resisting medium. 
FEBRUARY 15, 1917| 
these are due to nitric oxide generated.—Prof. J. W. 
Nicholson and Prof. E. Wilson: Magnetic induction and 
its reversal in spherical iron shells. A solution of 
problems which arise in the production of an effective 
magnetic shield for large spaces. These relate mainly 
to the effective demagnetisation of the shells of which 
the shield is constituted. Theoretical solutions of 
problems relating to the effects of indefinitely closely 
wound coils on various shells of such a shield are 
_ given, and compared with the experimental values for 
an actual coil. 
‘ The experiments supply an estimate 
of the deviations of Maxwell’s formula, for the field 
inside a spherically wound helical wire, from the true 
_ values, when the spacing in the helix is of importance. 
A study of the necessary interval between current 
reversals in the process of demagnetisation has been 
made, and it is shown that the delay in reversal of 
magnetic phenomena in considerable. masses of iron, 
due to eddy currents, is negligible when the magnetic 
inductions are fewer than 300 C.G.S. units.—S. Brodet- 
sky : The two-dimensional motion of a plane lamina ina 
Some of the types of motion of a 
plane lamina in a resisting medium, such as the air, 
are discussed. Experimental laws of resistance are 
used for varying circumstances of shape and motion. 
The motion is in two dimensions. Part i. deals with 
a lamina of large moment of inertia. For the case of 
no forces acting on the lamina other than the resist- 
ance of the medium, relations are obtained connecting 
the components of velocity, the rotation, and the time. 
In the case of a wide lamina an investigation is given 
of the oscillatory part of the motion. The graphical 
method is then extended to the case where forces in 
addition to the resistance act on the lamina, notably 
gravity. In part ii. the case of a lamina the moment 
of inertia of which is negligible is considered, and 
equations are found for correcting the paths found in 
Lanchester’s phugoid theory. Part iii. treats of the 
oscillations about a steady fall. The vertical fall of a 
lamina is shown to be unstable unless the centre of 
mass is at a distance below the centre of figure lying 
between two limits given by -a quadratic equation. 
The stability of a parachute with a hanging body 
attached to it is also considered. ' 
February 1.—Sir J. J. Thomson, president, in the 
chair.—Sir Ronald Ross and Miss H. P. Hudson: An 
application of the theory of probabilities to the study 
of a priori pathometry. Part ii. A number of hypo- 
thetical epidemics on the basis of the equations of 
part i. are constructed. The influence of some of the 
principal parameters is considered. The equations are 
generalised to include a wider range of a@ priori sup- 
positions as to the laws of the happening, and further 
suggestions are made as to the comparison of the 
results with mortality statistics. The following ten- 
tative conclusions are arrived at: The cases considered 
have led to exactly the series of curves required by 
the facts: 1. The steadily rising curve of a happening 
that gradually permeates the whole population (VII., 
iii.). 2. The symmetrical bell-shaped curve of an epi- 
demic that dies away entirely (VII., v.). 3. The un- 
symmetrical bell of a new happening that begins with 
an epidemic and settles down to a steady endemic 
level (VII., v.). 
rise and fall due to a seasonal disturbance (XI., iv.). 
5. The more irregular curve where there is recrudes- 
cence before the end of an epidemic, or where out- 
breaks differing in violence occur at unequal intervals 
(XI., v.). This suggests that the rise and fall of 
epidemics may be explained by the general laws of 
happenings as studied.—Dr. J. Brownlee: An investi- 
gation into the periodicity of measles epidemics in 
London from 1703 to the present day by the method 
of the periodogram. The statistics for the epoch 
NO. 2468, VOL. 98] 
4. The periodic curve with regular | 
NATURE 483 
EE ae 
of registration give the main periodicity of 
measles in London for the last seventy-two years as 
almost exactly ninety-seven weeks. The amplitude 
of this period is o-4 of the mean number of cases- 
Periods with amplitudes of about one-half of this are 
found for one year and for six months. These periods 
probably reflect the influence of the weather upon 
the-deaths from measles, though the evidence is not 
complete. There are two sets of periods grouped on 
either side of the main period in such a way as could 
be explained by interference with long waves of pre- 
valence or severity of the disease. These periodicities 
are probably the exvression of something in the life- 
history of the organism causing the disease.—Capt- 
J. Hammond: The causes responsible for the develop- 
mental progress of the mammary glands in the rabbit 
during the latter part of pregnancy. Experimental 
results show that the development of the mammary 
gland of the rabbit during the second half of preg- 
nancy is under the same influence as that which con- 
trols the development during the first half, namely, 
the corpus luteum. This gland is active during the 
second half of pregnancy. The further development 
of the corpus luteum is due to the influence of the - 
foetus. The view that milk secretion in pseudo-preg- 
nancy takes place in correlation with the involution 
of the corpus luteum is confirmed. Apparently the 
secretion of milk results whenever the influence caus- 
ing the glandular growth is removed or lessened in 
amount, provided that the initial development has* 
gone far enough.—F. H. A. Marshall and E. T. 
Halnan: The post-cestrous changes occurring in the 
generative organs and mammary glands of the non- 
pregnant dog. The uterus and mammary glands of 
the non-pregnant bitch undergo pronounced _post- 
cestrous development under the influence of the 
corpora lutea during a definite pseudo-pregnant period. 
Retrogressive changes do not set in with any of these 
organs until about thirty days after ovulation. The 
developmental changes are similar to those taking 
place during pregnancy. The relatively long persist- 
ence of the corpora lutea in the bitch is probably 
correlated with the moneestrous habit. This persistence 
elucidates the phenomenon of bitches which had not 
been impregnated secreting milk at or near the end 
of the pseudo-pregnant period. The changes which 
occur in the generative organs and mammary glands 
after cestrous are now brought into relation with the 
rest of the cestrous cycle. 
Physical Society, January 26.—Prof. C. V. Boys, 
president, in the chair.—C. O. Bartrum: A clock of 
precision. The principal feature is the employment of 
a ‘“‘slave” clock to do most of the work, leaving the 
master pendulum no function beyond that of control- 
ling the rate of the other. The master pendulum 
swings freely except for a short period every minute, 
during which it receives an impulse from a falling 
pallet electromagnetically released by the slave clock. 
At the end of its fall the pallet closes a second circuit 
and is restored to its initial position. The electric 
circuits also energise parts of the mechanism in the 
slave clock by which the latter is kept in time with the 
master pendulum. The lagging of correction behind 
error, with the resulting periodic fluctuation in the 
rate, is reduced almost to the vanishing point by a 
“negative backlash” in the control mechanism. A 
mathematical discussion of the best working condi- 
tions and of the possible magnitude of errors is 
given.—-Dr. F. Schwers: The effect of water 
vapour in the atmosphere on the propagation of 
electromagnetic waves. The probable influence of 
moisture in the atmosphere on the refraction of electro- 
magnetic waves round the earth’s surface is discussed. 
The conclusion of Kiebitz that the presence of mois- 
