Fan. 19, 1882] 
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NATURE 
265 
LETTERS TO THE EDITOR 
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Tidal Evolution and Geology 
Ir appears to me that the difference of opinion between Mr, 
George Darwin and his interpreter, Dr. Ball, is very small. Dr. 
Ball is careful to confine his large tides to the Eozoic rocks, and 
has not asserted their efficacy in Carboniferous times. 
The Laurentian rocks form nearly 19 per cent. of the total 
known thickness of strata of all ages, and occur at the bottom of 
all ; but we must ascend through nearly 66 per cent. of the total 
thickness before we reach the lowest bed of the Carboniferous 
period ; and it is plain that Mr. Darwin’s large tides may have 
existed (as Dr. Ball suggests) in the Eozoie period, and have 
become much smaller before the Carboniferous period began. 
The real importance, in my opinion, of a large tide considered 
as a geological agent, depends upon its rise and fall, and not 
upon its ebb and flow. The waves of the sea, agitated by the 
wind, make the ocean surface a vast planing-machine, acting 
upon the coast-lines ; and a great range of tide applies this 
planing-engine either twice or four times a day to every part of 
the coast laid bare by the rise and fall of tide. The effects 
must have been very serious when the day was six or eight hours 
long. 
The claim for priority made on behalf of Kant, by the meta- 
physicians, must be set aside, as Kant’s statement was not based 
on sound dynamical principles. SAMUEL HAUGHTON 
Trinity College, Dublin, January 17 
I was much interested in reading Prof. Ball’s lecture in 
NATURE, vol, xxy. p. 79, but failed to understand the follow- 
ing passage on p. 81:—‘‘ Zhe reaction of the earth tends to 
inerease that distance, and to force the moon to revolve in an 
orbit which is continually getting larger and larger.” In what 
sense does the reaction of the earth tend to ‘“‘drive away’’ the 
moon? Will the Royal Astronomer of Ireland, or some other 
friend of science, be so kind as to add a few words of 
explanation ? ape Be 
The Remarkable White Spot on Jupiter 
EARLY in the present month this singular object became 
obscured, so that on January 1 I could scarcely distinguish it at 
all, and on the 3rd, 5th, and 6th it was noted as extremely faint. 
The origin of the spot’s disappearance was obvious. A dark mass 
on the north border of the great south belt (and therefore in the 
same latitude as the white spot) appeared on December 14 ; it 
followed the white spot 1h. 4m., according to observations by 
Mr. A. S. Williams at Brighton. The dark spot moved with 
more rapidity than the latter, and soon overtook it, so that as 
the former swept over it, its disappearance was complete, On 
January 6 the white spot was seen struggling through the south- 
east limits of the dark patch. On January 7 it had further freed 
itself, and I saw it much plainer, though it still continued some- 
what faint. On January 9 it was bright, and evidently on the 
point of regaining its normal brilliancy, The dark patch re- 
ferred to is obviously of the same character as the train of black 
spots visible on one of the northern belts last winter ; they move 
with even greater velocity than the white spot, and are somewhat 
evanescent as regardsduration, They appear to be excrescences 
from the surface of Jupiter, and as they near the outer enve- 
lopes, are dispersed into longitudinal bands ; in fact, it is these 
dark spots which sustain the decided tone of the belts, for the 
latter show a disposition to become fainter, until reinforced by 
the commingling of these dark eruptions. 
As to the brilliant white spot, itis an object of notable per- 
manency ; and though it failed to come generally under notice 
until October, 1880, it had probably been a conspicuous mark- 
ing on Jupiter during the few preceding years. Certainly in 
1879 it was very bright, and several times observed by Dr. F. 
Terby at Louvain, and Mr. J. Gledhill of Mr. Crossley’s obser- 
vatory, Halifax. I computed back the dates of its conjunctions 
with the red spot, and found the following nights in 1879-80 
when it might have been well observed :— 
1879, September 1 1879, November 29 
», October 16 1880, January 13 
The date of November 29 is amply confirmed both by Dr. Terby 
and Mr. Gledhill as follows :— 
1879, November 27, 5h. 40m., a brilliant white spot (‘* Tache 
brilliante et blanche ”’) slightly east of the £ end of the red spot. 
