528 
NATURE 
which, except the fifth, is incontrovertible. He has himself 
pointed out that it is here that we differ, and that this difference 
may ultimately be traced to a difference in our doctrine as to the 
distribution of velocity among the molecules in any given portion 
of the gas. He assumes, as Clausius, at least in his earlier inves- 
tigations, did, that the velocities of all the molecules are equal, 
whereas I hold, as I first stated in the 7277. A/ag. for Jan. 1860, 
that they are distributed according to the same law as errors of 
observation are distributed according to the received theory of 
such errors. 
It is easy to show that if the velocities are all equal at any 
instant they will become unequal as soon as encounters of any 
kind, whether collisions or ‘‘ perihelion passages” take place. 
The demonstration of the actual law of distribution was given 
by me in an improved form in my paper on the J)ynamical 
Theory of Gases, ‘‘Phil, Trans.” 1866, and P/il. A/ag. 1867, 
and the far more elaborate investigation of Boltzmann has led 
him to the same result. I am greatly indebted to Boltzmann for 
the method used in the latter part of the sketch of the general 
investigation (see p.535) which was communicated in a condensed 
form to the British Association on Sept. 20, 1873 . 
J. CLERK-MAXWELL 
Mallet-Palmieri’s Vesuvius 
As Tam assured that it would be most undesirable as well as 
unbecoming of me to continue a scientific controversy in the 
tone of Mr. Mallet’s letter which appeared in NATURE of 
October 9, I would only beg those who have perused it to 
remember that my remarks were altogether directed to the 
assertions contained in Mr. Mallet’s introductory sketch, and 
not comments upon his theory of yoleanic energy of which, as he 
himself admits, we as yet know little or nothing. I would then 
ask them to compare its contents with the substance of my 
letter in NATURE, Sept. 4, and judge for themselves whether 
so far from its being any answer to my arguments, it does not, 
on the contrary, furnish additional ‘‘evidence of his confound- 
ing chemical constitution with percentage composition, &c.,” 
the very keynote of this discussion. 
Mr. Mallet writes—‘‘ Mr. Forbes appears to think that 
chemists, mineralogists, and geologists are the sole arbiters”’ of 
such questions ; a remark he could not have made had he read 
some of my publications ; yet I am quite willing to admit that I 
do place more faith in them collectively, than in any one 
physicist or mere mechanician whether theoretical or practical ; 
and I believe I am correct in asserting that no theory of volcanoes 
will be accepted by the scientific world until its doctrines are 
proved to be fully in accordance with the facts brought forward 
by these sciences. 
When the reasons for my delay in answering Mr. Mallet’s 
criticisms were fully stated, is it not, to say the least, most unjust 
of him to harp on this string, after having already taken more 
than a month to produce a rejoinder the reverse of an answer, 
and the style of which, peculiar to himself, is in complete har- 
mony with that of his introductory sketch, of which one of his 
favourable reviewers writes —‘* We do not cordially approve of his 
method of dealing with other writers. There is, if we may be 
excused the expression, a tone of bitterness all through his 
writing whlch gives the reader a most uncomfortable sensation, 
and leads a person altogether unbiassed to imagine a feeling of 
jealousy on the part of so distinguished a writer as Mr. Mallet 
which we are sure cannot exist in reality. After giving a sketch 
of the various authors who have ventured to give different 
and erroneous opinions on the subject of vulcanicity,” &c. An- 
other reviewer remarks that—‘“ While objecting to most of the 
views of geologists, which, however, he frequently distorts, Mr. 
Mallet claims the character of physical truth for his own ideas,” 
and adds, ‘what we chiefly object to inthis portion of the 
volume is the assumption on Mr. Mallet’s part of a conscious 
superiority to others, and a freely expressed contempt for all 
previous observers, especially for geologists.” Need I add 
more? DAVID FoRBES 
11, York Place, W. Oct. 20 
Oxford Science Fellowships 
As Mr. Perry’s letter, in the last number of NATURE, contains 
assertions calculated to impede the progress of science here by 
deterring persons, not graduates of Oxford, from competing for 
appointments in colleges, and also involves charges of, to say 
the least, discourtesy to himself, I trust you will find space in 
your next number for the following explanation. 
