162 Miscellaneous Intelligence. 



grain, you must place the heavy gold weight at five, and one of the 

 lighter ones at the extremity to counterpoise it ; and if it weighs 

 only 1, or 2, or 3, or 4-100ths of a grain, it will be counterpoised 

 "by one of the small gold weights placed at the first, or second, or 

 third, or fourth division. If, on the contrary, it weigh one grain 

 and a fraction, it will be counterpoised by the heavy gold weight 

 at the extremity, and one or more of the lighter ones placed in 

 some other part of the beam. 



" This beam has served me hitherto for every purpose ; but had 

 I occasion for a more delicate one, I could make it easily by taking 

 a much thinner and lighter slip of wood, and grinding the needle 

 to give it an edge. It would also be easy to make it carry small 

 scales of paper for particular purposes." 



Mr. Smithson observes, that the rings, or small weights, men- 

 tioned above, have the defect of their weight being entirely acci- 

 dental, and consequently most times very inconvenient fractions 

 of grains, and recommends instead that the weight of a certain 

 length of wire be ascertained, and then the length of it taken, 

 which corresponds to the weight wanted ; when fine wire is used, 

 a set of small weights may thus be made with great accuracy and 

 ease. This is a process, the value of which is well known to the 

 philosophical instrument maker. 



2. Tenacity of Iron, as applicable to Chain-Bridges. — The fol- 

 lowing results have been deduced from experiments made in Rus- 

 sia, and detailed by M. Lamb, in a letter from Petersburgh, Ann. 

 des Mines, x. 311. In the apparatus contrived for the purpose the 

 power was applied by a hydraulic press. 



The best iron tried supported 26 tons 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 

 appeared to increase in a geometrical ratio with arithmetical in- 

 crements of power. The worst iron tried, gave way under a ten- 

 sion of fourteen tons to the square inch of section, and did not 

 lengthen sensibly before rupture. By forging four bars of iron 

 of medium quality together, an iron was obtained which did not 

 begin to lengthen until sixteen tons had been applied, and sup- 

 porting a weight of twenty-four 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 

 subjected to a tension of sixteen tons per square inch, and bear it 

 without any sensible elongation. 



3. Moving Rocks of Salisbury. — Inconsequence of the interest 



