APRIL 21, 1923] 
by means of his equation given at p. 773 of NATURE 
ob December 9, 1922. 
Pressure of Fluidity. 

Kilos per sq. cm, 
1,080 
5,000 
19,200 
33,600 

__I have recently (with the generous aid of Mr. 
_R. H. H. Stanger of the Broadway Testing Labora- 
tories, and the following firms who prepared and 
presented the necessary three specimens of each 
metal) determined the pressures of fluidity of several 
metals by direct experiment, so it will be interesting 
to compare the results, remembering of course that 
the specimens were not made from the same piece of 
metal as those used by Mr. O’Neill. In the case of 
my tests the three specimens of each metal were made 
from the same piece. 
The British Aluminium Co. Ltd. supplied the 
specimens of aluminium. 
Messrs. David Colville and Co. Ltd. supplied the 
Specimens of mild steel. 
Messrs. Dewrance and Co. supplied the specimens of 
tin, lead, and zinc. 
The Elliott’s Metal Co. Ltd. supplied the specimens 
of copper. 
The Muntz’s Metal Co. Ltd. supplied the specimens 
of Muntz’s metal. 
The experiments were made not merely to determine 
the pressures of fluidity, but also to test an hypothesis 
to account for the phenomenon of pressure of fluidity. 
This hypothesis is far too long to reproduce here, but 
it will be found in the Transactions of the Society of 
_ Engineers for the quarter January-March 1923. It 
connects the pressure of fluidity with the ultimate 
shearing and tensile strength of the metal, and was 
devised in connexion with experiments with clay, 
and then found to apply to plastic metals as well. 
If p be the pressure of fluidity in kilos per sq. cm., 
f be the shearing stress in kilos per sq. cm., 
c be the ultimate tensile strength in kilos per 
ve 
sq. cm., 
_ then the hypothesis shows rationally on the assump- 
tions made that 
p=3°68c +5:21f. (1) 
The pressures of fluidity were determined by means 
of cylindrical specimens 70mm. in diameter and 70 mm. 
high, using a flat-nosed punch to mm. in diameter at 
the end and reduced in the shank to 9 mm. so as to 
clear the sides of the hole. 








Tensile | Sheari Pressure of| ” the calcu- ws 
Metal. Strength | Strengt Fluidity lated Value of | —*x100. 
« Ts p. fp. PA 
ER Aes II4'5 125 777 1,072 +27°5 
Lead-tin alloy 244°0 156 1,233 1,706 +277 
SAR 223 232 1,367 2,025 +325 
Aluminium 827 577 4,015 6,045 +33°6 
Copper . .| 2192 1445 10,860 15,590 + 3o'r 
Muntz’s metal | 3686 2004 | (16,800)* 23,9606 ————| 
Mild steel 4380 2990 | (22,140)* 31,625 Mean + 30°3 
Zime . 2r4 755 [7,760] 4,707 

All stresses are in kilograms per sq. cm. 
* These are not experimental values, but merely predictions, 
The relation given by equation (1) thus on the 
average gives results which need reducing by 30 per 
cent. to arrive at the actual values, and the maximum 
_ departure from this mean is 3:3 (aluminium). 
NO. 2790, VOL. IIT] 
NATURE 
535 
Zinc is a rank outsider as regards this hypothesis ! 
But zinc has no plasticity. It did not elongate or 
show any contraction of area under a tensile force. 
In shear even it failed by tension, and when the 
pressure of fluidity experiment was made, the 
specimen gradually burst by yielding in tension on 
several vertical planes. 
With regard to the variation of the figures in the 
last column, it must be remembered that these 
depend on the experimental values of f and c, which 
themselves vary. For example, in the case of the 
shearing tests, two experiments were made with each 
metal, the planes of shear being about one inch apart 
on the same specimen. For all this the values of f 
differed by 4:3 per cent. and 5°5 per cent. in the cases 
of tin and aluminium respectively. 
A. S. E. ACKERMANN. 
17 Victoria Street, Westminster, S.W.1, 
March 31. 

Use of the Millibar in Aerodynamics. 
THE millibar, introduced by Sir Napier Shaw into 
British meteorology, brings the same drastic simplifi- 
cation into the numerical relations between pressure 
and velocity in aeronautics. 
The accompanying diagram (Fig. 1) shows the 
t | 
oF 
MILLIBARS 
t 

Fic. 1. 
pressure distribution round a wing profile, calculated 
in accordance with Joukowsky’s theory. 
In C.G.S. units p—po= 4p. (v9?—v*) dynes/cm.? or 
microbars, where ~, v are the variable pressure and 
velocity at points on the profile, p», vp the values at a 
distance, and p the density of the air. 
Expressing p and v in M., Kg., S. units, which are 
more convenient for aeronautical measurements, 
pressure = $p. (vo?— v*) m.kg.s.-*m.-? 
4p . (v,? . 10-?— v®. r0-*)mb. 
=4p.10. (U9?, 10-*— v®.. 10-*) megadynes/m.*. 
The last two forms lend themselves to computation, 
since flying speeds usually lie between 10 m./s. and 100 
m./s. The absence of all extraneous factors save 
integral powers of ten is sufficient proof of the practi- 
cality of Sir Napier Shaw’s action. 
In the minority of cases where the forces considered 
are produced by the action of gravity on known 
masses, they are easily transformed, for the mega- 
dyne is 10/9'81 =1-02 kgm. weight, and the millibar is 
1000/981 =1-02 cm. head of water with an accuracy 
amply sufficient for aeronautical measurements. 
A. R. Low. 
London, March 22. 
