240 farmers' and mechanics' journal. 



The best iron tried, supported 20 toiis per square inch, without 

 being torn asunder. The bars began to lengthen sensibly when 

 two-thirds of this power had been applied, and the elongation ap- 

 peared to increase in a geometrical ratio with arithmetical isicre- 

 ments of power. The worst iron tried, gave way under a tension 

 of 14 tons to the square inch of section, and did not lengthen sensi- 

 bly before rupture. By forging four bars of iron of medium quali- 

 ty together, an iron was obtained which did not begin to Icngtiien 

 until 16 tons had been applied, and supporting a weight of 24 tons 

 without breaking. 



Taking these results as sufficient data, it was decided by the 

 committee appointed for the purpose, that the thickness of chains 

 in a suspension bridge, should be calculated so that the maximum 

 weight to be borne should not exceed eight tons per square inch 

 of sectional surface, and that before being used they should be sub- 

 jected to a tension of 16 tons per square inch, and bear it without 

 any sensible elongation. [Jour, of Science and Arts. 



Cementaiion of Iron by Cast Iron. 



Pure iron, when surrounded by, and in contact with, cast-iron 

 turnings, and heated, is carbonised very rapidly, so as to harden, to 

 temper, and, in fact, to exhibit all the properties of steel. M. 

 Gautier finds this a very advantageous process in numerous cases, 

 especially where the articles to be case-hardened, or converted 

 into steel, are small, as iron wire, or wire gauze. The tempera- 

 ture required is not so high as that necessary in the ordinary pro- 

 cess of cementation, and the pieces to be carbonised are not in- 

 jured in form. The kind of cast-iron used, should be the gray 

 metal, and the more minutely it is divided the more rapid and com- 

 plete is the operation. By covering the mass of cast metal, in 

 which the iron to be carbonised is enveloped, with sand, oxidation, 

 from contact of the air, is prevented, and the cast metal may be 

 used many times. Plumbago experimented with in the same man- 

 ner, does not produce the effect. 



[Jour, de Pharmacie, 1827, p. 18. 



For the Farmers' and Mechanics' Journal. 

 QUERY. 



A good way to acquire information, is, to admit our ignorance, 

 and frequently ask questions of those who have attended to the 

 subject. This may appear frivolous to those who care nothing 

 about the subject, or have no thirst for information. But almost 

 any question, if properly considered and answered, will elicit in- 

 formation. I would inquire, therefore, where is the centre of bear- 

 ing in the hub of a common carl-wheel, supposing the under side 

 of the axle a straight line and the wheels standing on a level with 

 each other ? And also, what is the best mode of tilting axle-trees 

 to the hubs of wheels for common roads, taking into consideratioa 

 the greatest strength and the least friction ? S. H. 



