JUNE 24, 1915] 
NATURE 
451 

graphic records of all the elements. One occurred 
Between 10.30 and 11.25 p.m. on the declination, the 
magnet swinging east, and the range being 38’. "This 
was preceded at 11.5 p.m. by an increase of H.F. of 
31’. A second Dee was recorded on this element at 
1.45 a.m. on June 18, the force decreasing by 11’. A 
small decrease of V.F. accompanied this movement of 
the H.F. 
The sun’s surface, though disturbed, had been almost 
free from spots between June 5-11. But from June 12 
when a group of spots appez ared i in bright faculze at the 
east limit, and almost on the sun’s equator, the solar 
surface became very active with spots, bright facula, | 
pores, and drifts of granulations. Individually the 
spots were not very large, but on June 17-18 there were 
no fewer than seven groups of spots visible, all dis- 
playing considerable changes of form. In particular 
there were two sy mpathetic groups, one, alre ady re- 
ferred to, extending i an latitude from +1: 5° to: + 4°, and 
in mean “longitude , and the other in latitude — 17° 
and longitude 46°. The whole region of the sun 
between these two groups was very active, the facule 
being visible even at the centre of the disc, with 
streams of granulations connecting the two groups. 
On June 17 the southern group passed the central 
meridian, and the northern group on June 18. The 
heliographic latitude of this northern group was almost 
exactly that of the earth as projected on the sun, so 
that on June 18 the spot group and the earth were 
radially opposite one another. Such a close approxi- 
mation of the position of the spot and the earth 
referred to the sun’s central meridian during a mag- 
netic storm is very unusual. It certainly has not 
occurred in any violent magnetic storm since the year 
1898. A; LL. Cortie. 
Stonyhurst College Observatory, June 20. 
ie} 
JO 
Man’s True Thermal Environment. 
Fottow1ne Dr. Hill’s article on healthy atinospheres 
in Nature of April a letter appeared in NaTUuRE 
of May 6 under the above heading, which suggests 
that too narrow a view has been talsen of this im- 
portant subject. Dr. Milne writes from a place where 
man exists in spite of the climate, and no doubt the 
robustness of the local race is largely due to genera- 
tions of selection under rigorous conditions that are | 
only overcome with the aid of ponderous clothing and 
heated dwellings.. At the outset we should inquire as 
to the thermal conditions that existed at the birth of | 
our race. No doubt man soon learnt to keep himself 
warm by artificial means, but he appeared first in 
association with a fauna almost tropical in character. 
It is in tropical regions that our race exists. to-day in | 
comfort with little or no protection and in spite of 
many adverse organisms that are also favoured by 
warmth. 
What results would Dr. 
zive us in these places? For it is*of importance if 
figures of any value are to be obtained that the 
methods should be generally applicable to habitable 
regions. It is not remarkable that methods bred in 
an extreme climate must fail in quite congenial regions 
but where the air temperature is often over 38° C. 
and sometimes exceeds 45° C. Here, no doubt, Dr. 
Milne’s ingenuity would preduce a metapsuchraino- 
meter to tell us what heat must be taken from a body 
to keep it at blood-heat. We should be the richer for 
a valuable device, but our knowledge of man’s true 
environment would not be much advanced. 
Meteorologists have succeeded very well in obscuring 
the significance of the wet-bulb temperatures by wrap- | 
ping them up in terms of relative humidity. The ‘ 
22 
22, 
Milne’s psuchrainometer 

relation of the dry- and wet-bulb reading, besides ; 
NO. 2382, VOL. 95] 
giving us the potential cooling power of the atmo- 
sphere as it affects a moist surface, enables us to arrive 
at the absolute humidity and the specific heat of the 
air. This last factor no doubt varies considerably 
with the moisture content, and must be of importance 
in the convection affecting the heated body of the 
psuchrainometer. 
Dr. Milne’s ¥ only takes into account the air tem- 
perature, specific heat and velocity, provided radiation 
effects are constant. It cannot be taken to represent 
the whole environmental effect, which depends also 
on the power of the air to take up moisture. The 
katathermometer figures appear most promising in 
this respect, but the present form of instrument is 
probably not complete ly suitable as ee climates 
 W. Grain. 
Khartoum, May 26 
A Continuous Spectrum in the Ultra-Violet. 

Tue following observation may be of interest in 
connection with Prof. E. P. Lewis’s letter in NatuRE 
of June to. 
During some recent experiments which I carried 
out in the Cavendish Laboratory, it was observed that 
the radiation coming from the gas in the path of the 
discharge between a-Wehnelt kathode and an iron 
anode was rich in ultra-violet light. The strength of 
the discharge current was between one and two 
amperes. With air in the bulb and the pressure re- 
duced as low as. possible with a Gerylk pump, the 
spectrum, which was photographed with a small 
Hilger quartz spectrograph, showed the nitrogen 
bands and the mercury line 12536. As the pressure 
was increased by admitting a small quantity of 
hydrogen a continuous spectrum made its appearance, 
the mercury line increasing in intensity relatively to 
the bands. By washing out the bulb several times 
with hydrogen and removing the air by means of 
charcoal and liquid air, a continuous spectrum was 
obtained which showed. no signs of the bands and 
lines. The spectrum extended beyond 2000 and 
gradually faded away, due to the absorption in the 
spectrograph. The pressure of the hydrogen in the 
bulb was about 2 mm. 
It is thought that this continuous. spectrum is the 
result of the bombardment of the hydrogen. molecules 
by slow-moving electrons, the energy of which is not 
sufficient. to produce ionisation in hydrogen. Further 
experiments are necessary to test this idea, and I 
hope to be able to carry them out on my return to 
America. James Barnes. 
The University, Manchester, June 19. 
The Names of Physical Units. 
To all who are interested in the improvement of 
scientific nomenclature the points raised by Dr. Guil- 
laume’s letter (NATURE, June 17, p. 427) are of great 
importance. In my opinion the case for rational 
pomenclaliire has been stated with both logic and 
humour by Dr. Guillaume, while Dr. Harker’s reply 
seems s re misapprehension of the main point. 
All good nomenclature should be unambiguous, and, 
if possible, self-explanatory. The terms masse volu- 
mique, volume massique, and stéradian Nave both 
these desirable qualities; no one with a knowledge: of 
physics and French could make any mistake as to the 
exact meaning of the first two, and the meaning of 
the third should be at once self-evident to anyone 
who knows the definition of a solid angle. I should 
not expect a chemist or a botanist to have anything 
but a hazy idea of the meaning of puissance massique, 
but even to an ordinary French engineer it should 
convey its meaning instantly. 
