310 Prof. 0. Reynolds on certain Dimensional [Feb. 6, 



cceteris paribus, depend on the difference of temperature, would also 

 depend on the relation between the density of the gas and the coarse- 

 ness of the plate, so that, cceteris paribus, the finer the plate the greater 

 the difference, and this conclusion was at once verified. 



A plate of meerschaum, "25 inch (6'3 millims.) thick, gave a differ- 

 ence, '25 inch with air, and "88 with hydrogen, at the pressure 

 of the atmosphere, while a plate of stucco of the same thickness as 

 the meerschaum only gave a difference of '02 inch with air and '14 

 inch with hydrogen. 



It also appeared from the theory that with the same plate and the 

 same gas, the difference of pressure should be a maximum at some 

 particular density, so that if the initial density was sufficient, the 

 thermal difference of pressure would increase as exhaustion proceeded 

 up to a certain point and then fall off, the density at which the 

 thermal difference would be a maximum depending on the coarseness 

 of the plate and the nature of the gas. These conclusions were 

 verified. 



Fio. 1. 



Mean Pressure in inches of Mercury. 



For with the meerschaum piate the thermal difference for air was 

 almost constant at pressures nearly equal to the atmosphere, but fell at 

 an increasing rate as the density diminished (this is shown by the curve 



