[Illustration:  FIG. 1.—­PRODUCTION OF SMOKE RINGS.]

As regards smoke rings, it is easy to produce these by puffing cigar smoke through a tube (Fig. 1).  But, in order to insure success, a few precautions are necessary.  The least current of air must be avoided, and this requires the closing of the windows and doors.  Moreover, in order to interrupt the ascending currents that are formed in proximity to the body, the operation should be performed over a table, as shown in the figure.  The rings that pass beyond the table are not perceptibly influenced by currents of hot air.  A tube 3/4 inch in diameter, made by rolling up a sheet of common letter paper, suffices for making very beautiful rings of one inch or more in diameter.  In order to observe the rings well, it is well to project them toward the darkest part of the room, or toward the black table, if the operator is seated.  The first puffs will not produce any rings if the tube has not previously been filled with smoke.  The whirling motion is perfectly visible on the exit of the ring from the tube, and even far beyond.

[Illustration:  FIGS. 2, 3, AND 4.—­VARIOUS ASPECTS OF SMOKE RINGS.]

As for the aspect of the rings projected with more or less velocity to different distances from the tube, Figs. 2, 3, and 4 give quite a clear idea of that.  Figs. 3 and 6 show the mode of destruction of the rings when the air is still.  There are always filaments of smoke that fall after being preceded by a sort of cup.  These capricious forms of smoke, in spreading through a calm atmosphere, are especially very apparent when the rays of the sun enter the room.  Very similar ones may be obtained in a liquid whose transparency is interfered with by producing a precipitate or rings in it.—­La Nature.

[Illustration:  FIGS. 5 AND 6..—­SMOKE RINGS BREAKING UP.]

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SHALL WE HAVE A NATIONAL HORSE?

To the Editor of SCIENTIFIC AMERICAN SUPPLEMENT: 

In your issue for August 13 is “A Proposition for a Government Breeding Farm for Cavalry Horses,” by Lieutenant S.C.  Robertson U.S.A., First Cavalry.  The article is national in conception, deep in careful thought, which only gift, with practical experience with ability, could so ably put before the people.  As a business proposition, it is creditable to an officer in the United States army.

The husbandman and agriculturist, also the navy and scientific explorations, each in turn present their wants before the government for help in some way, and receive assistance.  The seaman wants new and improved or better ships, and the navy gets them; but the poor cavalryman must put up with any kind of a craft he can get; the horse is the cavalryman’s ship—­war vessel on land.