Philosophical Transactions. 107 



cohesion of blistered steel to be I33,000lbs. (Philosophical Trans' 

 actions for 1818.) 



But in the very hard bar, the strain which produced permanent 

 alteration was 51,000lbs,for a square inch, find the absolute cohe- 

 sion only 85,000lbs. 



From these comparisons I think it will appear, that in the 

 hardening of steel, the particles are put in a state of tension 

 among themselves, which lessens their power to resist extraneous 

 force. The amount of this tension should be equal to the dif- 

 ference between the absolute cohesion in different states. Taking 

 Mr. Rennie's experiment as the measure of cohesion in the soft 

 state, it will be 133,000— 115,000= 18,000lbs. for the tension 

 with a straw-yellow temper ; and 133,000 — 85,000 = 48,000lbs. 

 for the tension in hard steel. And if this' view of the subject be 

 correct, the phenomena of hardening may be explained in this 

 manner, which nearly agrees with what Dr. Young has observed 

 in his Lecture I, p. 644 : after a piece of steel has been raised to 

 a proper temperature, a cooling fluid is applied capable of ab- 

 stracting heat more rapidly from the surface than it can be sup- 

 plied from the internal parts of the steel. Whence the contraction 

 of the superficial parts round the central ones which are expanded 

 by heat ; and the contraction of the central parts in cooling, while 

 they are extended into a larger space than they require at a lower 

 temperature, produces that uniform state of tension, which di- 

 minishes so much the cohesive force in hard steel. The increase 

 of bulk by hardening agrees with this explanation ; and it leads 

 one to expect, that any other metal might be hardened if we could 

 find a means of abstracting heat with greater velocity than its con- 

 ducting power. 



A short Account of some Observations made with Chronometers^ in 

 two Expeditions sent out by the Admiralty, at the recommenda' 

 Hon of the Board of Longitude, for ascertaining the Longitude of 

 Madeira and of Falmouth, In a Letter to Thoma.s Young, M.D., 

 F.S.R.S., and Secretary to the Board of Longitude. By Dr. 

 John Lewis Tiarks. 



The results of these observations are given at p. 270, Vol. XVII. 



Oftlie Effects of the Density of Air on the Rates of Chronometers ^ by 

 George Harvey, Esq. 



Wb have elsewhere shortly noticed the contents of this paper*, 

 which occupies forty of the quarto pages of the Philosophical 

 Transactions ; we must therefore refer those who are interested 

 in the minute details of such an inquiry to the original document. 



* Vol. XVII. p. 272. , ^<»^ ! 



