MECHANICS AND USEFUL ARTS. 105 



Coating of Cast Iron. — Herr "W. Lieke, of Hanover, has made 

 a series of practical experiments upon the various processes for 

 covering cast iron with a protecting varnish. Tiie author's obser- 

 vations were made with the view of discovering some new method 

 of protecting cast-iron objects from oxidation or rust when ex- 

 posed to the damp atmospliere. In the first place, he* observed 

 that "zinc dust,*' which is now extensively produced as a waste 

 product of zinc furnaces, can be applied with considerable advan- 

 tages. Half an ounce of this zinc dust mixed with 1 ounce of oil 

 varnish, and rubbed several times upon 1 square foot of cast iron, 

 will, he finds, preserve the metal from rust in a variety of circum- 

 stances ; but it is not entirely satisfactory when the iron is subjected 

 to soap water or other alkaline liquids. 



To be effective against the action of these solutions, the iron 

 must be coated with 2 parts of water-glass (silicate of soda) , em- 

 j)loyed in solution, marking 20° Baume, and 1 part of zinc oxide 

 intimately mixed together. This material, laid on as a thick var- 

 nish, gives the iron a kind of enamelled appearance, and the pro- 

 tective coating will not yield to soap water. — Scientific American. 



Improvement in Water-Wheels. — Mr. James P, Collins, of Troy, 

 N. Y., enamels all portions of any water-wheel exposed to the 

 action or force of the water with some suitable material, or com- 

 bination of materials, thereby giving a smooth and glazed surface, 

 over which the water flows with greatly diminished friction ; of 

 course adding proportionally to the efficiency of the wheel. All 

 chemical action of the water must be entirely prevented by such 

 a coating. 



The Durometer. — At the Paris Exposition an instrument was 

 exhibited designed for testing the relative hardness of steel rails. 

 This •' durometer," as it is styled, is virtually a small drilling ma- 

 chine, Avorking by hand or machine power, which registers the 

 number of revolutions of the drill spindle, and also the amount of 

 feed, the latter being given by the apjDlication of a known weight 

 to the back of the drill spindle. The friction of the machine and 

 the state of the cutting edges are supposed to be constant quan- 

 tities, and, as such, are thrown out of the calculation. The hard- 

 ness of any rail is considered to be inversely proj^ortionate to the 

 depth of feed obtained with a given number of revolutions. 



Drilled vs. Punched Holes. — A large number of specimens of 

 steel plates were recently tested at Chatham Dockyard, to deter- 

 mine the difference in strength between steel plates with punched 

 and drilled holes. Although the pieces were so prepared that 

 they should break at the smallest part, they all, without exception, 

 fractured at a place where two small holes had been punched. But 

 when the holes were drilled, and in the largest sectional area of 

 the steel, they as uniformly broke in the smallest part, exactly the 

 reverse of the previous trial. From this and other experiments, 

 the advantage in tensile strain, gained when the holes are drilled 

 rather than punched, was calculated to be 22.5 jDcr cent. 



Immense Machines. — The largest planing machine in the United 

 States, if not in the world, has just been completed at the gov- 

 ernment workshop at Charlestown. It has been several years in 



