500 Dr. Lees and Mr. Chorlton on the Thermal 



The numbers in the 4th column are found by subtracting 

 alternate times in the 3rd column from each other and 

 dividing 4° 0. by the differences. 



The numbers in the 5th column are obtained from those in 

 the 4th by multiplying by 1140 grams, the mass of the lower 

 disk, and by *093, the specific heat of brass, i. e. by 106. 



The numbers in the 6th column are obtained by dividing 

 by the total radiating surface of the lower disk, plus half the 

 radiating surface of the nonconducting ring = 148*5 + 5*5 

 = 154 sq. centims. 



The values of h are obtained from these numbers by dividing 

 by the excess of the temperature of the lower disk over that 

 of the air. 



Experiments. 



The following account of an experiment on a plate of glass 

 will show the method of treatment in each case : — 



A disk of plate-glass of diameter equal to that of the brass 

 plates — 11*4 centims. — was placed between them, and contact 

 made by means of thin mercury films. Good contact was 

 easily secured between the glass and the lower plate ; but 

 with much more difficulty in the case of the upper plate. 

 Contact was eventually obtained by covering the surface 

 of the upper plate with mercury, so that it adhered in pendent 

 drops, at the same time placing a few drops of mercury on 

 the surface of the glass, and then carefully lowering the 

 upper plate on to the glass and allowing the excess of mercury 

 to run out at the edges. 



Steam was then passed through the box attached to the 

 upper brass plate, and thermometers inserted in the upper 

 and lower plates, the temperature of the air being indicated 

 by a third thermometer placed as described above. 



After a time, the duration of which depended on the con- 

 ductivity and thickness of the substance experimented upon, 

 the thermometers became steady and the temperatures were 

 observed. 



2 = temperature of upper plate = 96' 1° C. 



X = „ „ lower „ =92-4°C. 



6 = „ „ air =20-0°C. 



Thickness = *292 centim. 



