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XL.— THE PHENOMENA OF SKATING AND PEOFESSOE J. 

 THOMSON'S THEEMODYNAMIC EELATION. By J. 

 JOLY, B.E. 



[Read, December 15, 1886.] 



Professor J. Thomson's Thermodynamic Relation 



dt _ T(v - Vj) 

 dp L 



entails that in the case of a substance such as ice, in which the 



consequence of the transference to the substance of a quantity of 



dT 

 heat, L, is to produce a negative change of volume, the value — 



is negative, and a lowering of the melting-point, results from the 

 application of pressure. 



I would suggest that to the many phenomena which have 

 found an explanation in] this physical fact might be added those 

 attending skating, i.e. the freedom of motion, and, to a great 

 extent, the " biting " of the skate. 



The pressure under the edge of a skate is very great. The 

 blade touches for a short length of the hog-back curve, and, in the 

 <ease of smooth ice, along a line of indefinite thinness, so that until 

 the skate has penetrated some distance into the ice the pressure 

 ■obtaining is very great ; in the first instance, theoretically 

 infinite. But this pressure involves the liquefaction, to some 

 extent, of the ice beneath the skate, and penetration or bite 

 follows as a matter of course, the amount of penetration 

 being roughly a measure of the extent to which liquefaction ob- 

 tains. As the blade sinks an area is reached at which the pressure 

 is inoperative, i.e. inadequate to reduce the melting-point below 

 the temperature of the surroundings. Thus, estimating the pres- 

 sure for that position of the edge when the bearing area has 

 become -^ of a square inch, and assuming the weight of the 

 skater as 140 lbs., and also that no other forces act to urge the 

 blade, we find a pressure of 7000 lbs. to the square inch, sufficient 

 to insure the melting of the ice at - 3'5° C. With very cold ice 



