294 



Mr. Mallet on the Physical Conditions 



Desormes, its tenacity sinks to zero. So that, if we take the normal strain of 

 rupture, for good wrought-iron, at 24 tons per square inch, the proportion in 

 which the total contractile force is divided, between contractile strain at the 

 extremities after cooling, and elongation of the bar previously, is — 



292. All which, is in perfect accordance with the principle, ut tensio sic vis. 

 This indicates, therefore, that in practice, there is no objection to shrinking-on 

 the successive rings, at temperatures much higher than theory, based on the 

 usual incorrect view of the relation of expansion and contraction by heat to 

 the molecular constitution of metals, would fix as the limit; besides which, the 

 most simple means are at hand, to regulate to any extent the final tension of 

 each ring upon the preceding one ; for, if the internal and external diameters 

 of any two rings to be superimposed, be made such that, when heated to the 

 previously fixed upon temperature, they shall be precisely alike, the tensions 

 and elongations shall bear to each other the ratios of the preceding Table ; but 

 it is in our power, to make the internal diameter of the outside ring exceed 

 the external diameter of the inside one, when both are at this temperature, 

 by any small fraction we please, and thus permit a certain amount of unresisted 

 contraction to occur in the outer ring, when superimposed, before it finally 

 grasps, and begins to compress the inner one, and thus, with perfect prac- 

 tical facility, all the rings of any series, however large or numerous, may be 

 shrunk-on over each other, and caused to produce any assignable degrees of 

 extension and of compression, (within the possible limits), and yet all the rings 

 be shrunk-on at one temperature, and that one a full red heat. 



293. This latter temperature would seem more advisable in every instance, 

 than one considerably below it, at least than any one below a low or " cherry- 

 red" heat, however small may be the amount of final contractile strain desired; 

 for at or a little below this latter temperature, the molecular condition of 



