March 9, 1871} 
NATURE 
369 

perpetual motion. That there are forces in nature which can and 
do produce it, is a matter of daily, yearly, and secular experience. 
If I am a perpetual-motionist in this sense, i am in good com- 
pany. You will find that Sir W. Thomson, in the Pzlosophical 
Magazine for February 1854, described a machine by which a 
steam-engine or water-wheel could produce thirty-five times the 
heat commonly considered as equivalent to the force used ; or the 
corresponding amount of cold. At that time, then, two years 
after his paper read to the British Association (to which you refer 
me), he certainly did not hold such an opinion with regard to the 
mechanical equivalent of heat as to exclude the possibility of such 
an engine. 
The final judgment of the question I confidently leave to time 
and facts. When any of the “grand founders of a rapidly pro- 
gressive science” can spare time from their investigations to refute 
my fallacies, I shall gladly retract them. H. HIGHTON 

The Spectrum of the Aurora 
In the sketch appended to my letter on this subject in last 
week’s NaturE, I notice that the engraver has made the line 
at 4°I much too sharp and definite in both spectra. It really 
shades off rapidly to the more refrangible side, at least in the 
spectrum of the vacuum tube, and possibly also in that of the 
aurora. Though much the brightest line in the auroral spectrum, 
it is not the most conspicuous in that of the tube, but the rela- 
tive brightness of lines frequently varies much at different tem- 
peratures. The band at 8 in the auroral spectrum is also repre- 
sented too narrow. Those who have practical experience with 
the spectroscope will appreciate the great difficulty of repre- 
senting faint spectra correctly in a woodcut. 
Henry R. PROCTER 

Science Teaching for the People 
THE subject of Science Teaching in our elementary schools 
having been ably brought forward by Mr. Henry Ullyett in a 
recentnumber of your Journal, and the scientific instruction 
under the Science and Art Department, South Kensington, | 
having been at various times the subject of consideration in it, 
I venture to ask for a short space in your columns in order to 
submit the following proposition for the consideration of your 
readers, many of whom, have, probably, special opportunities of 
coming to a correct judgment on the point. The question I wish 
solved is this : Is the spread of scientific education, under the 
auspices of the Science and Art Department, likely to be best 
promoted by the whole of the Department’s assistance to any 
one town being dispensed by a single committee, by whom a 
central school shall be provided, of which all other schools es- 
tablished, or that may be established, in various districts of the 
town, shall be considered only as branches, and be subject to the 
control of the central committee, on whose books the names of 
all students would be borne, and through the one secretary of 
which all the returns and other communications to and from the 
Department would have to pass ? 
Is it not better that the schools established in various districts 
of a town, say in connection with each elementary school, should 
each have their separate organisation of committee and secretary, 
at least in so far as the teachers connected with each are diffe- 
rent? In this town, in common with a great many others, the 
latter plan has been the rule, but a suggestion has now been 
made that all these committees should be amalgamated, together 
with those of the art-classes also, the plea being that it might 
lead to the erection of a central building for the purposes of an art 
and science school. 
Now, however necessary for art it is that there should bea 
central building for the provision of higher instruction than can 
be given in the night classes, I cannot see that there is anything 
in the study of science that demands greater facilities than can 
easily be provided by any district school, and the possession of 
which is indeed required by the Science Directory before any 
science school receives the approval of the Department. 
Why, then, the science committees should be asked to unite 
in this town alone, by which a most dangerous precedent would be 
established, I cannot understand, for I do not believe that sucha 
course would be beneficial to the town at large, while it would 
be very prejudicial to the interests of the existing district schools, 
and of any persons wishing to commence teaching in future. - 
Perhaps some of your readers will favour us with their views 
on the question I have stated, which I venture to think involves 
a principle of very general interest to the science schools of the 
kingdom. A MEMBER OF A SCIENCE COMMITTEE 



A Rare Moth 
Ir may be interesting to know that the rare and beautiful 
moth, Pe/lephila Galit, appeared somewhat plentifully in the 
neighbourhood of Derby during the past summer. I have in 
my possession a fine male and female which were cap'ured at 
Long Eaton, some few miles from Nottingham, whilst flying in 
company over a bed of geraniums. Is not this the first recorded 
instance of their appearance in this locality? Zeuzera Asculi 
also appeared in unusual abundance in the same garden, upwards 
of fifty specimens being taken by one person. W. H. G, 

Measurement of Mass 
WILT, you allow me a few words in explanation of a sentence 
in my last letter, which has strangely been misunderstood by 
Prof. Everett. In defending the system which makes the 
standard pound a unit of force on the ground that although not 
the most philosophical. it simplifies the conception of ma s which 
is always difficult for beginners, I said, ‘‘ The assumption of a 
hypothetical force of gravity not dependent on latitude, seems to 
stand on the same footing as the employment of a mean solar 
day,” meaning, of course, that just as we assume (for con- 
venience) that a solar day is the same length at whatever per’od 
of the year we take it, so we may assume a mean force of 
gravity (the actual force of gravity in latitude 45°) which is the 
same all over the earth. Such an assumption will enable us to 
explain the unfamiliar notion of mass by the familiar one of 
weight, and when it does become necessary to take into account 
the variation in the force of gravity at different points of the 
earth’s surface, the correction is easily made. 
Prof. Everett seems to think that I suppose that the average 
length of the apparent solar day is not the same at all places on 
the earth. Will you allow me to quote the following passage 
from the original in support of my first assertion, that Prof, 
Everett’s tacit assumption that everybody knows what mass is, is 
less likely to lead to clear ideas than the explanation given by 
Deschanel. He says—‘‘Un corps a une masse plus grande 
qu’un autre lorsque la méme force lui imprime une vitesse plus 
petite, et reciproquement, . . . .  sinous considérons en 
particulier le poids d’un corps, on aura, entre ce poids, la masse 
et laccélération de la pesanteur, la relation fondamentale, 
Eee: 
Cette formule nous montre que dans le méme lieu le poids est 
proportionnel a la masse, parceque g a la méme valeur pour tous 
les corps. Il n’en est pas de meme quand on passe d’un lieu a 
un autre ; mais comme aprés tout les variations sont extrémement 
petites, en réalité la masse et le poids sont deux quantités toujours 
sensiblement dans le méme rapport. Toutefois il faut se rappeler 
que ces deux expressions correspondent a des notions distinctes, 
et, abstraction faite de toute évaluation numérique, la masse d’un 
corps est quelque chose qui lui est propre et qui est indépendant 
du poids. La pesanteur n’existerait pas quil n’en serait pas 
moins vrai qu’une sphére de plomb a une masse plus grande 
qwune sphére de liége de méme diamétre, Nous reconnaissons 
ce fait ordinairement a ce que la poids de la premiére sphere est 
plus grande que celui de la seconde ; mais a déefaut de la pesan- 
teur, l'emploi de toute autre force pourrait nous conduire au 
méme resultat.” W. M. W. 


PHOTOGRAPHS OF THE ECLIPSE 
P= me to call your attention to the position of 
the woodcut illustrating my remarks on the Eclipse 
Photographs. The south point is where the north 
should be, As what F have now to say refers to the 
picture I shall feel obliged if you will permit its reinsertion 
in its true position.* With reference to the power of the 
light of the Corona, I used the word actznic, not active as 
rinted. 
The readers of NATURE may perhaps be glad of the 
opportunity to compare for themselves tracings of the 
American and of my own photographs, which I now give 
in outline in illustration of remarks in your second article 
* This vexing mistake was due to a blunder of the printer in reversing 
the block after ithad been placed on the machine. Its re-insertion this 
week will rectify the mistake.—Ep. 