—Terby. 
1879, November 29, 6h. 30m., a bright gap into north border 
of the great south belt. It is situated about a quarter the dis- 
tance from the middle to the g. end of red spot.—Gledhill. 
In two days the white spot traverses an extent of longitude 
equivalent to half the length of the red spot, so that the above 
observations are quite consistent, and there can be no doubt that 
they relate to the curious object at present visible. Mr, Gled- 
hill’s drawing of November 29 shows the spot to be some 
twelve hours past conjunction with the red, so that the pheno- 
menon probably occurred on the morning of November 29, 
which is not far from the computed time. The ensuing conjunc- 
tion on January 13, 1880, is confirmed by Dr. Terby. On 
January 11, 6h. 16s., he saw a brilliant white spot occupying 
the same longitude as the fend of the red, which is exactly the 
computed place, and there can be little doubt that these white 
spots are identical with each other and with the white spot of 
to-day. 
Mr. Gledhill’s drawings supply other interesting facts. Thus 
at 6h. 45m. both on November 13 and December 8, 1879, there 
was a brilliant white spot or gap (in the north side of the great 
southern belt) about 3h. past the central meridian. These ob- 
servations again conform to the positions of the present spot, 
which in the interval between the two dates mentioned had per- 
formed sixty-one rotations. It is curious that at periods of 
twenty-five days (equal November 13 — December 8 as above) 
the transits of the white spot recur at very nearly similar times. 
Mr. Gledhill’s observed conjunction of November 29, 1879, 
compared with my own similar observations on December 24 
last year proves that the white spot had completed seventeen 
revolutions of Jupiter in the 756 days! 
If possible it is important to trace still further back the 
apparitions of the white spot. The special brilliancy of this 
object and its unique position indenting the north side of the 
southern belt, could hardly escape notice unless indeed the spot 
was temporarily obscured, as it sometimes is, when the dark 
patches sweep over it. This brilliant spot should have been 
nearly in the same longitude as the red spot on the following 
dates in the last half of 1878 :— 
July 29 
September 11 
Can observers furnish any additional links in the previous 
history of this wonderful object ? W. F, DENNING 
Ashley down Bristol, January 10 
October 26 
December 10 
Fossil Insects of the Dakota Group 
THERE are till now, as far as I know, no fossil insects out of 
the Dakota group published. Among a large number Of fossils 
belonging to this group, and collected by Mr. Chas. H. Stern- 
berg, some of the leaves show insect galls and mines, the latter 
mostly of a decided Tineid and Tortricid character. I'erhaps a 
list of those plants may be of interest. The determination of 
the plants is by Mr. L. Lesquereux:—<Aspidiophyllum trilo- 
batum, 6 specimens ; Sassafras cretacewm, 1; Araliopsis grosse- 
dentata, 4; A. cretaceum, 23 A. mirabile, 4; A. acutiloba, 1; 
A. Haskenanum, 1; Ficus primordialis, 1, Mr, Sternberg in- 
forms me that this is only a partial list of his fossil plants, 
which were all collected in Central Kansas. Among the plants 
figured in the Cretaceous flora by Mr. L. Lesquereux (Hayden’s 
‘© Survey,” vol. vi.), I find on the following plants insect!mines 
or galls :—Menispermites obtusiloba, Greviopsis Heydenti, Pro- 
tophyllum Sternberzii, Platanus recurvata and [eerit, Liguid- 
ambar integrifolium. All from Kansas or Nebraska. Mr. F. 
B. Meek (‘‘ Cretaceous Invertebrata” in Hayden’s Rep., vol. ix. 
p. xliv.) says: ‘‘ The evidence respecting the exact part of the 
European Cretaceous series to which the Dakota group belongs 
X I am informnd by Mr. Lesquereux that a large number of Magnolia 
leaves from the Tertiary of Alaska show serpentine trails not larger than a 
thread running all over the leaves, apparently under the epithelium. 
