156 PHYSICS. 
on the mirror, which must be inclined at an angle of 45°, when the balls 
and sparks are visible to him. During a rapid revolution of the mirrors, 
the sparks appear elongated, the middle ones somewhat displaced towards 
the external ones. From the amount of this displacement, the rapidity with 
which the mirror was rotated, &c., Wheatstone calculated that the electric 
current traversed 288,000 miles in a second, light moving at the rate of 
192,000 miles in the same time. 
Several interesting experiments may be performed by means of the 
current of air which sets off from a point discharging electricity. One of 
these is illustrated by fig. 76, pl. 20. A pointed rod, cp, is fixed in the top 
of a prime conductor, and upon its upper extremities is balanced a 
horizontal wire, tt, with the two points bent in opposite directions in a 
horizontal plane. The re-action of the air from the points causes a rapid 
rotation of the wire in an opposite direction. (By mistake of the Engraver 
two figures are numbered 76. The one here referred to stands nearly in 
the centre of the plate.) 
Electricity may be developed in other ways than by simple friction. 
Thus it may be produced by pressure, as by pressing a plate of metal with 
an insulating handle, on a piece of oiled silk; on removing it after a few 
minutes, the former will be negatively and the latter positively electrified. 
A slight pressure will develope it likewise in calcareous spar, topaz, fluor 
spar, &c., which will be sensible for several days. It is produced also by 
heat, as in the case of the tourmaline. Heat this mineral, and one end will 
become positively electrified and the other negatively, the same polar 
condition being presented in the fragments, just like the fragments of a 
single magnet. The limits of temperature between which this electricity 
is exhibited in the tourmaline are 50° and 300° F. On cooling a tourmaline 
thus treated, the electricity disappears for a time, then re-appears, but with 
inverted poles, and remains until the temperature sinks below 50° F. 
C. GaLVANISM. 
1. Development of Electricity by Contact. 
Electricity may be developed, not only by friction and the other methods 
just mentioned, but also by the contact of different bodies. This kind of 
electricity has been called Galvanism, from its discoverer, Galvani of 
Bologna. It is exhibited, however, only in the case of very good conductors, 
metals for instance. If two different metals are connected by the nerves 
of certain muscles, sudden convulsions are produced in the latter when the 
two metals are brought into contact. This experiment was first performed 
by Galvani on the prepared legs of frogs, and the contortions were 
supposed to result from the existence of certain currents of an animal 
electricity. His countryman, Volta, first showed that it was common elec- 
tricity which caused the phenomenon. Pl. 20, fig. 47, represents the 
experiment instituted by him, namely, a pair of frog’s legs, connected 
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