16, 1882] 
as to the nature of the law by which the density increases 
internally." In the article in Mateure, I adduced the argument 
on which Mr, Williams comments, as a slight corroboration of 
the conclusions as to the physical constitution of the planet, 
which have been derived from telescopic inspection, and from 
observation of the ellipticity of figure. 
From the latter part of Mr. Williams’s letter I must beg leave 
to dissent. If one were to describe the oceanic tides on the 
earth as a reeling motion of the solid earth within the sea, it 
would surely be asomewhat obscure description of the facts, but 
the reeling of the Jovian nucleus caz only be a tidal pheno- 
menon.? Now the masses of the Jovian satellites are so small, 
that they can only raise very small tides, except indeed on one 
hypothesis, of the truth of which we haye no evidence, and 
which would not tend to explain the belts if it existed. The 
tide raised by a small satellite can only be large when the 
“*free” period of oscillation of the gaseous or liquid ocean is 
nearly the same as the ‘‘ forced” period. If this were the case 
with one of Jupiter's satellites, it certainly would not be so 
with the others. Although tides accompanied by fluid friction 
do tend to produce a longitudinal current adverse to the planetary 
rotation, yet no current of a millionth part of the velocity re- 
quisite for the production of the belts could possibly be occa- 
sioned by the tidal friction due to Jupiter’s satellites. 
For these reasons I quite dissent from Mr, Williams’s explana- 
tion of the belts, and of the unequal solar rotation. 
Sir William Thomson has recently pointed out, in a paper 
read before the Physical Society of Paris, a probable cause of the 
reinforcement of an atmospheric tide in the earth, due to an 
approximate agreement of free and forced periods of oscillation. 
He remarks that the semi-diurnal constituent of the barometric 
oscillation is nearly everywhere very much larger than was to be 
expected, and he shows that the sun and earth together con- 
stitute a thermodynamic engine whereby the earth’s rotation is 
accelerated. Rough numerical calculations are given, where- 
from it appears that the amount of this acceleration may not be 
entirely negligeable, when we consider the degree of refinement 
to which modern astronomy has arrived. G. H. DARwINn 
R.M.S.S. Medway, Southampton, Feb. 2 
The Search for Coal under London 
IN a recent communication to this journal I dwelt upon the 
importance of a systematic search being made for the Carboni- 
ferous rocks under London, by a series of borings running from 
north to south, and only a few miles apart ; but I pointed out at 
the same time that much of the expenditure required for such a 
search might be saved by a judicious selection of sites for the 
first two or three borings. I then quoted the opinions of Mr. 
Godwin-Austen and Prof. Prestwich as to the localities at which 
such explorations might be undertaken with the greatest chance 
of success. My friend, Prof. Prestwich, has written to me ex- 
pressing general agreement with the views I have put forward on 
the subject, but calling my attention to some other suggestions of 
his as to the points at which borings might be executed, with 
fair hopes of success. Writing in the Reports of the Coal Com- 
mission in 1870 (p. 162), Prof. Prestwich expressed himself as 
follows :— 
“* The direction of the great underground coal trough is, we 
think, likely to be on a line passing through North Wilts, 
Oxfordshire, thence across Hertfordshire, South Essex, the 
north-east extremity of Kent, onwards towards Calais, near to 
which place it is thrown out by the rise of the underlying rocks, 
but resumes again at Thérouanne. Or in case of the anticlinal 
axis taking a more southern course we should look for the coal 
basin or basins along a line passing from Radstock, through the 
Vale of Pewsey, and thence along the North Downs to Folke- 
stone and near to Calais.” . 
Some years later Prof. Prestwich wrote as follows :— 
“*In short, while there is every reason to hope that on the 
south of London we may yet find in the Lower Greensand, 
beneath the Tertiary Strata and Chalk, a source of large and 
valuable water-supply for metropolitan purposes, there is strong 
* Monthly Notices of R.A.S. Dec. 1876, ‘“‘On an Oversight in the 
Mécanique Céleste, and on the Internal Densities of the Planets.” 
* The expression ‘‘reeling’’ would at the first glance lead one to suppose 
that a diurnal tide is referred to,in which the fluid parts are carried rela- 
tively to the nucleus in the direction of the disturbing satellite, but without 
change of superficia form, technically a spherical harmonic deformation of 
the first order. But it is well known that this class of displacement must be 
non-existent, and _ therefore it must be presumed that Mr. Williams does not 
intend this. 
