752 
NATURE 
A 
[June 2, 1923 

Cape York, where his survey began. He continued 
his work to alout lat. 82° N., but was forced to 
abandon his project of charting Peary Land. A large 
tractor proved very useful for transport, and easily 
pulled over snow a sledge loaded with food supplies 
and ten barrels of petroleum. After several hundred 
miles it broke down and had to be abandoned. Pro- 
gress then became difficult. 
influenza in the Cape York district has caused such 
heavy mortality among the Eskimo that it is im- 
possible at present to start any expedition from that | 
base. Mr. Koch intends to return to Denmark in the 
course of the present summer. 
THE arrangements for the International Air 
Congress to be held in London at the Institution of 
Civil Engineers, Great George Street, London, S.W.1, 
on June 25-30, are now approaching completion. 
The papers to be presented cover every field of 
aeronautical development, and are thoroughly inter- 
national in character, as contributions have been 
received from America, Belgium, Denmark, France, 
Holland, Italy, Spain, and Sweden, among other 
countries, in addition to Great Britain. Applications 
for membership of the Congress will be accepted up 
to Saturday June 9. A number of visits to various 
Government experimental and research establishments 
and Royal Air Force Stations have been arranged, and 
several of the leading aircraft and engineering firms 
have expressed their readiness to receive members of 
the Congress at their works. Communications re- 
garding the Congress should be made to the Inter- 
national Air Congress, London, 1923, at 7 Albemarle 
Street, London, W.1. 
Bulletin 54 S issued by Messrs. Watson and Sons 
(Electro-Medical), Ltd., Sunic House, 43 Parker 
A bad epidemic of | 

Street, Kingsway, London, W.C.2, is a descriptive 
list of second-hand X-ray and electro-medical appar- 
atus which the firm has for sale. Complete units for 
X-ray work are offered, in addition to numerous ac- 
cessories, such as induction coils, Coolidge filament 
transformers, mercury interrupters, X-ray tubes, 
screens, and so on. 
THE Medical Supply Association, Ltd., of Gray’s 
Inn Road, London, now supply ‘‘ Radio-Wave "’ re- 
ceiving apparatus of all kinds, from a “junior” 
crystal set, up to a ‘‘ Radio-Wave Plutocrat”’ re- 
ceiver with a range of 300 miles. Modern radio sets 
are now so simple that no special education or skill 
is required to work them. Some of the sets also can 
receive the roughest usage without damage. The 
receivers made by the Association are of the approved 
type and the valve apparatus su pplied is fully licensed. 
The lengthy list given of radio parts in ene catalogue 
will be welcomed by amateurs. 
Messrs. LONGMANS AND Co. have in the press, ‘Hoe 
appearance in their series of ‘‘ Manuals of Telegraph 
and Telephone Engineering,’’ ‘‘ The Inspection and 
by 
Testing of Materials, Apparatus, and Lines,” 
F. L. Henley, which will describe the methods em- 
ployed in the British Post Office in the inspection and 
testing of supplies of the various materials used in line 
construction, cables, telephone and Morse telegraph 
instruments, and furnish the inspector with a basis of 
sound information upon which to form a judgment in 
those cases where electrical, mechanical, or chemical 
tests are either not available or are not conclusive. 
In the same publishers’ ‘‘ Rothamsted Monographs on 
Agricultural Science” 
Grasslands for Hay,’ by Dr. Winifred E. Brenchley 
of the Rothamsted Experimental Station. 
Our Astronomical Column. . 
THE DIAMETERS OF SATURN’S SATELLITES.—Major 
P. H. Hepburn contributes a paper on this subject 
to B.A.A. Journ. for March. The only one that is 
large enough to measure with a micrometer is Titan, 
for which Barnard and Lowell agreed in finding a 
diameter of some 2600 miles, say 0°7” in arc. Major 
Levin found 3500 miles by the eclipse of Rhea by 
Titan, April 8, 1921 ; probably the truth lies between 
the two. For the other satellites estimates can be 
made (1) from the masses determined by H. Struve, 
combined with assumed densities, and (2) from the 
stellar magnitudes of the satellites determined by 
Guthnick and at Harvard, combined with assumed 
albedoes. In practice each method was found to 
help the other ; it was established that all the smaller 
satellites must have small densities, not much greater 
than that of Saturn itself, which is 4 of the earth’s 
density, or o°7 of water. In particular, Mimas must 
have both low density and high albedo, and Major 
Hepburn suggested, half seriously, that it might be 
a gigantic snowball; he had in a former paper 
suggested that the Ring might be composed of ice 
crystals. The values of the diameters considered 
most probable are: Mimas, 300 miles; Enceladus, 
450; Tethys, 700; Dione, 800; Rhea, 1ooo. Data 
are lacking for trustworthy estimates of Hyperion | 
and Tapetus. As Jupiter's satellites have also low 
densities, some colour is given to the old suggestion 
that the outer parts of the solar system are built of 
NO. 2796, VOL. 111] 

less dense materials than the inner parts. It will be 
remembered that the inner satellites of Saturn are 
much the smallest bodies the masses of which have 
been determined gravitationally, hence determination 
of their densities throws new light on our knowledge 
of that of small bodies generally. Bodies that have 
never been in a molten state might well have inter- 
stices between their particles which would be filled — 
up in a molten condition. 
ANNUAIRE DEL’ OBSERVATOIRE ROYALDE BELGIQUE, 
1924.—Publication of this well-known Annual was 
suspended during the War, but the present volume 
is a very successful effort to make up for the lost 
years. It begins with the ordinary calendar, astro- 
nomical, and tidal information for the year, all given 
in great detail, and then follows a full summary of 
astronomical progress since 1915; thus Wolfer’s 
monthly sunspot numbers are given for six years ; 
they show a clearly defined maximum in the middle 
of 1917. The stages in the dissolution of the great 
eclipse prominence of May 1919 are fully illustrated. 
A full account is also given of work on the polarity 
of sunspots, the magnetic field of the sun, and 
Einstein’s theory, and the discussions to which it — 
has led. There are, in addition, tables and detailed — 
accounts of planetary, cometary, and stellar work — 
during the six years dealt with. The volume is thus — 
very useful as an index and guide to contemporary 
astronomical progress. 
will appear ‘“‘ Manuring of 

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