ON THE SUN^S HEAT. 395 



and floor and ceiling and one another, and remain- 

 ing in a constant average condition of denser crowd 

 just over the floor and less and less dense up to 

 the ceiling. 



In this constant average condition the average 

 velocity of the marbles will be the same all 

 through the crowd, from ceiling to floor, and will 

 be the same in all directions, horizontal, or vertical, 

 or inclined. The continually repeated blows upon 

 any part of the walls or ceiling will in the aggregate 

 be equivalent to a continuous pressure which will 

 be in simple proportion to the average density 

 of the crowd at the place. The diminution of 

 pressure and density from the floor upwards will 

 be precisely the same as that of the density and 

 pressure of our atmosphere, calculated on the 

 supposition of equal temperature at all heights, 

 according to the well-known formula and tables 

 for finding heights by the barometer. 



In reality the temperature of the atmosphere 

 is not uniform from the ground upwards, but 

 diminishes at the rate of about i C. for every 

 162 metres of vertical ascent in free air, undis- 



