When we

a finger to the conductors in a dark room. bring the finger near enough to the conductor we obtain a spark, and it is this phenomenon, and not the brush discharges, that we perceive in lower latitudes in the case of an ordinary thunderstorm.

The work done in these so-called silent discharges, like the aurora or in the ordinary brush discharge, is small compared with that done by the lightning discharge, as we can see roughly by the differences in the intensity of the light produced. I have tested this question of difference of work in the following manner: A Leyden jar, with its outer coating slit so as to produce alternate spaces of tin foil and glass, was charged to a sufficient degree to produce a spark between terminals a fixed distance apart. The spark was examined by a revolving mirror, and the number of oscillations or surgings to and fro was noted. At each discharge between the spark terminals a brush discharge occurred between the slits in the coating of the jar. When the jar was placed in oil this brush discharge ceased, but no essential diminution could be perceived in the energy manifested in the spark. The number of oscillations were the same, and the duration of the spark was not apparently modified.

CHAPTER XVII.

WAVE MOTION.

OUR study of electricity leads us now to the general subject of wave motion, which up to the time of the laying of the Atlantic cable seemed to be very little in touch with practical life. It was a subject for mathematicians and the natural philosophers, and it seemed to have no commercial importance. In signalling, however, through the cable the practical man was speedily confronted with problems of wave motion, and with the invention of the telephone the study of wave motion became instantly of importance to the practical electrician. The progress of electricity is steadily in the direction of the economical production of wave motion.

"By a wave is understood a state of disturbances which is propagated from one part of a medium to another." Energy pauses, and not matter. Waves are free or forced. An example of a free wave is afforded by that of the wave running into the Bay of Fundy, which is almost free from the influence of the sun or moon; while the ocean tide is a forced wave, since it depends upon the continued action of the moon and

sun.

It has been computed that waves on the ocean of about three hundred feet long travel at the rate of

nearly forty feet per second, or twenty-seven miles per hour. Their disturbance, however, is merely superficial. Even if they are forty feet high, the disturbance of a water particle at a depth of three hundred feet is not quite half an inch from its mean position. The depths of the ocean are practically undisturbed by such waves on the surface (Prof. Tait).

Although the study of wave motions of heavy fluids, like water, or even air, may provide us with analogies by means of which we can illustrate wave motions in an attenuated medium like the ether, we must bear constantly in mind the fact that the viscosity of water or that of the air greatly modifies the circumstances of wave motion.

Our ideas, however, of waves in the ether of space, which are believed to convey the energy of the sun to

FIG. 30.

us, are primarily obtained from contemplation of the wave motions which we perceive in water and the air. The electric spark has been used in an interesting manner to make manifest waves in air which otherwise would escape our senses. Prof. Boys by its aid has photographed the waves caused by the motion of a bullet. His method is substantially as follows:

C is a plate of window glass (Fig. 30) with a square foot of tin foil on both sides. This constitutes the con

denser, and it is charged until its potential is not sufficient to make a spark at each of the gaps, E and E',

*Nature, March 9, 1893.

though it would, if either one of these were made to conduct, immediately cause a spark at the other; c is a Leyden jar of very small capacity connected with C by a wire-as shown by the continuous lines—and by a string wetted with a solution of chloride of calcium, as shown by the dotted line. So long as the gap at B is open this little condenser, which is kept at the same potential as the large condenser by means of the wire and wet string, is similarly unable to make sparks both at B and E', but it could, if B was closed, at once discharge at E'. Now, suppose the bullet to join the wires at B, a minute spark is made at B and at E' by the discharge of c. Immediately C, finding one of its gaps, E', in a conducting state, discharges at E, making a brilliant spark which casts a shadow of the bullet upon the photographic plate, P. The wet string suffices to charge the jar c, but acts like an insulator when the discharge takes place at E' and B. The photograph is a silhouette, but it serves to define the wave of air caused by the bullet.

Prof. Boys remarks that the wave revealed by the photograph shows just as in the case of waves produced by the motion of a ship, which become enormously more energetic as the velocity increases, and which at high velocities produce an effect of resistance to the motion of the ship far greater than that of skin friction, that the resistance which the bullet meets increases very rapidly when the velocity is raised beyond the point at which these waves begin to be formed. Scott Russell has shown by diagrams and experiments what happens when a solitary wave meets a vertical wall. As long as the wave makes an angle with the wall it is reflected perfectly, making an angle of incidence equal to the angle of reflection, and the reflected and incident waves

are alike in all its parts. When the wave front nearly perpendicular to the wall runs along nearly parallel to it, it then ceases to be reflected at all. The part of the wave near the wall gathers strength; it gets higher, travels faster, and so causes the wave near the wall to run ahead of its proper position, producing a bend in the wave front, and this goes on until the wave near

B

FIG. 31.

A

the wall becomes a breaker. To see if a similar phenomenon could be traced in the air, Prof. Boys arranged three reflecting surfaces (as seen in Fig. 31). Below the bullet two waves strike a reflector at a low angle, and they are perfectly reflected.

The left side of the V-shaped reflector was met at nearly grazing incidence. There is no reflection, but the wave near this reflector is of greater intensity; it has bent itself ahead of its proper position, just as the water wave was found to do. The stern wave has a piece cut out of it and bent up at the same angle. Prof. Boys points out that if the wave was a mere advancing thing the end of the bent-up piece would leave off suddenly, and the break in the direct wave would do the same. But according to Huyghens's hypothesis, the wave at any epoch is the resultant of all the disturbances which have started from all points of the wave front at any preceding epoch. The reflector, where it has cut this wave, may be considered as a series of points of disturbance arranged continuously on a line, each coming