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Progress in Science . [July 
accumulate until it has sufficient tension to leap over a resisting medium, and 
that, as the thickness of this resisting medium increases, the spark obtained 
becomes greater. Evidently, on these occasions, the charge has time to 
att r adl unlike and repel like electricity in the core, and if a conductor in con- 
nection with the earth be presented to this core the like electricity will escape : 
hence a spark will result. As soon, however, as the tension has become suffi- 
cient to leap over the layer of air, it will be necessary to restore equilibrium 
in the core. Hence there will be a return spark in the opposite direction. 
From these experiments Mr. Thompson concludes that the phenomena observed 
may be explained by the ordinary laws of induction. 
On June ioth Mr. W. J. Wilson explained a reflecting tangent galvanometer, 
which he has recently designed, for the purpose of exhibiting the indications 
of the instrument to an audience, and so arranged that the divisions on the 
scale show, without calculation, the relative strengths of different currents. 
The beam of light, after passing through a small orifice traversed by cross 
wires, is reflected vertically by a fixed mirror ; the ray then passes through a 
lens, and is again reflected from a small plane mirror parallel to the first, which 
is rigidly fixed below a small magnetic needle. By this means the ray be- 
comes again horizontal, and, since the light now falls on the second mirror 
always at the same angle, the extent of motion of the ray is identical with 
that of the needle, and, if the scale be one of equal parts placed in the mag- 
netic meridian, the indications on it will be proportional to the tangents of the 
angles, and, therefore, to the strengths of the currents. The needle and mirror 
are suspended by a silk fibre, and a bent strip of aluminium, the ends of which 
dip into water in an annular trough, is attached to the needle in order to check 
its oscillations. A series of observations, taken with varying resistances 
introduced into the current, showed that the indications are very reliable. 
Prof. G., Fuller, C.E., described his “EleCtric Multiplier,” an instrument 
which may be looked upon as an automatic eleCtrophorus. An insulated plate 
of vulcanite is supported in a vertical position, and on each side of it is an 
insulated metallic plate, and these can be moved together to and from the 
vulcanite by rotating a handle. When these plates are far apart, two me- 
tallic arms, provided with points, are made to pass one on each side of the 
vulcanite plate. One of these is insulated, and is provided with a rod termi- 
nating in a knob, which at a certain point in its path almost touches the 
metallic plate on the opposite side of the sheet of vulcanite. The other arm 
is in connexion with the earth. The adtion of the instrument is as follows: — 
A charge of, say, negative eledtricity having been given to the insulated arm, 
it is passed over the face of the vulcanite, while positive is drawn up from the 
earth and thrown upon the opposite face by the uninsulated series of points. 
These arms are then removed, and the two metallic plates are brought into 
contadt with the vulcanite. Call the side of the plate charged with negative 
eledtricity A, and the other B. The negative of A induces positive on the 
near face of its metallic plate, and repels the negative. This passes, by a strip 
of tin-foil joining the two faces of the vulcanite, to the other metallic plate, 
neutralising its free positive ; and when the plates are moved away from the 
vulcanite, that from A is charged with positive, and that from B with negative. 
Before reaching its extreme position, this latter communicates its charge to 
the insulated arm by the brass knob, and the eledtricity is then distributed 
over the face A. At the end of its path, B is momentarily connedled to earth. 
It will be evident that the effedt of again bringing the plates in contadt is to 
increase the charge of positive eledtricity on the metallic plate opposite the 
face A. With the small model exhibited Prof. Fuller has frequently obtained 
sparks an inch in length. 
Prof. Guthrie exhibited Prof. Mach’s apparatus for sound reflexion, which is 
one of an interesting series of appliances designed by him for the demon- 
stration of certain fundamental principles in physics. It consists of a mathe- 
matically exadt elliptical tray, highly polished, and provided with a close-fitting 
glass cover. The tray is covered with pulverised dry silicic acid, and a Leyden 
jar frequently discharged between two small knobs at one of the foci, when 
the silicic acid arranges itself in fine curves around the other focus. 
