458 
NATURE 
[uLy 2,° roe4 
in which the strongest immigration occurred 
under such conditions the period of the chief 
movement was protracted to about nineteen 
(instead of about nine) days. 
Dr. Defant has clearly proved that a certain 
type of weather is peculiarly favourable for the 
spring immigration of Austrian birds. He is to 
be congratulated on his clear and well reasoned 
treatment of very unwieldy data and on having 
made a valuable contribution to a difficult subject. 
His further speculations are, however, open to 
serious criticism. He believes that the important 
factor in this type of weather is the wind and that 
birds prefer to fly with it behind them. His 
reasons as to why birds should do so seem to us 
to be wholly beside the point, and as the much 
more comprehensive results already obtained in 
this country are entirely opposed to Dr. Defant’s 
theory, we cannot accept it, however temptingly 
obvious it may seem, on such very slender 
grounds. 
The alternative theory is that “the winds and 
the performance, or non-performance, of the 
migratory movements are the effects of a common 
cause—namely, the particular type of weather 
prevailing at the time, which may be favourable 
or unfavourable for the flight of birds. 2 
(Eagle Clarke, “Studies in Bird Migration,” 1912, 
Pitt 78) eau 
In the British area there are certain types of 
weather favourable respectively for migration 
between the British Isles and northern Europe, 
between the British Isles and Iceland, and be- 
tween the British Isles and south-western Europe. 
The winds accompanying these types of weather 
may or may not be in the same direction as the 
movements concerned. Jurthermore, the same 
types of weather favour these movements both in 
autumn and in spring, the direction of flight being 
reversed, while the prevailing winds remain the 
same. 
In Dr. Defant’s simpler case it so happens that 
the favourable type of weather he has discovered 
produces a wind in the same direction as that of 
immigration from the Balkan Peninsula (his 
region “A”’) to Austria; but the immigration from 
the south-west (““B”’) probably forms a large part 
of the movements which were the subject of in- 
vestigation. It is unfortunate that only the spring 
migrations have been dealt with. Should the 
same type of weather with its accompanying 
winds prove to be favourable to the autumn emi- 
gration (as in the case of the British movements), 
Dr. Defant’s theory would be quite untenable. 
In the meantime, Dr. Defant’s selection of wind 
as the important factor is purely speculative, and 
his view entails an entirely different relation be- 
tween birds and weather im Austria from that 
existing in the British Isles. While rejecting his 
theory of the importance of wind as unproven and 
improbable, we feel grateful for the. new facts 
which he has added to our knowledge of bird 
migration by his most laborious and thorough 
research. A. LANDSBOROUGH THOMSON. 
NO. 1; WOLSIQ3) 
922 
—0)ro) 
METROLOGICAL RESEARCHES.} 
(eke volumes referred to below contain par- 
-ticulars of recent metrological researches 
made at the international bureau of weights and 
measures. As the result of a long series of in- 
vestigations on the length of invar wires used for 
geodetical measurements, it has been found that 
wires made of metal taken from the same tapping 
or ladle and treated in the same manner have 
practically the sarne coefficients of expansion; 
the differences from the mean lying . within 
+o°03 x 107% If the same coefficient of expan- 
sion were adopted for all such wires it would need 
a difference of temperature of 30°C. to introduce 
an error of one part in a million. There is now 
no difficulty in procuring invar wires having a 
coefficient of expansion as small as o'r x 107°, 
At the fifth general conference on weights and 
measures held in Paris in October last, a resolu- 
tion was passed to the effect that in view of the 
fact that the force of gravity is not precisely the 
same at sea-level for all places having the same 
latitude, it was undesirable that the value adopted 
for the normal force of gravity (viz., 980°665 
em. sec.~*) should continue to be defined as that 
corresponding to a particular latitude (45°). In 
the reduction of observations the theoretical factor 
given by Clairaut’s formula in.the amended form 
now. usually adopted should no longer be em- 
ployed, but merely the numerical ratio of the 
normal force of gravity to that,at the place of 
observation, the latter being determined directly 
if possible. 
The normal scale of temperature hitherto 
adopted at the international bureau of weights 
and measures has been that of hydrogen at con- 
stant volume. The fifth general conference re- 
solved that the absolute thermodynamic scale 
shall be substituted for the hydrogen scale as soon 
as the table of reduction from one scale to the 
other has been determined with sufficient cer- 
tainty. It was also recommended that a number 
of thermometric fixed points be ascertained with 
as great accuracy as possible, in order to facilitate 
the calibration of thermometers. A meeting of 
the principals of the various national laboratories 
is to be arranged at Sévres for the purpose of 
deciding what these fixed points shall be and how 
they are to be determined, as well as to promote 
their general recognition. 
The question of the determination of the length 
of the metre in terms of wave-lengths of light 
was considered at the fourth general conference. 
It was decided that investigation on this subject 
had not then reached the stage for the conference 
to adopt any particular number of wave- -lengths 
as representing the metre. Further researches 
made by physicists will be carefully studied at 
the international bureau with the view of ob- 
taining in the course of time a fundamental 
1 Comité internationale des poids et mesures. 
Deuxiéme série, tome vii., session de 1913. 
Villars, 1913.) 
Travaux et Mémoires da bureau international des poids et mesures: 
Tome xv. (Paris: Gauthier-Villars, 1913.) 
Procés-verbaux des séances® 
Pp. v+140. (Paris : Gauthier- 
