416 
NATURE 
[DECEMBER 4, I913 
were :—‘‘ Citizenship and Technical Instruction" (a 
subject almost entirely ignored in schemes of tech- 
nical instruction), by Mr. T. P. Gill; ‘‘ Apprenticeship 
Classes,” by Mr. B. O’Shaughnessy; ‘‘ Technical In- 
struction: its Achievements and Possibilities,’’ by the 
Rev. Canon Arthur Ryan; and ‘‘ Domestic Economy : 
the Family Budget,’ by Mr. G. Fletcher. The 
method of treatment and the importance of the sub- 
jects considered give the report a high value, and 
make it worthy of the serious attention of educationists 
on this side of the Irish Sea. 
SOCIETIES AND ACADEMIES. 
Lonpon. 
Royal Society, November 27.—Sir Archibald Geikie, 
K.C.B., president, in the chair.—Prof. B. Hopkinson : 
A method of measuring the pressure produced in the 
detonation of high explosives or by the impact of 
bullets. A steel shaft about 14 in. diameter and 4 ft. 
long is suspended horizontally from strings so that 
it can swing in a vertical plane as a ballistic pendulum. 
At one end it carries an end-piece of the same diameter 
and several inches long. The end-piece is held on by 
magnetic attraction; the surfaces of the joint are care- 
fully faced. If a bullet be fired at the other end a 
wave of pressure travels along the shaft, the length 
of which represents the duration of the blow on the 
scale 1 in.=5x10~-° second approx. The wave passes 
the joint without change and is reflected as a tension- 
wave from the free end. If length of wave exceeds 
twice that of end-piece, the tail of pressure-wave will 
have passed the joint when the head of tension-wave 
reaches it and the piece will fly off, having trapped 
within it the whole momentum of the blow, leaving 
the shaft at rest. By experimenting with different 
lengths of end-piece and finding that which is just 
long enough to stop the shaft, the duration of blow 
can be determined. The end-piece is caught in a 
ballistic pendulum and its momentum measured; thus, 
knowing the time, the average pressure is determined. 
Applied to investigation of the blow given by a lead 
bullet, the method gave results in close accord with 
those expected on the assumption that the bullet be- 
haves as though it were liquid, the measured duration 
of blow being nearly that required by the bullet to 
travel its own length. Measurements by the same 
method of pressures produced by detonation of a 1-oz. 
dry guncotton primer showed that, at a distance of 
= in. from surface of cotton, the pressure is practically 
all gone in 1/50,000 second, the average pressure 
during that period being about 25 tons per sq. in., and 
the maximum of the order of 45 tons per sq. in.— 
J. H. Jeans: Gravitational instability and the nebular 
hypothesis. The work of Maclaurin, Jacobi, Poincaré, 
and Darwin on rotating fluids has applied only to the 
abstract case in which the mass is considered perfectly 
incompressible and homogeneous. To estimate the 
bearing of their results on astronomical problems, it 
is important to know to what extent these results 
remain valid for actual, compressible, heterogeneous 
masses. The result of the present investigation is 
summed up concisely by saying that the ideal mass 
of incompressible fluid has been found to supply a 
surprisingly good model by which to study the be- 
haviour of the more complicated natural systems con- 
sidered in astronomy.—B. A. Keen and A. W. Porter: 
The diffraction of light by particles comparable with 
the wave-length. A suspension of finely-divided sul- 
phur, obtained by precipitation from a solution of 
thiosulphate of soda by the addition of acid, ordinarily 
diffracts an excess of blue light, so that a white source 
of light seen through it looks red. One of the authors 
discovered that if the particles be allowed to grow 
NO. 2301, VOL. 92] 
the red image gradually changes over in colour, be- 
coming at one stage a deep indigo blue, and after- 
wards passing through various shades of green to 
white. The present investigation was undertaken to 
obtain quantitative information in regard to this 
phenomenon.—Prof. R. J. Strutt: Note on the colour 
of zircons, and its radio-active origin.—Prof. W. H- 
Bragg : The influence of the constituents of the crystal 
on the form of the spectrum in the X-ray spectrometer. 
The energy of the pencil of X-rays which falls on the 
crystal of the X-ray spectrometer is in part spent 
within the crystal through absorption, which implies 
the production of kathode and characteristic X-rays, 
and in part is scattered, producing the reflected ray 
when circumstances are favourable. It is found that 
where there is much absorption there is little reflection. 
The best reflectors are therefore those crystals of which 
the absorption coefficients are smallest in comparison 
with their weights or their scattering powers. For 
this reason alone the diamond must be a very good 
reflector.—\W. L. Bragg: The analysis of crystals by 
the X-ray spectrometer. By a quantitative comparison 
of the intensities of the successive orders of reflection 
by various crystal faces, it is shown that the X-ray 
spectrometer can be made to give a very complete 
analysis of the crystal structure. The structures par- 
ticularly investigated in the paper are those of the 
isomorphous sulphides, pyrites, and hauerite, and of 
the series of compounds which compose the calcite 
family of minerals. By a study of these last com- 
pounds, it is concluded that the diffracting power of an 
atom is proportional to its atomic weight.—Dr. T. H. 
Havelock : Ship resistance: the wave-making proper- 
ties of certain travelling pressure disturbances. The 
paper contains a theoretical comparison of the wave- 
making resistance associated with certain distributions 
of surface pressure. Various inferences are drawn in 
regard to variation of resistance with speed, and the 
speeds at which typical interference effects occur. In 
particular, types are examined which are similar in 
general form to those associated with the motion of 
ship models in recent work at the William Froude tank 
in the National Physical Laboratory.—Dr. R. A. 
Houstoun : The mathematical representation of a light 
pulse. The object of this paper is to direct attention 
to a new series of expressions representing the initial 
form and dispersion of a light pulse. They have been 
suggested by one of Kelvin’s hydrodynamical papers, 
and are derived from his instantaneous-plane-source 
solution in the conduction of heat. 
Zoological Society, November 11.—Dr. S. F. Harmer, 
F.R.S., in the chair—Dr. W. T. Calman: Fresh- 
water Decapod Crustacea (families Potamonide and 
Palzmonidz) collected in Madagascar by the Hon. 
Paul A. Methuen. One new species of Potamon and 
five varietal forms of P. madagascariense were de- 
scribed. It is suggested that the river-crabs of Mada- 
gascar may have had an autochthonous origin from 
some form resembling P. madagascariense. No clear 
affinities can be traced with the Potamonidz of Africa 
or of Peninsular India, but it is pointed out that in 
the present state of knowledge the river-crabs appear 
to be a hazardous subject for zoogeographical specula- 
tion.—G. A. Boulenger: A collection of reptiles and 
Batrachians made by Dr. Spurrell, in the Colombian 
Choco. The series of specimens was of great interest, 
and contained several new species.—C. Tate Regan: 
A revision of the Cyprinodont fishes of the subfamily 
Peeciliine. A number of new genera were defined 
and several new species were described; the structure 
of the intromittent organ was found to be of great 
systematic importance.—Prof. W. N. Parker: Investi- 
gations on a growth of Spongilla lacustris in the 
Cardiff Waterworks system. The author described 
FOE 
