HEAT. 149 



advantage is taken of the property of bodies in conducting 

 heat, or, what is the same thing, in confining heat. 

 The brick-maker covers his kilns of unburnt bricks with 

 a coating of burnt bricks, plastered over with clay and 

 sand to keep the heat in, these substances being bad 

 conductors. Furnaces are coated with the same mate- 

 rial, or with charcoal, which is much better. Double 

 windows are to be seen in Boston during the winter 

 season, the use of which is founded upon the same prin- 

 ciples the air which is confined between them being a 

 bad conductor of heat, prevents the warm air of the room 

 from being cooled from without. 



Upon the same principles we may explain several phe- 

 nomena which, although of daily occurrence, are but 

 seldom understood. If we apply our hand in cold 

 weather to wood, iron, or stone, the iron feels colder than 

 the wood, and yet they have both been exposed to 

 the same temperature, and of course if exposed to the 

 thermometer, would both show the same degree of heat. 

 The iron feels the coldest because, being the best con- 

 ductor, it carries off the heat of the hand much faster than 

 the- wood, which is a bad conductor. For the same 

 reason when iron and wood are heated to the same tem- 

 perature before a fire or otherwise, the iron to the hand will 

 feel much the hottest. Workmen who are exposed to 

 great heat or cold, should always wear flannel, as it is a 

 slow conductor of heat, and in cold weather will keep the 

 heat of the body in, and in warm weather will keep the 

 external heat out. For the same reason, woollen or 

 worsted mittens, or gloves, are much warmer in winter 

 and cooler in summer, than leather. 



OF THE DISTRIBUTION OF HEAT. 



Every person knows by experience the common law of 

 heat, that a warm body cannot remain long near a colder 

 one, without being deprived of a part of its heat, which 

 passing to the colder one makes it warmer. 



Experiment. If we mix a quantity of water at 212, 

 with the same quantity at 32 , the mixture will be found 

 to be 122, which is the exact mean of the two; of 

 course the hot water has lost 90 of its heat, while the 

 cold water has gained 90 . 



VOL. I. NO. VI. 14 



