Philosophical Tramactions, 105 



end of the bar bore with a constant pressure on the specimen, 

 and the graduated arc over which the index moved was divided 

 into indies, tenths, and hundredths ; and thousands were mea- 

 sured by a vernier scale on the end of the index. By a screw 

 at the lower end of the vertical bar, the index was set to zero, 

 when necessary. 



The first trials were made with a bar of blistered steel of a very 

 good quality. It was drawn out by the hammer to a proper width 

 and thickness, and then filed true and regular. It was then 

 hardened, and tempered to the same degree of hardness as com- 

 mon files. 



The total length of the bar was 14 inches ; the distance be- 

 tween the supports 13 inches ; the breadth of the bar 0.95 in- 

 ches, and the depth 0.375 inches : the thermometer varied from 

 55° to 57° at the times of trial. 



With a load of 54 lbs. the depression in the middle was 0.02 in. 



82 0.03 „ 



110 0.04,, 



The last load remained on the bar some hours, but produced no 

 permanent alteration of form. 



The temper of the bar was then lowered to a rather deep straw 

 yellow, and it was tried again ; when the same loads produced 

 exactly the same flexures as before. 



The temper was then lowered till the colour was an uniform 

 blue, or spring temper ; and the trials were repeated with the 

 same loads ; but the flexures were still the same. 



It was now heated to redness and very slowly cooled. In this 

 state the same loads still produced the same flexures ; and the 

 load of 110 lbs. caused no permanent change of form. 



The bar was hardened again, and made very hard ; in this 

 state the same loads produced the same flexures ; and 



With a load of 300 lbs. the depression in the middle was 0.115 in. 



350 0.130 „ 



580 - broke. 



When the bar was relieved from the load of 350 lb?, it retained 

 a permanent flexure of 0.005 inches, which increased to 0.01 

 with the addition of 10 lbs. to the load. 



Finding that a bar of much greater length might be tempered 

 without difficulty, Mr. Tredgold had another bar made of the 

 same kind of steel ; the length of which being 25 inches, about 

 double the flexure could be given with the same strain upon the 

 material ; and therefore any small degree of difference in the 

 elastic force might be more easily detected, for the preceding 

 experiments showed that if there be any diff'erence, it must be 

 extremely small. 



The breadth of this bar was 0.92 inches ; the depth 0.36 



