62 / NATURE 

ELECTRICAL DiIsTRIBUTION ON Two SPHERICAL - 
Conpuctors.—In a recent paper to the Physical 
Society Dr. Alexander Russell returns to the electro- 
static problem of two spherical conductors. The most 
interesting feature of the investigation is the fact that 
Kelvin’s method of images is not the best way to 
attack the problem. The author shows how Poisson’s 
method can be made to yield useful results, amenable 
to easy and accurate numerical computation. The 
principle of the method is as follows. A functional 
form is postulated for the potential at any point of 
the actual but unknown electrical distribution on each 
sphere, and the constancy of the potential on each 
sphere is used, coupled with the theory of inverse 
points. The two cases, (1) one sphere inside the 
other and (2) the spheres outside one another, are 
discussed in. detail, and applications are made to find, 
the force between the spheres, the energy, and the 
stress in the medium. 
Aw AuToMATIc VOLTAGE REGULATOR.—For electric 
lighting it is essential that the voltage of the dynamo 
should vary only between very narrow limits. In 
country house lighting where the direct current 
dynamo is driven by a petrol motor the voltage 
variations sometimes cause serious and very objection- 
able fluctuations of the light. Messrs. Isenthal 
and Co., Ltd., now manufacture an automatic rapid 
action voltage regulator suitable for use with dynamos 
up to 50 kilowatt capacity. The device is extremely 
simple, and by its use the voltage can be maintained 
practically constant even when the driving speed 
and the load vary very suddenly. The principle 
employed is practically the same as that used in 
other vibratory regulators some of which, the Tirrill 
regulator, for example, are extensively used in electric 
lighting stations. Messrs. Isenthal’s regulator is,’ 
however, applicable to quite small machines, and 
should prove very useful. 
GuipinGc }Fiicut.— Die Naturwissenschaften of 
October 6, 1922, contains an article by Prof. C. 
Runge, of Géttingen, entitled ‘‘ Uber den Segelflug,”’ 
reproducing a lecture delivered at Wasserkuppe 
during the German gliding contests last August. 
Prof. Runge gives a clear account of the main prin- 
ciples underlying gliding or sailing flight. After 
emphasising the point that a steady horizontal wind 
is useless for the purpose, the author divides useful 
winds into two categories, (1) steady winds in upward 
directions, and (2) variable winds. The former can 
be used in gliding flight if the vertical component is 
at least equal to the rate of vertical fall of the glider, 
and Prof. Runge points out that upward winds are of 
frequent occurrence; that, in fact, ordinary air 
movements in the form of wind are primarily vertical, 
but are horizontal more or less to us because we live 
at that stratum of the atmosphere affected by the 
earth’s solid crust. In the case of variable winds, 
several kinds of variations are possible : thus different 
air layers may have different speeds, or the air in any 
one layer may have different speeds at different times. 
An attempt is made to explain non-mathematically 
how such variation can be used for flight. The effect 
of dimensions is also considered briefly. 
Loss oF HEAT FROM SURFACES.—At the request of 
the Engineering Committee of the Food Investigation 
Board, Dr. E. Griffiths and Mr. A. H. Davis of the 
National Physical Laboratory have carried out a 
series of measurements to determine the laws of gain 
or loss of heat by solid surfaces in contact with air at 
temperatures which differed from their own, and the 
NO. 2776, VOL. 111] 

[ JANUARY 13, 1923 
results are embodied in the recently issued Report 
No. 9 of the Board (H.M.S.O., price 1s. 6d.). It is 
shown that the loss or gain is mainly due to the con- 
vection currents set up in the air cl6se to the surface, 
and that the amount of heat transmitted per unit 
time and area is proportional to the 5/4ths power of 
the difference of temperature of surface and air, a law 
first stated by Prof. L. Lorenzin 1881. Unfortunately 
the factor of proportionality is not independent of the 
shape, size, and orientation of the surface, the loss 
for the same difference of temperature being greater 
per unit area and time for a small surface than for a 
large, and for a horizontal surface facing upwards than 
for one facing downwards. The authors give curves 
from which the proper value of the factor can be 
obtained in any practical case, so that the Lorenz 
law may be readily, used by heating, ventilating, and 
refrigerating engineers. 
UPPER-AIR WINDS IN INnp1IA.—Memoirs of the 
Indian Meteorological Department, vol. xxiii. Pt. 
III., which has just reached us, contains mean monthly 
characters of upper-air winds deduced from the 
flights of pilot balloons at thirteen stations in India 
during the period 1910 to 1919. The discussion has 
been carried out by Mr. J. H. Field, director of the 
Agra Observatory, and 
direction of the Director-General of Observatories. 
It contains nearly 100 foolscap pages of figures, with 
two pages of explanation. At many of the stations 
the observations are for a few months only and it 
is not easy to select a period with consecutive 
observations at several stations. The stations are 
distributed fairly well over India. Good comparisons 
can be obtained for the greater part of 1919, and these 
observations show a general increase in the speed 
of the wind with height, which is greater in winter 
than in summer, although this varies with the 
latitude, being more marked in northern India, 
according to the observations in February and 
August at Lahore, Agra, and Akyab, than in the 
south as shown by Bangalore. Naturally there is 
also much variation in the direction of the wind 
of the upper air at different seasons. To plot 
graphically the observations for the several stations 
for the several months and for varying heights would 
involve considerable labour, but where air-ways are 
to be used such plotting seems essential. The data 
will supply much which is of interest relative to the 
movement of the upper-air over India, and associated 
with what is known at the earth’s surface, it will 
afford most useful and instructive information. mY 
A New Beck Microscopre.—Messrs. R. and J. 
Beck, Ltd., 68 Cornhill, E.C., have submitted to us 
an example of their Model 22 ‘‘ London Microscope.” 
The instrument is simple in design and is supported 
on a modified horse-shoe base with widespread 
limbs, which gives complete stability even in the. 
horizontal position The coarse adjustment is of 
the spiral rack and pinion pattern, the fine adjustment 
is of the vertical type with milled head. The stage 
is a large one measuring 4 in. x3} in. With one 
eye-piece, and a low-angled § in. and a } in. objectives 
in canvas-covered case, the price is the moderate one 
of 8/. 17s. 6d. Double or triple nose-pieces are 
supplied at an additional cost of 1/. Is. and 1/. Ios. 
respectively. 
condenser of the Abbe type with centring screws, 
cell, and iris diaphragm costs an additional 2/. 7s. 6d. 
The instrument is well made and thoroughly efficient ~ 
and is suitable for all ordinary microscopical work. 
Additional objectives and other fittings can be 
supplied if desired. 
is published under the 
A spiral, screw focussing, swing-out — 

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