a 
SECTIONAL TRANSACTIONS.—A. 367 
molecule in the free state. A brief account will be given of the application of 
this method to the problem of the isotrimorphism of the tervalent metallic 
acetylacetones and the structure of the acetylacetene group. 
(f) Mr. S. H. Pirer.—X-ray Crystallographic Methods as an Aid 
to Chemical Research. 
(g) Prof. W. L. Brace, F.R.S.—Eahibit of Models illustrating 
Crystal Structure. 
27. Prof. V. Bserxnes.—Lecture on The Forces which Lift Aero- 
planes. (Illustrated by experiments.) 
Now that aviation has become a reality much work is being expended upon 
the investigation of the hydrodynamic forces which make flying possible. The 
result has in one respect been surprising : these forces are seen to belong to an 
extensive class which was investigated long before the period of aviation in 
connection with a totally different problem, that of action-at-a-distance. 
After Newton’s discovery of universal gravitation, action-at-a-distance was 
for two centuries the leading idea of natural philosophy. But a decided change 
. of our ideas came about thirty years ago, after a development connected with 
the names of Faraday, Maxwell, and Hertz. In the period before this reversal 
had taken place C. A. Bjerknes, the father of the lecturer, found, first by 
mathematical investigations and later by experiments, that spherical and 
cylindrical bodies moving in a fluid exert apparent action-at-a-distance upon 
each other. These actions form part of a remarkable analogy which exists 
between hydrodynamic and electromagnetic phenomena. After the theory had 
been generalised by the lecturer, and made independent of every special sup- 
position concerning the form of the bodies, this analogy may now be stated 
tkus : . 
1. Regarding geometric structures full identity can be established between 
eee cegmamic fields of motion and static or stationary electromagnetic fields 
of force. 
2. The mechanical forces which act in corresponding hydrodynamic and 
electromagnetic fields are oppositely equal to each other. 
This direct geometric and inverse dynamic analogy can be illustrated by 
striking experiments. Regarding the apparent actions at a distance, the experi- 
ments will show the following phenomena, the opposite sign of the forces being 
tacitly understood : pulsating bodies act upon each other as if they were electri- 
fied, oscillating bodies as if they were magnets; neutral bodies take induced 
oscillations from the fluid and become subject to the same forces as iron and 
bismuth ; rotating cylinders act upon each other like electric currents. 
The forces which act upon the rotating cylinder are the same as those that 
carry the wing of an aeroplane. Theoretically the simplest aeroplane should 
have rotating cylinders instead of rigid wings. The lift depends upon the 
circulation of the air round the wing. We produce and regulate at will the 
required circulation by rotating the cylinders, while in the case of the common 
rigid wings we get it by a spontaneous process. 
Cosmican Puysics SUB-SECTION. 
28. Dr. J. S. Owens.—The Automatic Measurement of Atmospheric 
Pollution. 
Refers especially to results of the automatic recorder designed by the author 
_ for the Advisory Committee on Atmospheric Pollution. The function of this 
is to measure the pollution of city air by smoke. A short description and 
references to fuller descriptions are given. The results obtained in London 
_ by this apparatus are compared with those of the author’s dust counter (Proc. 
_ Roy. Soc. A., Vol. 101, 1922) and show a good correspondence. Curves obtained 
_ by both methods in investigating the effect of suspended matter on obstruction 
of light are given; the relation between obstruction and dust content is shown 
