NATURE 
[May 7, 1885 
and the nature of which may be inferred from the fact that I 
advised the author to withdraw his paper and submit the subject 
to a renewed investigation, with the aid of improved methods. 
With regard to the question, which Mr. Lowne raises, as to 
my competency to form any opinion at all, on the ground that 
I had not myself devoted any special attention to the literature 
of the subject, I may remark that the points which had practi- 
cally to be decided were (1) whether Mr. Lowne’s statements 
were in themselves probable, and (2) whether they were corrob- 
orated by his preparations. Had I not felt myself qualified to 
form an opinion on these points I should not have accepted the 
reference. FE. A. SCHAFER 
The Late Prof. Clifford’s Papers 
In the ‘‘ Mathematical Papers” (pp. 628-37) I was able to 
print the syllabuses of a series of ten lectures delivered by Prof 
Clifford to a class of ladies at South Kensington in the spring 
and summer of 1869. Whilst turning over a collection of mis- 
cellaneous papers, in a box, Mrs, Clifford and I had the good 
fortune to light upon a manuscript quite ready for printing, 
and this (‘‘ Mathematical Papers,” p. 628) subsequently formed 
part of the volume on “ Seeing and Thinking ;” but we could 
not find any trace of any more manuscript of the above-men- 
tioned series of lectures. Just before the recent Easter holidays 
Prof. Karl Pearson returned to me a few pages of manuscript 
bearing on the International Scientific Series volume which I 
had lent him, and with them he sent me a large note book 
which had been in the late Prof. Rowe’s hands. On open- 
ing this book I at once saw that it contained very. full 
notes of other lectures of the course. In fact, Lecture II. 
(‘*On Plane Surfaces and Straight Lines”) is quite ready 
for press, as is also, I think, Lecture IIT. (‘‘On the Rota- 
tion of Plane Figures”); Lecture IV. (“‘ Of Similar Figures ”) 
is a fragment, and still more fragmentary is Lecture V. (‘‘ The 
First Principles of Calculation”). Of Lecture VI. (‘‘ The 
Theorem of Pythagoras ’’) there are two loose sheets of figures : 
on one sheet is ‘‘the Bride’s Chair,” and the figures on this and 
the other sheet show that my information was correct, and that 
the remarks on pp. 633, 637 are ad vem. As Lecture IX. 
(‘* On the Shadows of a Circle’’) is very fully illustrated in the 
recent volume edited by Prof. Pearson, we see that we are in 
possession of a fairly complete presentment of Prof. Clifford’s 
views on the subjects of the course of lectures. 
Messrs. Macmillan have stated their willingness to publish the 
MS. of the second part of ‘‘ The Elements of Dynamics,” and 
I hope to be able, after a re-examination of it, to put the work 
into their hands for printing. When this book is got out, and 
the above lectures published in some shape yet to be determined, 
the mathematical world will be in possession of all that we can 
now Jook for from the hands of this great master. 
University College School, April 25 R. TUCKER 
Sir Wm, Thomson and Maxwell’s Electro-magnetic 
Theory of Light 
SHORTLY after writing my former letter I saw a copy of the 
verbatim report of Sir Wm. Thomson’s lectures in Baltimore, 
and would have written to you to that effect and to apologise to 
Mr. Forbes for having doubted the accuracy of what I thought 
was his report, only that I met him in London about that time, 
and he then desired me not to do so, Sir William Thomson has 
now himself stated that the passage is correctly quoted, and I 
can only regret that he has expressed himself in the way he did. 
I certainly think that anybody reading the passage would 
imagine that the velocity of propagation of electro-magnetic 
disturbances upon Maxwell’s electro-magnetic theory of light, 
which he showed to be the same as the velocity of propagation 
of light, and to be a true velocity of wave-propagation—any 
one, I say, would suppose that this was the same thing as that 
Sir Wm. Thomson calculated in the year 1854. 
Sir Wm. Thomson certainly says, ‘‘ That is a very different 
case,” but the rest of this sentence is rather ambiguous as to what 
the “it” after ‘‘ putting” refers to, and I am afraid that many 
people will imagine that, in Sir William Thomson’s opinion, 
Maxwell has made some unjustifiable assumption. I believe, 
however, that all he thinks is that Maxwell has not made a 
satisfactorily definite thing of the so-called electro-magnetic 
theory of light. 
