SCIENTIFIC SUMMARY. 
283 
After working out the complete theory as to light, on the assumption 
that the displacement currents are rotated as well as the conducted, he finds 
the result very satisfactory. He thinks there are strong grounds for sup- 
posing the two phenomena to he the same. 
A new form of Siren is described by Lord Rayleigh in the Philosophical 
Magazine. Some years ago he observed that a light pivoted blade is set in 
rapid rotation when exposed to wind. The phenomenon is of the same char- 
acter as the rotation of a slip of paper falling freely in air, which was dis- 
cussed in 1854 by Professor Maxwell. In both cases, the rotation may 
occur in either direction, proving that its cause is not to be looked for in any 
want of symmetry. The present is an application of a principle, the ex- 
planation of which he thinks has yet to be discovered. A blade is cut out of 
sheet brass, and provided with sharp projecting points, bearing in hollows at 
the end of two set screws, adjusted till the blade can turn in a small 
wooden frame freely, but without shake. Pieces of cardboard, or metal, 
are made to fit the blade pretty closely, so that when all are in one 
plane, the aperture is almost closed. The blade, on turning, acts as a revolv- 
ing stopcock. He has made several sirens on this plan, which perform well. 
If the wind from the bellows is admitted symmetrically, they will revolve in 
either direction, and soon acquire sufficient speed to give a note of moderate 
pitch. The position of maximum obstruction is, for small displacements, one 
of stable equilibrium. If a larger displacement is made, the vibration tends 
of itself to increase up to a certain point, or even to pass into continuous 
rotation ; but the precise behaviour in this respect probably depends on the 
details of construction. 
In the same paper he also describes experiments for demonstrating — 
The Acoustical Shadow of a Circular Disk. — In Poisson’s experiment, a 
bright point is observed in the centre of the shadow of a circular disk, on 
which waves of light are directly incident. To obtain the acoustic analogue 
of this phenomenon, it is advisable to use sounds of very high pitch, which 
have the advantage of readily exciting sensitive flames. The best results 
were obtained with a squeaky toy-reed. A bird-call was blown, with a 
pressure of four inches of water, and placed about twenty inches from a disk 
of fifteen inches diameter. The observation was made at a distance of 
twenty-four inches on the other side of the disk, and succeeded, both with 
the ear and with a sensitive flame. In the former case a plate of wood, 
bored with a hole about ^-in. diameter, was held against the side of the head 
so that the hole was opposite the ear-passage. The head was moved until 
the position of maximum sound was determined. To verify the fact that the 
position of maximum sound was really at the centre of the shadow, a hole 
bored through the centre of the disk was closed with a cork during obser- 
vation. This was afterwards removed, and then the eye could see the source 
of sound through the two holes. The most suitable flame was that from a 
pin-hole burner, brought near to flaring-point by gas pressure of about ten 
inches. In a subsequent experiment, a toy-reed was substituted for the bird- 
call, and answered better with the flame than with the ear. 
The Electromotive Forces in free jets of Water have been studied by 
Julius Elster, under the guidance of Professor Quincke. He claims as 
results : — 
