SCIENTIFIC SUMMARY. 
379 
your hat.” We have found that articulate speech can he reproduced by the 
oxy hydrogen light, and even by the light of a kerosene lamp. The loudest 
effects obtained from light are produced by rapidly interrupting the beam by 
the perforated disc. The great advantage of this form of apparatus for 
experimental work is the noiselessness of its rotation, admitting the close 
approach of the receiver without interfering with the audibility of the effect 
heard from the latter; for it will be understood that musical tones are 
emitted from the receiver when no sound is made at the transmitter. A 
silent motion thus produces a sound. In this way musical tones have been 
heard even from the light of a candle. When distant effects are sought, 
another apparatus is used. By placing an opaque screen near the rotating 
disc the beam can be entirely cut off by a slight motion of the hand, and 
musical signals, like the dots and dashes of the Morse telegraph code, can 
thus be produced at the distant receiving station.’ 
The Action of Hollow, as compared with Solid , Steel Magnets, is considered 
by Herr Holtz in Wiedemann’s Annalen. He had already come to the con- 
clusion that solid bars do not form good permanent magnets, because the 
core, or central part, absorbs much of the magnetizing force, and because it is 
equivalent to an armature joining the two poles. Tubes can be forged out of 
sheet steel, and need not be welded at the junction. A rod and such a tube, 
12^ centims. long and 13 millims. in diameter, were compared, the thickness of 
the tube-wall being If millims. They were magnetized to saturation, and 
the magnetism of the rod was found to bear the ratio to that of the tube of 
1 : 1*6. When the rod and tube were 32 centims. in length and 35 millims. in 
diameter, the difference was as 1 : 1*5. When the tube was filled with a core of 
soft iron, it hardly retained enough magnetism to obey the directive action of 
the Earth. After six months of rest, the same magnets were again com- 
pared, with the result that in the larger pair the solid magnet retained 1, 
the hollow 2*5. In the smaller pair, the rod held 1, the tube 2*9. He pro- 
mises to make further observations on the subject. 
An Electro-dynamical paradox is described by M. Gerard-Lescuyer in the 
Comptes Rendus of J uly. If a current produced by a dynamo-electrical be 
sent into a magneto-electrical machine, the latter begins to move, but after a 
time it suddenly slackens, stops, and starts in the opposite direction. This 
reciprocating motion lasts as long as the producing current. The motive 
current must evidently change in direction, — a fact proved by introducing a 
galvanometer into the circuit. This occurs even without change in velocity 
of the generating machine. If it be supposed that the receiving machine 
receives a periodical increase of velocity, it would, by virtue of this, give rise 
to a current of its own, which would traverse the motor instrument in an 
opposite direction to that emanating from it, and reverse the polarity of the 
inductors. This is shown to be the case by applying a brake to the receiver 
sufficient to prevent its increase of velocity : the phenomenon does not then 
take place. With inductors of cast iron, as in the Gramme machine, the ex- 
periment is less certain, owing, as he thinks, to the residual magnetism of 
the cast-iron, which offers resistance to the reversal of current. Soft iron 
inductors, on the other hand, permit the action at the first attempt. 
The Earth's Rotation was demonstrated by means of the pendulum by 
Leon Foucault in February 1851. He was permitted to hang a bob of 28 
