RADIATION OF MEAT. 



Let the endless tube represented in 

 the accompanying figure be supposed 

 to be entirely filled with water, and 

 the heat of a fire be applied to the 

 lower portion of it at a, which is 

 twisted into a spiral form, the water 

 will immediately be set in motion,, and 

 made to circulate through the tube, 

 from the expansion and ascent of the 

 portion in a, and the whole of the water 

 in the tube will be brought in succes- 

 sion to the source of heat. The tube 

 may be led into an apartment above d y 

 and being twisted into another spiral at 

 by a quantity of the heat of the circulating water will be dis- 

 charged in proportion to the extent of surface of tube exposed. 

 Water of a temperature considerably above 212 is made to 

 circulate in this manner through a very strong drawn iron tube 

 of about one inch in diameter, for the purpose of heating houses 

 and public buildings. A slight waste of the water is found to 

 occur, so that it is necessary to introduce a small quantity every 

 few weeks by an opening and stopcock c, in the upper part of 

 the tube. Tubes of larger calibre, with water circulating below 

 the boiling point, are likewise much used for warming large 

 buildings. 



Air and gases are very imperfect conductors. Heat appears 

 to be propagated through them almost entirely by conveyance, 

 the heated portions of air becoming lighter, and diffusing the 

 heat through the mass in their ascent, as in the case of liquids. 

 Hence, in heating an apartment by hot air, the hot air should 

 always be introduced at the floor or lowest part. The advantage 

 of double windows for warmth depends, in a great measure, on 

 the sheet of air confined between them, through which heat is 

 very slowly transmitted. In the fur of animals and in clothing, 

 a quantity of air is detained among the loose fibres, which mate- 

 rially enhances their non-conducting property. In dry air, the 

 human body can resist a temperature of 250 without inconve- 

 nience, provided it is not brought into contact with good con- 

 ductors at the same time. 



Radiation of heat. Heat is also emitted from the surface of 

 bodies in the form of rays, which pass through a vacuum, air, 



