There are, moreover, many other questions requiring to be elucidated by experiment, especially as regards the arrangement of the conducting wires: but it is needless to dwell further upon this subject, which has been ably treated by many English men of science--for instance, Dr. Siemens and Professor Ayrton. Nevertheless, for further information the author would refer to the able articles published at Paris, by M. Mascart, in the _Journal de Physique_, in 1877 and 1878. The author would gladly have concluded this paper with a comparison of the efficiencies of the four systems which have been examined, or what amounts to the same thing--with a comparison of the losses of power which they occasion. Unfortunately, such a comparison has never been made experimentally, because hitherto the opportunity of doing it in a demonstrative manner has been wanting, for the transmission of power to a distance belongs rather to the future than to the present time. Transmission by electricity is still in its infancy; it has only been applied on a small scale and experimentally. Of the three other systems, transmission by means of ropes is the only one that has been employed for general industrial purposes, while compressed air and water under pressure have been applied only to special purposes, and their use has been due much more to their special suitableness for these purposes than from any considerations relative to loss of power. Thus the effective work of the compressed air used in driving the tunnels through the Alps, assuming its determination to be possible, was undoubtedly very low; nevertheless, in the present state of our appliances it is the only process by which such operations can be accomplished. The author believes that transmission by ropes furnishes the highest proportion of useful work, but that as regards a wide distribution of the transmitted power the other two methods, by air and water, might merit a preference. * * * * * THE HOTCHKISS REVOLVING GUN. The Hotchkiss revolving gun, already adopted in the French navy and by other leading European nations, has been ordered for use in the German navy by the following decree of the German Emperor, dated January 11 last: "On the report made to me, I approve the adoption of the Hotchkiss revolving cannon as a part of the artillery of my navy; and each of my ships, according to their classification, shall in general be armed with this weapon in such a manner that every point surrounding the vessel may be protected by the fire of at least two guns at a minimum range of 200 meters." * * * * * THALLIUM PAPERS AS OZONOMETERS.