of the direction of the star's diurnal motion. If arranged with a driving-clock, so as to follow the star in its motion, it will give a bright point, the photographic image. If we wish to make a picture of the sky, we must register the stars by such points as these. But the trails have various advantages, one of which it that they can not be mistaken for dust or for pin-holes on the plate itself; to avoid the liability to which, the dot-pictures are always repeated. The position of dots in latitude and longitude can be very accurately measured; the latitude of the star can be even better determined from its trail, but its longitude must be determined by special devices. The proper length of exposure for a star of the first magnitude is not more than 51000 of a second. For a star just visible to the naked eye, half a second is enough; for stars of the tenth magnitude, twenty seconds; of the twelfth, two minutes; of the thirteenth, five minutes; of the fourteenth, thirteen minutes; and for the faintest stars visible, an hour and twenty-three minutes.
Fire-Proof Mixtures.—The processes employed to make cloths and woods uninflammable ought to satisfy the following conditions: 1. The preservative substance or mixture should be cheap and easily used. 2. It should not change either the cloths or their colors. 3. It should not be poisonous or corrosive. 4. Cloths or woods impregnated with it should remain uninflammable after having been exposed for a month to a temperature of 100° and over. "La Nature" gives a few of the preparations which seem best to satisfy the different conditions required. A mixture applicable to all light fabrics consists of pure sulphate of ammonia, 8 kilogrammes; pure carbonate of ammonia, 2·5 kilogrammes; boric acid, 3 kilogrammes; pure borax, 2 kilogrammes; starch, 2 kilogrammes, or dextrine, or gelatine; water, 100 kilogrammes. Cloths should be dipped in the solution at a temperature of about 84°, till they have soaked it well up, then partly dried in the air, and afterward dried enough to be ironed like starched clothes. The quantity of starch, dextrine, or gelatine, may be varied according to the degree of stiffness it is desired to give the goods. This mixture is good for ball-dresses. A quart of it will serve for the preparation of about sixteen yards of goods. A mixture applicable to canvas that is already painted and to mounted scenery, to wood-work, furniture, curtains, bedclothes, cradles, doors, and windows, and which can be mixed with dyes, consists of sal-ammoniac, 15 kilogrammes; boric acid, 5 kilogrammes; glue, 50 kilogrammes; gelatine, l·5 kilogramme; water, 100 kilogrammes, with lime enough to give the proper consistency. It should be employed at a temperature of from 122° to 140°. The pieces may be dipped into it or painted with it. In case of decorations already painted it is enough to whitewash the backs of the canvases, and the frames on which they are hung, with the preparation. A kilogramme of it will paint five square metres of surfaces. A mixture applicable to heavier canvases, cordage, straw-work, wood, and carpentery, consists of sal-ammoniac, 15 kilogrammes; boric acid, 6 kilogrammes; borax, 5 kilogrammes; water, 100 kilogrammes. It is used at a temperature of 212°. The immersion should continue fifteen or twenty minutes, after which the piece should be aired and then dried. Another mixture is applicable to plain or printed papers. It consists of sulphate of ammonia, 8 kilogrammes; boric acid, 3 kilogrammes; borax, 2 kilogrammes; and water, 100 kilogrammes. It is used at a temperature of 122°. To resolve the problem completely—that is to reduce the action of heat on combustible articles to a simple calcination and render them uninflammable, and consequently incapable of starting or supporting a fire—the compositions should protect the fibers of the cloth or wood from contact with the air during the whole continuance of the heat; and the combustible gases disengaged by the heat should be mixed with so strong a portion of other, incombustible gases as to be no longer inflammable. Therefore, the cloth or the wood should be painted with a very fusible substance which, on the first impression of heat, will cover the surface of the fibers, adhere to them, and prevent the contact of the air. Salts that crumble under the action of heat, or of long-continued dryness, those that are hard to melt, efflorescing and hygrometric substances, are, therefore, not the most suitable for these
