably fully half as broad as the belt, 

 but I have never seen it broader than that. 



Changes in the width of the space between the south 

 edge of the equatorial belt and the north edge of the 

 spot, are more readily detected, as the space can be 

 easily compared with the breadth of the spot. This 

 space is generally equal to ^ the spot's breadth, yet it 

 is sometimes nearly one-half as broad as the spot. I have 

 seen it diminished to one-sixth. These changes are due 

 either to a swelling out of the spot or a broadening of 

 the equatorial belt. It is more likely due to changes in 

 the spot. I have on several occasions estimated that the 

 distance between the southern edge of the equatorial 

 band and the southern edge of the spot was about equal 

 to one-third the distance from the south pole to the 

 equatorial belt. 



There are sometimes slight changes in the general 

 form of the spot ; at times the ends are blunt or rounded, 

 again they are cigar shaped. One end has been seen 

 rounded while the other was very much pointed. The 

 sides are at times a little flattened, but are generally 

 slightly rounded. On July 24 the south-side was curved 

 or convex, while the north-side was somewhat flattened. 

 It is sometimes long and lanky and then again it is fat 

 and " chubby " — neither of these have been carried to 

 extremes. Faint continuations, or trails, have been 

 visible, sometimes from one end and then from the 

 other. These have on several occasions been seen trail- 

 ing from both ends at once, but are not always seen 

 without close looking. At times the spot is a deep solid 

 brick color ; then again it is lightish red and pale. I 

 have never, for certain, seen any detail on the surface of 

 the spot, but I have sometimes thought that there was 

 detail but just too indefinite for my aperture. The out- 

 line of the spot is always clean — no diffusion. 



These observations ate from notes and sketches which 

 I have made this year with a 5-inch Byrne refractor. 



E. E. Barnard. 



Nashville, Tenn., December 27, 1880. 



NOTE. — The motions of the spots on Jupiter, in an 

 article by me in "Science" No. 24, are referred to an 

 assumed rotation period of 9h. 55m. 27.08s., which 

 should have been stated in that article. E. E. B. 



PENNULE'S COMET. 



The following position of this comet was obtained by 

 ring micrometer, on December 30, 1880, 7 h . 01.2 m., 

 Nashville m. t. : 



R. A. ic-h. 55m. 38.5s. 

 Dec.+ 18 52' 39.6" 



It is several minutes in diameter and very brightly 

 condensed. E. E. Barnard. 



Nashville, Tenn., Jan. 2, '81. 



