i 
OcTOBER 23, 1913] 
NATURE 
433 
type. The tables published by Lockyer and Pickering 
give no indications of lines in the positions calculated, 
but further examination of the photographs is highly 
desirable. 
It should be noted in conclusion that Dr. Bohr’s 
theory has not yet been shown to be capable of explain- 
ing the ordinary series of helium lines. 
A. Fow er. 
Imperial College, South Kensington, October 14. 
Azolla in Norfolk. 
A very interesting case of the rapid spread of an 
introduced species is afforded by Azolla caroliniana, a 
North American species. So far as the Norfolk 
Broads are concerned, this free-floating water-fern has 
hitherto been confined to a single ditch or “dyke” 
near Horning Ferry, on the river Bure. Here the 
plant flourished greatly, covering the entire surface, 
but owing to the isolation of the “dyke” was pre- 
vented from spreading. According to an inhabitant 
of the neighbouring village of Ranworth, the plant 
has been observed in this one spot for the past fifteen 
years. I have no evidence as to its original intro- 
duction. The disastrous floods of August, 1912, car- 
ried some of the plant into the Bure, and its increase 
during the past twelve months has been extraordinary. 
Distributed by the tide it is now abundant in several 
of the Broads, and is carried by the tide in large 
quantities along the Bure and its tributaries, the 
Thurne and the Ant. It has found the still waters of 
South Walsham and Ranworth Broads particularly 
suited to its needs. It is most partial to the reed 
swamps of Typha angustifolia, so characteristic of the 
borders of our fen-lakes, and with this protection it 
is seen in large crowded expanses. More and more 
of the marsh and fen “dykes” are being invaded. 
It seems probable that the spread of the species to 
the other rivers of the Norfolk system, the Yare and 
the Waveney, will be prevented by the brackish nature 
of the water below Acle Bridge. I understand that 
a hard winter would probably kill the plant off, but its 
abundant sporocarps would carry it over to the suc- 
ceeding spring. 
Undoubtedly ecologists will soon find it necessary 
to include Azolla caroliniana in the local open reed- 
swamp association as a subdominant. It is a highly 
ornamental plant, being pale green in spring, and 
exhibiting a hundred shades of brown and red in 
autumn, 
It would be interesting to know the result of com- 
petition between Azolla and members of the 
Lemnacez, and I am at present carrying out experi- 
ments to test this point. W. E. Patmer. 
Great Yarmouth. 
The Theory of Radiation. 
I owe Prot. Nicholson an apology. 
of course, earlier than Dr. Bohr’s, and is actually 
cited by the latter. The wording of my letter 
(Nature, October 9) implies the reverse. 
S. B. McLaren. 
University College, Reading, October 18. 
His work is, 
RESEARCH IN AERODYNAMICS. 
HE fourth volume of researches from the 
Institut Aerodynamique de Koutchino covers 
the period 1910-1912, and deals mainly with deter- 
minations of the air-resistance of various bodies 
and with comparisons between the results obtained 
at Koutchino with those of observers elsewhere. 
A change in the standard temperature correspond- 
NO. 2295, VOL. 92] 
ing to the published results has been made since 
the publication of the three earlier volumes, the 
later determinations being referred to Tee 
instead of o° C. to bring the results to a form more 
easily comparable with those of other experimental 
establishments. 
An examination into the velocity standard of the 
institution has been carried out, the ultimate 
standard being the movement of the end of a 
whirling arm 16 ft. in radius. Three independent 
methods of estimating and correcting for the 
motion of the air in the room were used prior to 
the calibration of various anemometers on the 
whirling arm. The anemometers were divided into 
two groups, the first containing ‘vane instru- 
ments ” and the second “ pressure tubes.” 
It was found, when the anemometers were trans- 
ferred to the wind-channel of the laboratory, that 
the vane type of anemometer gave somewhat lower 
results than the pressure tubes, and it was con- 
cluded that the latter were more trustworthy, since 
the centrifugal effect of whirling on the vane instru- 
ments might easily account for the differences 
found. 
Using the new calibration of the air-channel 
resulting from these experiments, a series of deter- 
minations of the resistance of square plates normal 
to the air-current was made. The plates were 
12°5, 25, and 50 millimetres side, and the values of 
the absolute coefficient of resistance are given as 
0'58, 0°57, and 0°57 respectively. This is some- 
what higher than the value hitherto accepted for 
plates of this size, and is more nearly equal to 
that previously given for plates of from 300 to 500 
millimetres. 
The same plates were also tested at inclinations 
to the air-current, the curves obtained for the 
normal force showing the well-known maximum 
at an inclination of about 35°. 
Amongst the theoretical investigations is one 
entitled, ‘“‘ Méthode des variables de dimension zéro 
et son application en Aerodynamique.” Reference 
is made to papers by Lord Rayleigh and others, 
but, curiously enough, there does not seem to be 
any indication throughout the paper that the 
author considers the method to have any further 
importance than that of convenience. Approached 
from another point of view, the method of no- 
| dimensional variables arises directly from the 
principles of dynamical similarity, and is only one 
of the many uses of the laws governing similar 
motions. The importance of the physical mean- 
ing behind the mathematics appears to have been 
| overlooked. 
In the articles in this volume which deal with 
comparisons with other observatories it is con- 
_ cluded that the type of channel having enclosing 
walls is preferable to that of Eiffel, and that the 
channel used at Géttingen is more steady than 
that at Koutchino. 
An attempt was made to repeat an experiment 
by Rateau on a discontinuity in the centre of 
pressure variations of an inclined plate. Between 
inclinations of 25° and 50° Rateau found a sudden 
; change, whilst at Koutchino a continuous and well- 
