
The Principles of Aerodynamics. 667 
temperature, the weight of one litre of a hypothetical mercury 
vapour having a vapour-pressure measured by one millimetre 
of mercury, and the actual pressure corresponding to the 
weights of vapour observed. There is added a series cf 
values computed from the formula 
p=ab*, in which log a=4°6064, and log b=0°02856, 
which agrees sufficiently well with the observations. 





Tempera- Weight of one | Weight of one | Vapour- Vapour- 
By tare litre of mercury litre of mercury) _ pressure, pressure, 
vapour. vapour at lmm.| _—_ observed. computed. 
Bo mg. | mg. / mm. mm. 
0 so ; eae | sae 0-0004 
| 10 | = ) is oes 00008 
| > | ce | Si ) 0-0010 00012 
2 | a ) oti eh 00015 
| 30 0-628 ) 10°60 00027 9-005 
a | 0054 10°26 0-0052 0-006 
| 50 | 0-112 G4 TT PESO UIs 0-011 
> | 0°206 9°65 : 0-0214 0021 
) 
7 ) 0378 | 9°37 ) 0-0404 0:040 

It will be observed that the formule of Hertz and of 
Ramsay and Young give values comparing well with the 
results of actual measurements, except Hertz’s values for 0° 
and 10°, which are sensibly too small, though the difference 
is expressed in ten-thousandths of a millimetre. The values 
given by van der Plaats are ten times as large as my own ; 
the latter have now been found the same, in experiments 
made in three different years and with many modifications of 
apparatus. 


LXXIV. On the Principles of Aerodynamics and their Appli- 
cation, by the Method of Dynamical Similarity, to some 
special Problems. By Prof. M. SmoLucHowskI-SMOLAN, 
PhD., LLD., University, Lemberg*. 
oe HAT is the characteristic feature of aerodynamics, 
contrasting it with hydrodynamics ? Compressi- 
bility, of course ; but itis not mere isothermic compressibility 
which makes up the difference: there comes in, in general, as 
a factor of equal order of magnitude, variation of temperature, 
as produced by the motion of the gas. 
To appreciate its importance, consider, for example, the 
difference between Newton’s and Laplace’s formula for the 
* Communicated by the Author. 
