a & 
—— ae 
—— 
'our declination magnet, which is of 
JANUARY 22, 1914] 
periments in St. Paul’s, repeated by Desaguliers, as 
described in the ‘‘ Principia,” lib. ii., prop. x1. 
Aristotle is speaking of motion such as of a rain- 
drop or hailstone falling vertically in the air, or of a 
smoke particle up the chimney; also of a_ stone 
dropping in water, or a bubble rising. 
But in ‘‘De Motu Gravium Naturaliter Accelerato,” 
Galileo is discussing the start of such a body from 
rest, while getting up speed, like a steamer or train 
from a station, when the motion is slow enough for 
_ resistance to be insensible, as he verified on the 
Leaning Tower of Pisa, dropping lead weights. 
A train starts from the station with the full Galilean 
acceleration of the net pull of the engine, but as the 
speed and resistance increases the acceleration falls 
off, and finally, at full speed for the most part of the 
journey, Aristotle’s state of motion is attained, and 
the inertia is eliminated, in the language of the 
engineer. d 
Galileo versus Aristotle can be shown off in a 
tumbler of soda-water, where a bubble starts up from 
the bottom with double Galileo’s gravity acceleration, 
but before it reaches the surface the velocity has 
attained very nearly the terminal velocity of Aristotle. 
I hope Capt. Hardcastle will be encouraged to 
devote his learned leisure to the preparation of a 
“Defence of Aristotle’s Dynamics,’ on the lines of 
Duhem’s recent book, ‘‘ Les précurseurs parisiens de 
Galilée.”’ G. GREENHILL. 
1 Staple Inn, W.C., January 14. 
Tungsten Wire Suspensions for Magnetometers. 
Owi1nG to the troublesome changes of zero and 
torsion constant of the silk suspensions of magneto- 
meters, experiments have been made at the Royal 
Observatory, Greenwich, with the view of finding a 
satisfactory substitute. Quartz fibres were first tried, 
but were too rigid in proportion to their tensile 
strength. Success has, however, been obtained with 
tungsten wires such as are used in metallic filament 
electric lamps. These were suggested to us by Mr. 
F. Jacob, of Messrs. Siemens Bros., who kindly ob- 
tained various samples of wire for us; of these a 
tungsten wire of circular section, and diameter 
20 microns, has been adopted as the suspension for 
the ordinary 
Elliott pattern, weighing about 50 grams. This wire, 
about 25 cm. in length, has now been in use for five 
months, during which time its zero has not changed 
within the limits of measurement, i.e. certainly less 
than 10°; the effect of 90° torsion on the wire is to 
turn the magnet through 4’ (it may be noted that a 
thicker wire, of diameter 51 microns, which was also 
tried, gave a deflection of the magnet of more than 
2° for go° torsion). : 
This success encouraged us to try a similar wire for 
the vibration experiment in the determination of abso- 
lute horizontal force, also with satisfactory results. 
The deflection of the magnet for go° of torsion is 
53’, and the zero is constant, 
For determining the moment of inertia of the de- 
flecting magnet the latter wire was too weak, the 
inertia bar doubling the weight carried. A wire of 
diameter 30 microns is therefore used for this purpose, 
in a separate box. The advantage of tungsten wire 
for moment of inertia experiments is that the torsion 
constant does not vary with the weight borne by the 
wire; with silk suspensions this is not so. 
The ends of the. wire are held by simple squeezing, 
the lower end being gripped between grooved metal 
cheeks held together by a screw collar just as pre- 
NO, 2308, VOL. 92] 
NATURE 
585 
viously for the sill fibres. Another device was 
adopted for the top end, consisting of a spring clip 
with a sliding collar; any method involving soldering 
is unsatisfactory. The wire used here can be bought 
for 3d. per foot. S. CHapMan. 
W. W. Bryant. 
Royal Observatory, Greenwich. 
The Pressure of Radiation. 
In his letter of January 1 Prof, Callendar gives his 
reasons for doubting the formula for the pressure of 
radiation as it is usually accepted. He makes use of 
Boltzmann’s proof of the fourth power law for the com- 
plete radiation, extends it to each separate frequency, 
; and deduces that the energy in every frequency ought 
to be proportional to the fourth power of the absolute 
temperature. Since this is known to be untrue he 
concludes : ‘‘ Either Carnot’s principle does not apply, 
or E/v is not equal to 3p for each separate fre- 
quency,” and chooses the latter alternative. But it 
would appear that Prof. Callendar’s use of Carnot’s 
principle is somewhat questionable. For, in order to 
investigate the pressure in an enclosure it is essential 
to alter its volume, and any change of size will bring 
' the Doppler effect into play and cause a small change 
in the frequency of the radiation. If this be taken 
into account, the result leads straight to the displace- 
ment law of Wien—E,=f(AT)/A°—and beyond this 
gives no information. Moreover, a recapitulation of 
Wien’s work with a different law of pressure fails to 
give the displacement law, so that this law must be 
abandoned, if the pressure formula is to be altered. 
Prof. Callendar wishes to change the pressure for- 
mula in the hope of accounting for the observed radia- 
tion curve without making an open breach with our 
present electromagnetic theory. In his paper in the 
October Philosophical Magazine he extends his con- 
ception of caloric from matter to «ther, and obtains 
a formula which fits the radiation curve as well as 
Planck’s. However, his work involves a certain con- 
stant, b, the nature of which he does not discuss very 
fully, and this constant appears to be identical with 
h/k in Planck’s theory, so that ‘‘ molecules of caloric” 
are very closely related to Planck’s quanta. Thus 
the work, which has established that the electro- 
magnetic equations lead inevitably to Rayleigh’s for- 
mula, proves also that according to those equations b 
should vanish; in fact, that in any finite region of the 
zether there ought to be an infinite number of mole- 
cules of caloric. If my reading of his paper is cor- 
rect, it would appear that in extending the caloric idea 
to the zther Prof. Callendar has invented a new and 
helpful way of regarding Planck’s quantum hypo- 
thesis. C. G. Darwin. 
The University, Manchester. 
** Atmospherics ’’ in Wireless Telegraphy. 
Wit reference to Prof. Perry’s interesting letter 
on ‘‘atmospherics’ in Nature of January 8, the fol- 
lowing experience may be of interest. 
Whilst at my instruments on December 12, 1913, 
I was tuning in the Eiffel Tower signals to read the 
7 a.m. press news when the atmospheric disturbances 
became so great that Paris was entirely unreadable, 
the phenomenon continuing for fifteen minutes with- 
out cessation. The aérial was only 35 ft. high, and 
sheltered by other buildings. 
RecinatD F. Durrant. 
121 Broadway, Cricklewood, N.W., 
January ty. 