In Sir Wm. Thomson’s article in Nichol’s ‘‘ Cyclopzedia” 
he puts the matter very clearly indeed. He says:—‘‘ The law 
of this phenomenon [transmission of electric signals] is identical 
with that which Fourier . . . found as the law of propagation 
of summer heat and winter cold to different parts of the earth,” 
z.c. it obeys the laws of a diffusion and not of a wave-propaga- 
tion; and again:—‘‘ Now it is obvious from these results 
[experimental results] that the supposed velocity of transmission 
of electric signals is not a definite constant like that of light :” 
and afterwards he says that, when an initial current is started, 
the potential rises simultaneously at all points, and that the 
apparent velocity would depend on the delicacy of our instru- 
ments. All these obviously distinguish between the propaga- 
tion of a variable current in a conductor and a true wave- 
propagation. 
He has also clearly pointed out a direction in which to look 
for a true wave-propagation. It will make his position clearer, 
and also Maxwell’s, to use his analogy between water in an 
elastic tube and a conductor of electricity. I will suppose the 
water contained in a tube bored out of a very large lump 
of india-rubber. He enumerates three electric qualities con- 
cerned, and their hydrodynamic analogues :—(1) ‘* Charge” or 
electrical accumulation in a conductor subjected in any way to 
the process of electrification. (2) ‘‘ Electro-magnetic induction” 
or electromotive force excited in a conductor by variations of 
electric current. (3) Resistance to conduction through a solid. 
The hydrodynamic analogues are :—(1) Accumulation of a 
greater or less quantity of water in any part of the canal or tube. 
(2) Inertia of the water. (3) Viscosity or fluid friction, He 
explains that a true wave-propagation arises from the compressi- 
bility of the water, combined with its inertia, and that if the 
tube be elastic, like india-rubber, there would also arise a waye- 
propagation. ‘‘ Accordingly,” he says, ‘‘a definite velocity of 
propagation of electric impulses, depending on the inertia 
and the capacity for charge, is to be looked for, as has been 
done in a first article, published by Kirchhoff, on the subject.” 
Now, in all this discussion Sir Wm. Thomson omits to men- 
tion the only thing that is at all analogous to Maxwell’s propa- 
gation of wave disturbances in non-conductors, and it arises 
from his considering the water as contained in a tube like 
ordinary india-rubber tubes, instead of in a tube bored in an 
indefinitely large lump of india-rubber. If we consider this 
case it is evident that one of the conditions to be considered is 
the propagation of waves in this lump of india-rubber. In Sir 
Wm. Thomson’s tube there would of course be a velocity of 
wave propagation in the india-rubber, but that isa very different 
matter from the propagation of disturbances away from the 
neighbourhood of the tube by which energy would be carried 
away from it. To do this Sir Wm. Thomson should have in- 
cluded the propagation of sound in the air or whatever he sup- 
posed surrounding the outside of his tube. Without including 
this, he was not including anything a bit analogous to Maxwell’s 
electromagnetic theory of light. In Sir Wm. Thomson’s tube 
the whole state of affairs at any time could be expressed in terms 
of variables that represented bodies near the tube, while in the 
other case it would be absolutely necessary to introduce variables 
representing every part of the india-rubber which I have sup- 
posed of indefinite extent. This is just the difference between 
Sir Wm. Thomson’s and Maxwell’s views. According to Max- 
well’s view there is a great deal more going on outside the con- 
ductor than inside it, and it is evident that the inertia of the 
water is a very bad analogue to electromagnetic induction, for 
this latter depends essentially upon the form of the circuit, and 
not only upon its section and length. Maxwell has shown that 
light may be a wave-propagation of what are on his theory 
analogous, though probably utterly z/ike the distorsional waves 
propagated in the india-rubber, and has shown that a medium 
which would only transmit disturbances analogous to these 
would explain electric and magnetic phenomena. It is to be 
remembered that Maxwell’s theory gets rid of all action at a 
distance, and that the only expevimentum crucis between theories 
of action at a distance and of action through a medium is that 
in this latter case the energy may be propagated in time through 
the medium, while in the former it cannot. 
I cannot conclude without protesting strongly against Sir Wm. 
Thomson’s speaking of the ether as /4e a jelly. It is in some 
respects ava/ogous to one, but we certainly know a great deal 
too little about it to say that it is Z/te one. May be Maxwell’s 
conceptions as to its structure are not very definite, but neither 
are any body’s as to the actual structure of a jelly, and there is 
no real difficulty in supposing a medium whose condition is 
