POPULAR MISCELLANY. 



283 



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 advan- 

 tages, 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 lati- 

 tude of the star can be even better deter- 

 mined 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 y-^iT 

 of a second. For a star just visible to the 

 naked eye, half a second is enough ; for 

 stars of the tenth magnitude, twenty sec- 

 onds ; 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 unin- 

 flammable 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 impreg- 

 nated with it should remain uninflammable 

 after having been exposed for a month to a 

 temperature of 100 and over. " La Na- 

 ture " 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 kilo- 

 grammes, 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 mixt- 

 ure applicable to canvas that is already 

 painted and to mounted scenery, to wood- 

 work, furniture, curtains, bedclothes, cra- 

 dles, 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 kilo- 

 gramme ; 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 canvas- 

 es, cordage, straw-work, wood, and car- 

 pentery, consists of sal-ammoniac, 15 kilo- 

 grammes ; 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 con- 

 sists of sulphate of ammonia, 8 kilogrammes ; 

 boric acid, 3 kilogrammes ; borax, 2 kilo- 

 grammes ; 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 unin- 

 flammable, and consequently incapable of 

 starting or supporting a fire the composi- 

 tions 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 im- 

 pression of heat, will cover the surface of 

 the fibers, adhere to them, and prevent the 

 contact of the air. Salts that crumble un- 

 der the action of heat, or of long-continued 

 dryness, those that are hard to melt, efflo- 

 rescing and hygrometric substances, are, 

 therefore, not the most suitable for these 



