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LETTERS TO THE EDITOR. 
The Editor does not hold himself responsible for opinions ex- 
pressed by his correspondents. Neither can he undertake 
to return, or to correspond with the writers of, rejected 
manuscripts intended for thts or any other part of NATURE. 
No notice ts taken of anonymous communications. | 
The Intake of Carbon Dioxide—a Correction. 
WILL you give me the opportunity of making the following 
correction in my Presidential Address to the Chemical Section 
of the British Association. 
I stated incidentally that Mr. F. F. Blackman, in his well- 
known experiments on the intake of carbon dioxide into the two 
sides of an assimilating leaf, employed air enriched with that 
gas to the extent of 4 per cent. and upwards. 
Mr. Blackman has pointed out to me that in the experiments 
in question he used air containing only from 1°8 to *33 per 
cent., and that since the publication of his earlier results he has 
still further reduced this amount. In fact he is of the opinion 
that his method is applicable to the measurement of the intake 
of carbon dioxide of even a much greater degree of dilation. 
The error was an inexcusable one on my part, but does net 
affect the main argument that the natural rate of intake cannot 
be directly deduced from experiments in which the carbon 
dioxide content of the air materially departs from the normal 
amount 0’03 per cent. Horace T. BRowN. 
52 Nevern Square, Kensington, S.W., September. 
Geological Time. 
In his Presidential Address to Section C at Dover, Sir A. 
Geikie has offered a bold challenge to Lord Kelvin and those 
who agree with him by calling upon them to give due weight to 
geological phenomena in forming an estimate of geological time. 
Permit me to say what I think about it. 
It seems to me probable that, when the grand idea of the 
universal dissipation of energy had occurred to Lord Kelvin, he 
saw that the principle must be applicable to the earth, and that, 
if the law of conduction of heat could be used, he might from it 
obtain an estimate of the world’s age. He then instituted his 
important experiments to determine the conductivity of rocks 
zm s?tw, and found the value 400 (more or less), the units being 
a foot a year, and a degree Fahr. But it was necessary, for his 
calculation to succeed, that he should assume the earth to be 
solid. If I do not misjudge him, I think he then sought for 
arguments to prove this point. Now, I cannot but think that 
the proofs of solidity on which physicists rely are by no means 
convincing ; and if the earth is not solid, Lord Kelvin’s esti- 
mates are without foundation. Moreover, it is not sufficient 
that the earth should be solid at the present, to which these 
proofs refer, but it needs to have been so from the beginning of 
the time to which his estimates go back. 
Prof. G. Darwin, in his book on the tides (p. 237), has done 
me the honour of referring to my ‘‘ Physics of the Earth’s Crust ” 
as if I am an arch-heretic on this question of solidity. Whether 
my arguments are beneath notice, or whether there 1s a difficulty 
in answering them, I do not know; but they have never been 
refuted, while they are held to be of decided force by some 
geologists, and among these by the Indian Geological Survey. 
Harlton, Cambridge, September 25. O. FISHER. 
The Terrestrial Gegenschein. 
I po not know whether the phenomenon I am about to 
describe has ever been noticed. The circumstances under which 
it is noticeable must occur rarely. They are these :— 
I spent some time of the summer of 1898 on an isolated 
mountain peak, surrounded by lower mountains whose sides 
were densely wooded. The result was that near the time of 
sunset the shadow of my own mountain peak was visible on a 
mountain side which might have been three or four miles 
distant. One evening I amused myself by watching the shadow 
of the peak as the sun was descending. My attention was 
attracted by an illumination in the direction opposite the sun 
so strikingly resembling the astronomical gegenschein, that at 
the first glance I saw in it an explanation of the latter. It 
consisted of a somewhat bright glow, which might be a degree 
or two in diameter, but which shaded off by such imperceptible 
gradations that a definite extent could not be assigned. A 
NO. 1562, VOL. 60] 
NALORL 
[OcToBER 5, 1899 
little study, however, showed an explanation. As I have said, 
the mountain on which the glow was seen was densely wooded. 
