TRANSACTIONS OF THE SECTIONS. 115 



being required in the large rolls for making the finest thin sheet iron. Chemically 

 speaking, cast iron and steel are of the same composition, viz. iron with a propor- 

 tion of carbon ; the proportion of the latter in cast iron being infinitely greater than 

 in steel. Here I would point out a remarkable difference between chilled cast iron 

 and steel. If the latter is heated red-hot and plunged into cold water it becomes 

 extremely hard ; if in this state it be again heated, it resumes its original softness ; 

 but if chilled iron be so treated it still retains its hardness. Whether this is caused 

 by mere mechanical arrangement or by the chemical combination of the atoms, 

 whether there be a metallic base of carbon in one case and not in the other, or by 

 whatever these diflferences are caused, is far too little understood ; the whole subject 

 is one deserving the close attention of those whose pursuits enable them to study 

 chemical analysis. Indeed, when we reflect on the fact, that without the peculiar 

 properties of iron and carbon, civilization could not have been carried on, it does 

 appear strange that the master-minds of the age have not acquired more knowledge 

 of the relative action and combinations of these two substances. It would be foreign 

 to our present object to enter upon the mode of manufacturing steel, but I may 

 state the fact, that it is extremely difiicult to procure any masses that are of uniform 

 density, whilst chill cast iron is easily produced with large homogeneous surfaces, 

 and this brings me to the main subject proposed for your attention, viz. the appli- 

 cation of it to the pivots of astronomical instruments. About four years since the 

 Astronomer Royal applied to my partners and self respecting the construction of the 

 mechanical parts of a new meridional instrument, the size of which so greatly ex- 

 ceeded anything of the same kind, that it became a serious question of what mate- 

 rial the pivots should be made : it was requisite that it should be both hard to resist 

 wear as much as possible, and homogeneous to ensure that whatever wear took place 

 should be uniform. . 



"The extensive use we make of chill cast iron suggested, that if the pivots were so 

 cast with the body of the axis in sand mould and all run together, an instrument 

 might be produced combining all the requisite qualifications. This has been suc- 

 cessfully accomplished, and the great transit circle or meridian instrument is now at 

 work in the Royal Observatory to the satisfaction of the Astronomer Royal, on 

 whose designs the whole has been constructed." 



After a rigid examination of the form of the pivots, the Astronomer Royal has 

 concluded that no correction for the shape of the pivots is required. Specimens of 

 trial castings for the pivots were laid before the meeting. 



Description of an Improved Safety Valve. By James Nasmyth, F.R.A.S, 

 Mr. Nasmyth described his improved safety valve for steam-boilers, in which he 

 sought to combine simplicity of construction and efficacy of action with the utmost 

 security which a safety valve can afford against explosion arising from undue pres- 

 sure. He prefaced his description by alluding to the main source of derangement 

 and uncertainty in the action of safety valves as hitherto constructed, namely, the 

 employment of a conical bearing surface in the valve and in its seat, which renders 

 the use of a spindle and guide-socket requisite so as to constrain the valve to rise 

 from its seat in a direction absolutely vertical to the seat or bearing. This g«ide- 

 spindle to be of any service has to fit the socket in which it works with considerable 

 precision, in consequence of which any mud or incrustation which may chance to 

 get upon the spindle of the valve tends to prevent its rise and so far arrest its action. 

 In order to remove this serious defect, a spherical bearing has been employed, which, 

 by permitting the valve to fit its seat in any position, dispenses with the necessity of 

 any guide or spindle. The grand feature in Mr. Nasmyth's improvement, however, 

 consists in the peculiar mode by which a constant shght movement is given to the 

 valve in its seat by employing the motion of the water during ebullition so to act 

 upon the valve as to furnish the means of preventing its ever becoming set fast in 

 its seat. 



This important object is attained in the most simple manner by attaching to the 

 bottom of the weight, which hangs down imide the boiler (and which weight is 

 attached to the valve by an inflexible rod), a sheet- iron appendage, which dipping a 

 few inches into the water, transfers the constant swaying motion of the water to the 



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