Fig. 6 represents a discharge that occurred under the following circumstances:  The disk, P, being metallic and connected with one of the poles, there was placed upon it a thin ebonite plate of the same dimensions as the photographic one, and then the latter with the sensitized pellicle upward.  Finally, the pellicle was put in contact with the upper conductor, L, which terminated in a ball and was connected with the other pole of the induction coil.

It will be seen that, despite the two dielectrics (ebonite and glass) interposed, and the opacity of one of them, the efflux that occurred around the disk, P, is quite sharply reproduced upon the sensitized plate by a circle like that which we observed in Figs. 2 and 3.  It will be seen, besides, that an infinite number of ramifications in every direction has been produced around the ball, and we can follow the travel of the spark that leaped between the ball and disk in two directions situated in the prolongation of one another.

Under the two principal and clearly marked lines that this spark made there are seen two other, very pale and much wider ones, that present no sinuosities parallel with the first.

The results of these experiments are very curious.  The position of the plates was varied in 18 different ways, as was also the form of the conductors.  We have spoken of those only that appear to us to present the most interest.  Unfortunately, notwithstanding the skill of the engraver, it is impossible to render with accuracy all the details that are seen upon examining the negative.  The proofs that have been printed upon paper present much less sharpness than the negative, for there are certain parts of the figures on the glass that do not show in the print.

[Illustration:  FIG. 6.]

We have been content here to make known the results obtained, without drawing any conclusions from them.  It is to be hoped that these experiments, which can be easily repeated by means of the apparatus described above, will be repeated and discussed by electricians, and that they will contribute toward making known to us the nature of the mysterious agent that will give its name to our era.—­G.  Mareschal, in La Lumiere Electrique.

* * * * *

THE TRUE CONSTANT OF GRAVITY.

Many of the readers of this journal may like to participate in the discussion of the following proposition.  The statement is this: 

The space through which a body, near the surface of the earth, at mean latitude, in vacuo, descends by virtue of the accelerating force of gravity in 1/1000 of an hour is precisely 2,500 geometric inches = 100 geometric cubits = the side of a square geometric acre.

[The geometric inch is taken, in accordance with the view of Sir John Herschel, at 1/1,000,000,000 of twice the polar axis of the earth, and equals 1-1/1000 English inches very nearly.]