586 



NATURE 



[April 21, 1898 



salient features of its eruption were recorded by an eye- 

 witness in the sixteenth century. The others probably 

 •date from prehistoric times ; otherwise, such a marvel as 

 an extensive eruption could not have escaped mention 

 by some Greek or Roman author. In the table, the 

 younger volcanic structures are arranged above those 

 •which are considered to be older, and lines connect 

 those between which it is thought that some evidence of 

 relative age can be detected. 



In my recent paper I have shown that almost all the 

 "hills of the Phlegra^an Fields are either the entire walls 

 of volcanic craters or of portions of walls of volcanic 

 •craters which have been to a greater or less extent 

 washed away by the action of rain or the sea, or have 

 been partially destroyed or buried by more recent vol- 

 canic eruptions. Secondly, these volcanic craters are 

 •distributed over a crescentic area round the Bay of 

 Pozzuoli ; the more recent ones are situated closer 

 to the shore of the bay than the older ones, indi- 

 cating a gradual march of volcanic vents along lines 

 radiating towards the centre of that bay. In other 

 words, the general effect of volcanic activity has been 

 the accumulation of new land around the bay of 

 Pozzuoli. Inasmuch as water seems to be a necessity 

 for a volcanic eruption, this progression of the volcanoes 

 seawards may stand in some relation to the channels by 

 which water has obtained access to the fires below. 



Thirdly, wherever crater rings of a larger and smaller 

 diameter overlap, we never find a larger ring super- 

 imposed on a smaller one. The smaller ring is always 

 superimposed on the larger. The conclusion to be 

 •drawn is, that the volcanic activity of the region has 

 been decreasing in intensity. The same proposition is 

 proved for a single locality by the Campana series of 

 craters. 



As it is impossible to make all the above-mentioned 

 points clear without a suitable map, I would draw the 

 attention of the interested reader to the surveys of the 

 region published by the Istituto Geografico Militare, and 

 to the contoured map published in the Geographical 

 Journal for October 1897. R. T. Gunther. 



THE PRESENT APPEARANCE OF JUPITER. 



IT may be interesting, now that the planet Jupiter is 

 very favourably situated, to call attention to a few of 

 the more conspicuous features which diversify its surface. 

 Many of these objects will be sure to have come under 

 the repeated observation of all those observers who have 

 been habitually examining the planet in their telescopes. 

 It is fortunate that at the present time there are a large 

 number of conspicuous markings on the planet. These 

 -are situated in different latitudes, and will allow some 

 excellent redeterminations to be made of the velocities 

 of the various longitudinal currents. 



There are a great many dark and bright spots visible 

 near the equator, on the north side of the southern equa- 

 torial belt. Some of these at the date of writing (April 

 18) are placed in the following longitudes, which are com- 

 iputed on the basis of System I. (daily rate 2,77°-()o 

 = 9h. som. 30s.) in Mr. Crommelin's ephemerides in 

 .Monthly Notices for January 1898. 



Long. 



Bright spots. 



... 80 .. 



... 146 .. 



. . . 209 



... 238 .. 



... 270 .. 



••• 349 •• 



»JO. i486, VOL. 57] 



Dark spots. 



.. 71 

 .. 94 



.. il6 

 .. 127 



- 15s 

 .. 216 



279 



319 



355 



These markings are moving at approximately the same 

 rate as the zero meridian System I., but I find that in 

 several cases there is a disposition of the spots to de- 

 crease in longitude ; hence I believe that their rotation 

 periods will prove to be slightly less than gh. 50m. 30s. 

 The bright spots exhibit great variations in their appear- 

 ance, and some irregularities of motion. A spot which 

 is conspicuously brilliant on one night may, a few even- 

 ings later, have declined so much that it can scarcely 

 be discerned. 



The great red spot is still visible, and its present 

 longitude is about 24"^, so that it follows the zero meridian 

 of System II. in the ephemerides by about forty minutes. 

 The spot has the aspect of a faint dusky ellipse with a 

 light interior, and it is connected on its south side with 

 a grey belt. The recent motion of the spot corresponds 

 with a period of 9h. 55m. 4r5s., and it may be expected 

 to drop further behind its computed place unless an 

 acceleration of motion should set in. Comparing an ob- 

 servation of the spot which I obtained on April 17, 1898, 

 I2h. 5m., with one of the earliest by Mr. Dennett, of 

 Southampton, on July 27, 1878, I find that I7,4I4 rota- 

 tions were performed in the interim of 7203 days, and 

 that the mean period of rotation was 

 9h. 55m. 39-45. 

 This value is identical with that found by Prof. Hough 

 from 14,505 rotations between 1879-96. 



In July 1898 the spot will have been visible during 

 twenty years, and under careful observation at each op- 

 position of the planet. Whether this object is identical 

 with other elliptical or reddish spots seen at various 

 times in the same latitude by Dawes (1857), Huggins 

 (1858-59), Gledhill (1869-70), Lord Rosse (1873), Cope- 

 land (1873), Russell (1876), and others, must remain an 

 open question. It appears highly probable that the spot 

 was the same as that observed by Russell in the summer 

 of 1876, but there is an absence of connecting observ- 

 ations in 1877. And the identity of two or more mark- 

 ings cannot be absolutely proved by their longitudes 

 when a fairly long interval has intervened between the 

 observations, because the proper motions of the object 

 are variable, and introduce great differences of longitude 

 not always in the same direction. The retarded westerly 

 drift of the red spot has caused it to lose more than 

 900 degrees of longitude relatively to its place in 1878, 

 so that the spot has really been displaced to the extent 

 of about two and a half circumferences of the planet. 



From my own observations of the spot since 1880, I 

 have obtained the following rotation periods for a certain 

 interval during each apparition : — 



Limiting observations. 



1886 Nov. 



1888 Feb. 



1889 May 



1890 May 



1 89 1 Aug. 



24 

 2 

 22 

 26 

 25 



1880 Sept. 27-1881 March 17 



1881 Tuly 8-1882 March 30 



1882 July 29-1883 May 4 



1883 Aug. 23-1884 June 12 



1884 Sept. 21-1885 July 



1885 Oct. 24-1886 July 

 23-1887 Aug. 

 12-1888 Aug. 

 28-1889 Nov. 

 22-1890 Nov. 



7-1892 Feb. 2 



1892 Aug. 15-1893 March 8 



1893 Aug. 9-1894 March 24 



1894 Nov. 25-1895 May 10 



1895 Aug. 24-1896 Feb. 22 



1896 Sept. 27-1897 April 25 



The red spot has been falling behind the zero meridian 

 during the last six or seven years, but at a very slow rate, 

 the mean annual retardation being only about six 

 minutes. 



There are a number of dark, elongated spots or short 

 belts placed in various regions of the planet. They are 



413 

 640 

 674 

 710 

 700 



659 

 609 

 462 

 439 

 451 

 432 

 495 

 548 

 401 



439 

 514 



Period, 

 h. m. s. 



9 55 35'6 

 9 55 38-2 

 9 55 39-1 

 9 55 39-1 

 9 55 39-2 

 9 55 4I-I 

 9 55 40-5 

 9 55 40-2 

 9 55 40-0 

 9 55 40-2 

 9 55 42-2 

 9 55 42-3 

 9 55 41-2 



9 55 41 '2 

 95541-3 

 9 55 40-8 