First, as to Mr. Perry’s general assertion’respecting fellowships. 
From the fact that a graduate of Belfast is ineligible for a 
Fellowship in AJerton College, Mr. Perry infers that “ outsiders 
are ineligible for Oxford Fellowships in Physical Science.” This 
is clearly illogical, and it is also untrue. 
Secondly, as to the special case of Mr, Perry. : 
The ordinances of Merton College state that “‘no person shall 
be eligible” for a fellowship’“ who shall not have passed all 
the examinations required by the University for the degree of 
Bachelor of Arts.” It appears a possible interpretation that 
Cambridge and Dublin B.A.’s, who can at any time incorporate 
in this University, may be candidates. If this be so, the reply 
of the Warden of Merton, as Mr. Perry gives it (of the actual 
correspondence I know nothing), may be correct, though perhaps 
not sufficiently explicit. This, however, is a legal question, and 
the college is taking steps to obtain the opinion of an eminent 
counsel, 
Mr. Perry was not left, as his letter would naturally lead ~ 
readers to infer, without warning as to this difficulty ; for in 
July I wrote to Mr. Perry strongly expressing my doubt as to his 
eligibility, but as I was away from Oxford I could not quote the 
words of the ordinance; I advised him to consult the sub- 
warden, but I believe he did not follow my advice. 
Mr. Perry received my letter, and replied to it on July 27. 
The great difficulties which Mr. Perry asserts to have been 
thrown in his way, simply arose from the fact that he only pro- 
posed to come to Oxford during the vacation. Now it is not to 
be expected that I should allow any person who chooses to apply 
to overhaul the physical apparatus of the University in my ab- 
sence, and it is unreasonable to suppose that, to suit the con- 
venience of such a person, I must give up engagements made 
long before, in order to assist him in a candidature for an office 
of emolument in a college. 
Tt must be borne in mind that there are nineteen colleges, any 
one of which may at any time offer a fellowship for proficiency in 
physics, and consequently to have to be at the service of out- 
siders, who may wish to be candidates, during the long vacation 
(the only time I have for real study) might become a serious 
matter, and to ask for such “assistance seems to me to make}a 
most unreasonable request. 4 
I must add that if Mr. Perry imagines he would have been at 
any appreciable disadvantage by not knowing the particular in- 
struments in the University cabinet (which it is by no means cer- 
tain would be used for a college examination), either he assumes 
that the examiners would be guilty of the absurdity and unfair- 
ness of puzzling candidates by new or peculiar apparatus, or he 
feels very uncertain about his own practical knowledge. = 
A Cambridge B.A. is a candidate for the Merton Fellowship, 
and I have every reason to think that he found the Oxford can- 
didates on exactly equal terms with himself in the practical exa- 
mination, H. B. CLirron 
Oxford, Oct. 18 
P.S.—Since writing the above I have been informed that a 
Cambridge graduate has been elected to a Science Fellowship in 
Magdalen College, Oxford. ‘This is a proof of the inaccuracy of 
Mr. Perry’s statement as to the ineligibility of outsiders for 
Oxford Fellowships. 
Harmonic Echoes 
] BELIEVE the echo observed by W. J. M. is of a different na- 
ture {rom mine and more analogous to one described by Oppel 
(Pogg. Ann, xciv. 357, 530). Each bar of the railing, when 
struck by the aérial pulse, diverts a small portion, which is scat- 
tered in all directions, much as if the bar were itself the source of 
sound. These derived pulses reach the ear of the observer at 
approximately equal intervals, and accordingly blend into a 
musical note, whose pitch, however, may not be quite constant. 
Oppel discusses the effect of different positions of the original 
source and the observer with respect to the grating, on which 
alone the pitch and its variations depend. It is evident that an 
echo formed in this way is in no sense selective. 
I have been asked several times how the Bedgebury echo 
would be affected by the character of the original sound. Of 
course, if my theory is correct, the octave could not be returned, 
unless it were originally present ; but the intensity of the echo 
was too feeble to give any promise of a successful observation 
with such an instrument as the clarinet. The experiment would 
be most interesting if a more powerful echo of the same class can 
be found. RAYLEIGH 
Terling Place, Witham, Oct. 
[ Oct. 23, 1873 1 
x 
