654 

static balance and gyroscope. As is well known, com- 
pressed air (at 150 kilos. per square centimetre) is used 
for the engines. On expansion it is very materially 
cooled, and a reference is made to a modern arrange- 
ment for re-heating the air by injection of an alcohol 
or petrol spray, together with a fine spray of water— 
a method which leads to a great increase in speed and 
run of the torpedo. The article deals further with the 
interesting experimental station of the Schneider firm 
—the Batterie des Maures—situated in the roads of 
Hyeres. 
In a recent article (NATURE, August 5) on modern 
methods for the preparation of hydrogen for balloons 
and dirigibles, the difficulties attendant on the use and 
transport of acid for the well-known methods of gene- 
ration from zinc or iron were mentioned. One of the 
greatest troubles arising from the use of gas prepared 
by these reactions has been due to impurities in the 
gas. An article in La Nature (July 10) deals specially 
with this method of preparation and the purification 
of the gas. Among the impurities are hydrogen 
arsenide, antimonide, and selenide, all poisonous gases, 
and which led to the death of two employees and 
serious illness of three others engaged at the Chalais- 
Meudon Aerodrome in 1900. Sulphuretted hydrogen, 
formed by reduction in the generators, has a very 
deleterious action on the varnish of the fabric, which, 
containing lead, is attacked, forming the sulphide, and 
further, by the action of moist air, on the tissue itself. 
Exposed metal in dirigibles is also subject to its 
attack. Other impurities are water vapour, carbon 
dioxide, and gaseous hydrocarbons, which seriously 
diminish the lifting power of the gas. Purification 
entails first washing the gas to remove acid spray, 
passage through a special purifier containing a moist 
mixture of iron sulphate and lime on sawdust, and 
finally through soda. The iron purifying material 
which has absorbed the sulphuretted hydrogen is re- 
generated by exposure on grids to the air. The cost 
of purification is stated to be 1-5 to 2 centimes per 
cubic metre. 
An investigation by Messrs. G. K. Burgess and 
P. D. Merica, of the Bureau of Standards, a pre- 
liminary account of which appears in the Journal of 
the Washington Academy of Sciences for July 19, 
appears to provide a complete explanation of the 
mysterious failure of tin fusible boiler plugs. It will 
be remembered that these plugs are inserted in the 
crowns of boiler furnaces or flues and that their 
function is to give warning of overheating of the 
boiler by their melting and admitting steam to the 
fires. It was found that the tin of plugs which had 
failed had been converted in service into SnO,, which 
melts at 1600°C. This oxidation was finally traced 
to the presence of zinc in the tin to the extent of 
o-3 per cent. Plugs with this amount of impurity, 
when heated to 190°C. for 500 hours, develop a 
cellular structure, the walls of the cells consisting of 
oxidised zinc, to which oxidised tin is added with 
time, and the unoxidised tin is enclosed in the cells. 
Even when the tin is melted the cell walls may be 
strong enough to withstand the boiler pressure. The 
authors conclude that tin of at least 998 per cent. 
purity should be used for such boiler plugs. 
NO. 2389, VOL. 95| - 
NATURE 

[AUGUST 12, I915 

Tue July number of the Bulletin of the American 
Mathematical Society (xxi., 10) contains particulars 
of the courses in mathematics announced for the 
summer semester in six of the German universities. 
Excluding such general items as ‘‘colloquium”’ and 
““seminar,’’? the statistics work out as follows :— 
Berlin, 13 courses by 8 lecturers; Bonn, 7 courses, 
4 lecturers; Frankfurt, 7 courses, 4 _ lecturers; 
Gottingen, 13 courses, 9 lecturers; Leipzig, 9 courses, 
4 lecturers; Munich, 11 courses, 8 lecturers. We also 
note the award of the Helmholtz medal of the 
Berlin Academy to Prof. M. Planck, and the 
celebration of the seventy-fifth birthday of Prof. F. 
Mertens, of Vienna, on November 7, 1914, and of 
the seventieth birthday of Prof. Georg Cantor, of 
Halle, on March 3, 1915. The lists of courses do 
not appear to differ to any substantial degree from 
those published in previous years, either in the 
number of courses or in the character of their sub- 
ject-matter. To English readers they should afford 
some indication of the way our enemies are main- 
taining their higher scientific educational systems in 
war-time. By ‘lecturers’? in the preceding list we 
mean professors in most cases, but include other 
members of the teaching staffs. 
WE are asked by Messrs. Constable and Co., Ltd., 
to state that they are the English publishers of 
‘Elementary Text-book of Economic Zoology and 
Entomology,’ by Profs. Kellogg and Doane, which 
was reviewed in the issue of Nature for July 1 last. 

OUR ASTRONOMICAL COLUMN. 
ABSOLUTE SCALES OF PHOTOGRAPHIC AND PHOTO- 
visuAL MaGnirupEs.—A great piece of photometric 
work on which the 60-in. reflector of the Mount Wil- 
son Observatory is engaged is the determination of 
absolute scales of photographic and photovisual mag- 
nitudes covering the whole range from brightest to 
faintest known stars. An account of the present posi- 
tion of the investigation has been communicated by 
F. A, Seares to the National Academy of Sciences 
(U.S.A.), vol. i., p. 307, 1915. The method employed 
involves the comparison of two series of images with 
a known variation of intensity between the exposures. 
The many practical difficulties have been successfully 
overcome at Mount Wilson by the use of wire gauze 
screens and circular diaphragms. The photographic 
scale for the intermediate stars (10-18 mags.) was 
first determined in two series of exposures, one set 
of eleven minutes and less, the other thirty to sixty 
minutes; numerous determinations were made. The 
average difference between the mean scales from the 
two series, derived from nine groups of stars between 
10-6 and 16:8, is only oo15 mag. The extension to 
the fainter objects was effected by plates which re- 
ceived two different exposures with the full aperture 
of 60 in., the longer exposures of four to five hours, 
the shorter approximately half an hour. The limiting 
photographic magnitude thus reached was about 20. 
The bright stars (brighter than 10 mag.) were photo- 
graphed with screens or diaphragms interposed pro- 
ducing images comparable with those of stars be- 
tween the tenth and fifteenth magnitude, obtained 
with the same exposure with unreduced aperture. 
The entire series of photographic magnitudes of 617 
objects was reduced to the international zero point 