NATURE 
364 
reason to believe in the probability of the discovery to the xorti 
of London of Carboniferous Strata, including possibly produc- 
tive Coal-measures.” ... (‘On the Range of the Lower 
Greensand and Palzeozoic Rocks under London,” by J. Prest- 
wich. ve Quart, Fourn. Geol, Soc. for November, 1878, 
p- 911. 
The discovery of Upper Devonian strata, both at Turnford 
and at Tottenham Court Road, in both cases dipping at high 
angles, lends not a little support to the view that a trough of 
Carboniferous strata may exist between those two localities. 
Prof. Prestwich authorises me to state that what he would 
now recommend would be a boring ‘‘a mile or two north of 
Kentish Town, not directly north, but north-east or north-west, 
so as to avoid the hills—say about Edmonton on the one side, or 
near Edgware on the other.” On the south side of London 
he would prefer to avoid the Lower Greensand, and would 
recommend a boring ‘‘just beyond its outcrop at Red Hill— 
somewhere between there and Horley.” But he thinks that if 
Coal-measures were found to extend beneath the Lower Green- 
sand, means might be found to sink through the latter, by the 
new appliances of which the Belgian engineers have so largely 
availed themselves, Joun W. Jupp 
Researches on Animals containing Chlorophyll 
1. Dr. BRANDT’S observations (Si/z.d. Berlin Physiol. Gesellsch., 
Noy. 11, 1881) are upon the green bodies of Hydra, Spongilla, a 
fresh-water planarian, and numerous infusors. He finds that 
these green bodies are masses of hyaline protoplasm, containing 
a nucleus and a cblorophyll-granule. Sometimes two to six are 
present, these he considers are states of division. He regards 
these facts as proving that those bodies are unicellular alge, and 
erects the genus Zoochlorel/a. He finds them survive isolation, 
and even develop starch in light. Specimens from Spongilla 
were taken in by infusors, but were either digested or ejected : 
those from a dead Hydra were, however, retained by Para- 
mecium, Coleps, &e. He believes that the chlorophyll never 
belongs to the animals, but always to algze. 
My observations deal with the yellow cells of quite different 
animals. I have, however, ventured the opinion that in most 
of the above cases, the green bodies do belong to the animals, 
and are not alge, and I do not yet see sufficient reason for 
withdrawing that view. 
2. For the yellow cells of Radiolarians and Ccelenterates (for 
the alga nature of which Dr. Brandt so ably argued in his 
former paper) he proposes the genus Zooxanthella, Were Dr. 
Brandt has doubtless priority. 
3. He observes that large Radiolarian colonies show no signs 
of digesting foreign bodies, that these and also Spongilla can be 
kept best in filtered water, and that the latter will not live in a 
half darkened room. These facts are doubtless new. 
4. Dr. Brandt concludes that the algze maintain their hosts ; 
that so long as the animals contain few or none, they feed in the 
ordinary way, but when sufficient algz are present, they are 
nourished like plants. He indicates an analogy to lichens (an 
hypothesis which, as I also state in my paper, was first ventured 
by Semper), and yet points out a distinction, since in a lichen 
there is an association of an alga with a true parasite, here a 
«*Symbiose” of aleze with animals accustomed to independent 
life, which they, however, give up, and take in no further nutri- 
ment. Thus in a morphological sense the algz, in a physio- 
logical sense the animals are the parasites. 
While welcoming Dr. Brandt’s interesting paper, and while 
not desiring to lay too much stress on such awkward facts for 
his view as that Hydra, Anthea, Velella, &c., are quite as voracious 
as their congeners unprovided with chlorophyll, or that the 
animal may possess its chlorophyll from development, and while 
giving him and his predecessors all due credit for their valuable 
observations and theoretic insight, I must point out that (1) the de- 
monstration of the truth of the view that the yellow cells of Radio- 
larians and Ceelenterates are alge, (2) the development of the 
hypothesis of the lichenoid nature of the alliance between alga 
and animal] into a theory of mutual interdependence, and (3) the 
transference of that view from the region of probable specula= 
tion into that of experimental science, remain with my paper. 
For it will not do to ignore, with Dr. Brandt, such weighty 
opposing evidence as (1) the recent direct statement of Hamann 
that the yellow cells of Ccelenterates are not algze, but unicellu- 
lar glands, (2) the observation of Krukenberg that Azthea viridis 
did not evolve oxygen, or (3) the failure of himself and others 
to prove the presence of cellulose and chlorophyll, or even te 
