76 
NATURE 
[May 27, 1886 
Leaving out of account frictional generation of heat (in other 
words, the effect which varies as the square of the current), let 
od be the heat generated in unit time by unit current in a uni- 
form copper bar, in passing from a section where the tempera- 
ture is @ + @@ to one where it is @ (see Thomson’s “ Papers,” vol. 
i. p. 246). Let x be distance along the bar in the direction of 
the current, andc the thermal capacity of unit length of the bar, 
The heat generated in a short length 5x is — osx, and the 
ax 
. : re pee a a0 se 
consequent rise of temperature in this portion is — — co This 
c dx 
is in unit time. Tence, putting ¢ for time, andz for 2, we 
¢ 
have— 
de _, a0 
dt ax 
If the limits of temperature are not very far apart, it is known 
that o and ¢ are sensibly independent of @; hence v may be 
treated as a constant. The integral of the above equation is 
then— 
6= F(x - vt), 
where / is a functional symbol such that @ = F(x) expresses 
the original distribution of temperature. The interpretation is 
that the original ‘‘temperature curve” travels forward with 
velocity v without flattening or any other change of form. 
Belfast, May 14 J. D. EVERETT 
Scientific Nomenclature 
SoME time ago Mr. ‘‘John O’Toole,” in the columns of 
NATURE, waged war against that hideous monstrosity Potential 
Energy, and he very aptly summarised his case against this term 
by saying that it involves, by the very signification of words, ‘‘a 
double remotion from actuality.” 
A few months ago it occurred to me that to express what is 
intended by the potential energy of any system the term Svadic 
Energy is not only logically unobjectionable but specially fit for 
the’ purpose. Thus, if a string or a membrane is stretched, a 
wire bent and twisted, or, generally, a body strained in any 
manner, the work which it can do against resistance in returning 
from its state of strain to its unstrained condition is the S/a/c 
Energy of the system in the strained condition. The work which 
a moving system can do in virtue solely of its motion is, of 
course, its Avvetic Energy. 
Thus we have simply Static Energy and Kinetic Energy, and 
these terms have the further advantage of harmonising with the 
ordinary subdivisions of dynamics. 
I may add that the term Statice Energy has received the 
approval of all the mathematical physicists to whom I have sub- 
mitted it. GEORGE M. MINCHIN 
R.I.E. College, Cooper’s Hill, May 19 
Pendulum Oscillation 
THE oscillations of a long pendulum are observed to describe 
an ellipse the axis of which tends to set itself at right-angles to 
the plane in which the pendulum was started. An explanation 
of the above phenomenon would much oblige. M. H. Maw 
Walk House, Barrow-on-Humber, Hull, May 18 
What is Histioderma ? 
Histioderma hibernica was described by Dr. Kinahan in 1858 
as an annelid, and most writers who have since mentioned it 
have also regarded it as such. Dr. Haughton notices and figures 
it in his ‘Manual of Geology,” Sir R. I. Murchison places it 
amongst annelids in his ‘‘ Siluria,” as also Dr, Bigsby in his 
““ Thesaurus Siluricus.” Writing from memory only, I think a 
description and figure will be found in W. H. Baily’s useful 
compendium of Paleozoic British fossils, published a few years 
ago. The fossil is mentioned with more or less detail by 
numerous writers on Cambrian strata. 
The above references will perhaps answer the letter signed 
“*S.,” but there still remains the question, What is Histio- 
derma? It is apparently a fossil impression in the rocks of 
Bray Head, Wicklow, Ireland. Only one species and only one 
locality is, I believe, known. It seems to me very doubtful] 
what kind of animal made the impression—whether an annelid, 
or otherwise. Iam inclined to doubt if the rocks are Lower 
Cambrian, as geologists generally suppose, because the evidence 
seems to indicate that the Bray Head rocks had become hard- 
ened and raised into land before the oldest Lower Cambrians of 
Wales and England were deposited. I would therefore pass 
on the question and ask, What is Histioderma ? 
4, Cowper Road, Acton, London, W. A. RAMSAY 
IN answer to the above question in your issue of May 20 
(p. 53), I refer your correspondent to the Proceedings of the 
Geological Society of Dublin, where he will find (Matural 
History Review, vol. v., Proceedings of Society, p. 150) the 
original description of Histioderma by Prof. Kinahan. Accord- 
ing toitsauthor, Histioderma is the tube of a cephalo-branchiate 
annelid. J. Vicror Carus 
Leipzig, May 23 
Black Skin 
In a letter lately received from Mr. Flinders Petrie, who is 
now in Egypt, are some remarks on ‘Black Skin,” which [ 
think very interesting, and perhaps the readers of NATURE may 
think them so too, so Isend the paragraph to you. 
Belvedere, Kent, May 24 F. C. J. SPURRELL 
“*Tn considering the use of a naturally or artificially black 
skin, we should not look so much to the requirements of the 
surface, which is constructed to bear variations, and has the 
means of cooling and maintaining a proper temperature within 
itself, but rather we should consider the far more delicate tissues 
beneath. We all know how translucent flesh is to strong light, 
and it can hardly be doubted that the rays of a tropical sun 
would light up a white man’s inside considerably ; whereas 
black skin would stop out the solar energy of light, heat, and 
chemical rays effectually. Skin heat is of no importance, as 
perspiration can always keep that down. May not the oiling of 
the skin in hot countries be partly to make it reflective, so that 
it should absorb less heat? And may not the regard white 
races have for clothing be partly for the purpose of keeping the 
insides of their bodies sufficiently in the dark?” 
Male Animals and their Progeny 
CAN any of your correspondents inform me whether any of 
the male wild animals in foreign countries show any love for, or 
recognition of, their progeny? In this country amongst the 
domestic animals it does not seem to exist, save in the case of 
the gander, who carefully guards the goose while sitting, and 
attends to the goslings when hatched ; but the cock pays no 
attention to the hen while sitting nor when with chickens, nor 
does the dog, the bull, the horse, nor the boar evince any sign 
of parental feeling under circumstances favourable to its de- 
velopment. 
One peculiarity of geese is, I think, worthy of notice, for it is — 
not possessed by ducks or fowls, who also live in flocks. If 
when goslings are hatched, they are permitted to run with a 
goose in‘company with the other geese of the flock, all chances 
of any eggs being laid by the other geese who have no goslings 
are over. 
Birds and Mirrors 
For six days I have been for two or three hours in the club 
reading-room, where tere is a large mirror. During all this 
time a cock and hen sparrow have been flirting with and 
bowing to their images in the glass with evident pleasure— 
rushing along the mantel-shelf, flying to the top of the frame, or 
resting for a time, always apparently happy in contemplation, 
never showing disappointment. IF. C. CONSTABLE 
Sind Club, Karachi, May 2 
SURGEON-MAFOR T. R. LEWIS 
R. TIMOTHY LEWIS, Surgeon-Major Army Medi- 
cal Staff and Assistant Professor of Pathology in 
the Netley Army Medical College, whose death, which took 
place on May 7, we announced last week, was a native of 
South Wales, and received his medical education at Uni- 
versity College, London, and graduated as M.B. at 
Burlington, Gardens. He was selected about twenty 
