NATURAL PHILOSOPHY. 213 







required for the supply of the room through a maze of hot-water pipes, 

 raising the whole of it to the temperature desired sixty or eighty de- 

 grees, as the case may be. If the room be thirty feet in height, and it be 

 desired to change all the air in it every fifteen minutes, enough air should 

 be pumped in above to cause a general descent of the whole body of air 

 in the room at the rate of two feet a minute. This would be an imper- 

 ceptible current. The exit should be made by numerous holes in the floor, 

 perhaps through the carpet, or the risers of the platform on which are the 

 members' chairs. Three important advantages would thus be gained : the 

 avoidance of all eddies, a nearly homogeneous and tranquil atmosphere, and 

 the immediate removal downwards of any dust from the carpet, which 

 would thus be prevented from rising, to be inhaled into the lungs. To 

 prevent the disturbance and contamination of the atmosphere by the gas- 

 lights, I would place them above the glass of the sky-lights the space 

 between those in the ceiling and those in the roof being separated from the 

 chamber into which the fresh air should be admitted. In summer, the 

 same apparatus which sends in warm air in winter would supply a con- 

 stant breeze ; and if the temperature of the external air was too high, it 

 might be cooled by jets of water from pipes in the air passages, or even by 

 melting ice. 



To recapitulate, then, I propose to place the ceiling at such a height as 

 to be within the limit of perceptibility, and thus strengthen the voice ; to 

 destroy reverberation between the ceiling and the floor by a thick carpet 

 on the latter ; to prevent echoes from the walls, by drapery ; from the 

 chairs and desks, by cushioning or covering them ; to keep out extraneous 

 sounds, by making the room an interior apartment, lighted only from 

 above ; to secure a tranquil atmosphere, uniform in its density and refrac- 

 tion of sound, by excluding all currents of hot or cold air ; to secure a 

 constant supply of pure air at the temperature desired for the room by 

 mechanical means, introducing the air through all parts of the ceiling, and 

 taking it out through all parts of the floor thus also to remove dust; to 

 prevent all interference with ventilation by the lights at night, by placing 

 them outside the room, above the sky-lights. 



ON THE PRESENT STATE OF OUR KNOWLEDGE OF RADIANT 



HEAT. 



At the British Association, 1854, Professor Powell presented a report of 

 the above subject, supplementary to his former, read in 1832 and 1840. It 

 consisted, in the first instance, of some preliminary remarks on the con- 

 fusion introduced into the subject from the neglect of those well-marked 

 distinctions which the author had long ago dwelt upon between the differ- 

 ent species of rays, all included under the common name of radiant heat, 

 but which had been shown to be materially different in their nature and 

 properties. He then adverted to the theory by -\vhich all those different 

 kinds of effects are ascribed to the absorption of rays emanating from hot 



