714 
NATURE 
[FEBRUARY 26, 1914 
one-fifth of the value of the magnetic moment, 
92-7 x 10-**, for the electron moving in a circular orbit. 
Sir Oliver Lodge has directed attention to the im- 
portance of all cases where commensurable numbers 
enter into physical problems. Mr. Chalmers thinks 
that the present result indicates that, in mag- 
netic materials, there is a unit of five (or 
ten) atoms, which has a constant number of mag- 
netons. Since in the last resort we must consider 
magnetism to be an atomic property, I should prefer 
to regard the result as affording further evidence for 
the view that the magnetic effects of the complex 
nucleus must be taken into consideration. The mag- 
neton may then result as a difference effect. 
To make this clearer, it may be worth while dis- 
cussing a simple illustrative model. Prof. Peddie has 
put forward some interesting suggestions as to the 
structure of the atom in the February number of the 
Philosophical Magazine. He supposes that the atom 
may be built up of concentric spherical shells of elec- 
trification, which may be in rotation round a common 
axis. Following this suggestion, suppose we have a 
uniform sphere of positive electrification of radius A 
rotating with angular velocity ©. Outside this, sup- 
pose we have a single ring containing n (from 1 to 8) 
valency electrons. The remaining negative electrifica- 
tion may be relegated to a central core having no rota- 
tion. Then the magnetic moment of the rotating 
sphere may be taken as }EA*Q, where E is the total 
positive charge, which we shall assume equal to Ne. 
We have no direct evidence as to the value of A*Q, 
but if for convenience we assume that it has the same 
value as a*w for a ring electron, we obtain the result 
that the magnetic moment of the rotating core is 
equivalent to 2N magnetons. The resultant magnetic 
moment for such a model would be the difference be- 
tween the 2N magnetons of the core and the 5n mag- 
netons of the ring. This is only intended to illustrate 
the way in which the magneton may be introduced as a 
unit for measuring magnetic moments without in- 
volving the necessity of a single magneton existing 
as an independent entity. If the core of an atom is 
built up of a and £ particles in orbital motion, the 
experimental results of Weiss indicate that the re- 
sultant magnetic moment of these particles can be 
expressed in terms of the magneton, 
H. S. ALLEN. 
Wheatstone Laboratory, King’s College, 
London, W.C. 
Origin of Structures on the Moon’s Surface. 
In Nature of February 5 Dr. Johnston-Lavis 
writes, ‘‘the more I compare the moon’s surface with 
volcanic vents, in different parts of this world, the 
less I see a resemblance between the two,” and “the 
more does the planetoid and meteorite projectile theory 
become acceptable.”’ 
G. K. Gilbert, in his address to the Philosophical 
Society of Washington, 1892, gives an extremely in- 
teresting summary of the various theories to account 
for the features of the moon’s face. After a very 
clear description of the phenomena to be accounted 
for, he accepts the meteorite theory with modifica- 
tions. He remarks that, if the so-called craters of 
the moon were due to the impact of meteors, their 
form would be for the most part elliptical, whereas, 
in fact, they are circular. His own theory is that the 
earth was at one time attended by a ring similar to 
that which encircles the planet Saturn, and that this 
afterwards ‘‘gradually coalesced, gathering first 
around’a large number of nuclei, and finally all 
uniting in a.single sphere,’’ the moon. 
to show that this hypothesis accounts for the facts. 
NO. 2313, VOL. 92] 
He attempts | 
| In Nature (vol. xxv., p. 243, 1862) I suggested 
that when. according to Sir G. H. Darwin’s view, 
the moon broke away from the earth and commenced 
an independent existence, thescar left by the great 
catastrophe forms now the basin of the Pacific Ocean. 
The same idea was elaborated five years later — 
by Prof. Pickering’ (Journal of Geol., vol. -xv., 
No. 1, 1907). It is evident that if the Pacific Ocean — 
indicates the place from which the moon departed, 
the continents surrounding it must have been then in 
existence and the earth covered with a solid crust. 
Both Prof. Pickering and myself make this assump-_ 
tion. The question which I would ask, therefore, is this : 
Does the moon’s surface bear traces of this mode 
of origin ? 
The material detached from the earth would have 
been partly solid, derived from the cooled crust, and 
partly liquid, derived from the molten substratum. 
The expulsion would have been probably explosive, 
owing to the gases dissolved in the substratum. The 
material would consequently have been scattered. 
Subsequently it would have collected about its centre 
of gravity, the smaller masses falling in last. The 
paths of the falling masses would have been radial. 
What the telescope now reveals would be the final 
effect after the mass had settled into the spherical 
form. May not, then, the circular so-called craters — 
have been caused by the impact of fragments of the 
solid crust, and may not the mountains of the moon 
be also angular portions of the earth’s crust projecting 
above the mean spherical surface of the moon? 
The above suggestions do not exclude the pos-— 
sibility of true meteors also having fallen, and left 
their mark upon the moon, but it is not probable that — 
many large meteors have struck the moon, because 
they are rare upon the earth, and if many siderites — 
had so fallen, the moon’s specific gravity would have 
been higher than it is, viz., about 3. ihe 
It has occurred to me to inquire at what distance 
from the earth’s surface the centrifugal force would 
balance the earth’s attraction, because it is difficult to © 
see how the material, which is supposed to have been 
detached from the earth to make the moon, could have 
got away until that distance was reached. 
This condition is expressed by the equation, 
3 
9 a . . 
ae = Bo where a is the earth’s radius, 20,926,202 ft., 
» the angular velocity, assumed in this case to be 
27/18,000 seconds, g gravity, 32 ft. a second, and r 
the distance from the centre required. From this I 
get the required distance from the earth’s surface, 
viz., r—a, equal to 53,205,200 ft., or more than double 
the earth’s radius. How this distance could be reached 
seems unaccountable. ; 
Sir G. H. Darwin, in the summary of his paper 
on the remote history of the earth, does not refer to 
this point; but since his theory has been adopted 
without demur by Sir Robert Ball and Prof. Picker- 
ing, the difficulty which strikes me must be more 
apparent than real. Can some reader of NATURE 
explain it? O. FisHeEr. 
Graveley, Huntingdon. = 
I HAVE read with a great amount of interest the 
letter by Dr. Johnston-Lavis which appeared in the 
issue of Narure for February 5, as regards the ray 
systems of the moon from a vulcanologist’s point of 
view. Even taking into consideration his explana- 
tions, however, I cannot now help comparing the 
moon with our own planet, especially when Laplace’s — 
nebular hypothesis, and subsequent theories in refer- 
ence to the relation of the moon to the earth are con- 
sidered, which seem to prove conclusively that the 
} Moon was once a portion of this world. Then with 