In such a case the shadows of those leaves and branches 
which the sun’s rays first reached fell upon the interior foliage 
and obscured it. But an observer looking from the exact 
direction of the sun will see through the foliage as far as the 
sun’s rays extend. In other words, the visible surface on 
«which he is looking will be entirely illuminated by the sun’s 
rays, whether this surface is formed of the outer strata of 
foliage or of a strata ever so far inside, which can be seen 
only through the crevices in the outer stratum. The shaded 
interior will be entirely invisible to him. But if his point of 
view is in a direction ever so little oblique, he will see only 
the outer foliage illuminated, while more or less of the interior 
foliage which he sees will be in the shadow. Thus the region 
exactly opposite the sun will be seen in its full brilliancy, 
while the neighbouring region will be a mixture of light and 
darkness. At a distance of several miles this compound of 
light and darkness will be fused into a single half-shade, 
strongly contrasting with the full brilliant light of the opposite 
point. 
It is clear enough that we cannot have such a state of 
things as this in the case of the astronomical phenomena. Yet 
the phenomenon seems to be of sufficient interest to warrant 
its being placed on record. S. NEWcoMB. 
The Cause of Undercurrents. 
In NaTureE of August 3, p. 316, is given a letter from Rear- 
Admiral Sir William Wharton, in which he states that he is 
diametrically opposed to my opinion about the double currents 
in the Straits. He says that ‘* Admiral Makaroff considers that 
difference of density of the water is the primary, and, indeed I 
gather he thinks, the only cause of these opposing currents ; 
but he brings no evidence beyond theoretical considerations in 
support of his belief” ; further, in his letter, Admiral Wharton 
refers particularly to the double current of the Bosporus, of 
which I spoke in my lecture at the Royal Society of Edinburgh. 
I cannot leave unnoticed remarks from so distinguished a 
hydrographer, who, during his long work, has contributed so 
much to the advance of science. My researches about the 
Bosporus are published only in Russian, in a book named 
“On exchange of water between Black Sea and Mediter- 
ranean” (St. Petersburg, 1885). Should Admiral Wharton 
know my language, he would easily come to the conclusion 
that my opinion about double currents in the Bosporus 
are based upon the observations made in 1881 and 1882. I 
then invented an instrument for measuring the current at 
different depths, and gave the name of ‘‘fluctometer”’ to it. 
The instrument consists of a propeller revolving on a horizontal 
spindle. A bell is attached to the propeller, and at every 
revolution of the propeller it strikes twice. As water is a very 
good conductor of sound, the number of revolutions could be 
counted through the bottom of the ship (provided the ship is 
not sheeted with wood) at all depths to which the instrument 
was lowered (40 fathoms), I used to anchor in the middle of 
the Bosporus for a couple of days at a time, and make a series 
of observations every two hours. In order to obtain more 
detailed data, I used to take the samples of water from the same 
depth to which the fluctometer was lowered. Twice I used to 
go along the Bosporus from the Black Sea to the Marmora 
Sea in order to learn in what depth is the limit of two currents. 
In volume xxii. of the Proceedings of the Royal Society of 
Edinburgh, Plate I. shows a position of the limit of two 
currents, mean velocity of both currents, and specific gravity of 
water. In Plate II. is given a sketch of my ‘‘ fluctometer.” 
I am sorry that the limits of this paper do not allow me to 
give particulars of my observations, but I believe some of my 
deductions, worked out from direct observations, would be 
interesting to English readers. 
Mean velocity of the upper current, 3} feet per second. It 
varies from © to 10 feet per second in certain places. 
Velocity of the upper current diminishes with every fathom 
of depth. : 
Limit between two currents close to the Marmora Sea is at 
11 fathoms. It gradually goes down to 27 fathoms close to the 
Black Sea. Limit between two currents is influenced by winds 
and by barometrical pressure, but not very much. 
Lower current has close resemblance with the river. Its 
velocity does not vary very much. We never found anywhere 
