MECHANICS AND USEFUL ARTS. 31 



believe, with the ancients, that certain waters are more adapted 

 to this operation tiian others. Tiie onl}^ difference lies in their 

 temperature. 



Mercury has no other property than that of being cold, and of 

 producing a hardness which can be obtained with water at the 

 same temperature. Tallow and oils, where carbon is one of the 

 constituent elements, produce an imperfect hardening, but pre- 

 vent a loss of c iibon. When, by over-heating, steel lias been 

 burned and dec.irourized, the oils and fatty matters are useful, 

 because they give back to the steel a part of the carbon lost in the 

 fire. Some acids, such as sulphuric, are justly considered as im- 

 parting more hardness to steel, by dissolving a film of iron ii'om 

 the surface and exposing the carbon. As for urine, alcohol, 

 brandy, and numerous other liquids extolled by ignorant work- 

 men, thc}^ are not worth as much as water, which has the advan- 

 tage of being abundant everywhere, cheap, and adapted to all 

 changes of temperature. 



M. Troost advocates the following method of obtaining a very 

 pure soft iron, which would be of great utility if oxygen can be 

 prepared at a cheap rate on a large scale. He melts cast iron in 

 a lime crucible by the oxj'-hydrogen flame ; when the metal is 

 fused, he increases the proportion of oxygen, and thus burns 

 away the silicon, carbon, and sulphur, the slag which is formed 

 being absorbed by the crucible; at the close of the process a 

 button of very pure metal remains at the bottom of the crucible 



ruthven's hydraulic propeller. 



Mr. Isaac Newton communicates to the " Journal of the Frank- 

 lin Institute " for March, 1868, a paper on this subject, in which 

 he compares at length the hydraulic propeller with the ordinary 

 screw propeller, showing that the former is much more liable to 

 be disabled by an enemy's shot, that it gives an inferior ma- 

 noeuvring or turning power, affords no advantages in reversing 

 power, and is more affected by the motion of the vessel in a sea 

 way ; the opposite of all of these has been heretofore claimed for 

 the hydraulic propeller. He states the following as the result of 

 the official trials: immersed midsection of *' Water witch," 347 

 square feet; horse-power, 777; speed, 9.2 knots, — that is, by the 

 usual calculation, 2.8 horse-power are required for each square 

 foot of immersed midsection to give a speed of 10 knots ; exten- 

 sive experience with our bluff freighting propellers shows that 

 but 1.8 horse-power per square foot of immersed section is neces- 

 sary for the speed of 10 knots. Hence, the hydraulic propeller in 

 the *' Waterwitch" is, as a propelling instrument, 36 per cent, less 

 efficient than the ordinary screw propeller ; or, in other words, it 

 wastes 36 per cent, more power than that method of propulsion. 



