5O Emission and Transmission of Heat 



rotation of the agitator had a very perceptible influence; the value 

 of a increased or diminished notably with this speed. When we 

 consider that the steam in condensing must cover the lower -sur- 

 face of the disc with a layer of almost motionless water it is very 

 probable that in these experiments this surface was not at the 

 temperature of the steam, and likewise that the upper surface was 

 not at the temperature indicated by the thermometer. The heat 

 was really traversing a sheet of metal, comprised between two 

 sheets of water, one of which was practically motionless and the 

 other was only slowly changed; and since the conductivity of 

 water is very low compared to that of the metals, the influence of 

 the conductivity of the metals disappeared. 



831. To verify this conjecture I abandoned the heating by 

 steam. I filled the upper vessel with water at OCand I immersed 

 the disc which formed its bottom to a depth of several millimeters 

 in a vessel filled with water at ordinary temperature. The interior 

 agitator was provided with brushes which grazed the surface of 

 the disc and the water in contact with the other surface was re- 

 newed by cords stretched in a frame to which a rapid reciprocat- 

 ing motion was imparted. 



With this arrangement the heating of the water in the vessel 

 was very slow and the water in contact with the disc could be 

 changed with great rapidity. 



With the discs of lead from one to twenty-five millimeters 

 thick, the values of a varied from .00060 to .00025. The other 

 metals gave similar results. 



From this I concluded that, by greatly increasing the rate of 

 renewal of the waters bathing the two faces of the disc, and by 

 employing thick discs of the metals with the lower conductivities, 

 coefficients would be obtained that would be inversely proportional 

 to the thicknesses. 



832. I devised for this purpose a new apparatus, in which 

 the interior agitator was driven by gearing; the external agitator 

 was also geared and consisted of a horizontal wheel, mounted ec- 

 centrically to the vessel and having spokes formed by tightly 

 stretched cords, which in their motion rubbed against the outer 

 surface of the disc. 



Figure 165 is a vertical section of this last apparatus, figure 



