178 



NATURE S TEACHINGS. 



It is well known that air is practically a non-conductor of 

 heat, and that therefore a layer of air between two very slight 

 walls is just as warm as if the wall had been made of solid 

 stone. Now, there are several inventions whereby the present 

 brick could be made half its present weight, twice its present 



NEST OF LITTLE E'IMINE-MOTH. 



strength, hard and smooth as earthenware, so that it could not 

 absorb water like our common brick, and pierced with holes 

 through which air could pass. 



Unfortunately, however, there is a stringent rule among brick- 

 makers and bricklayers that they are to play into each other's 

 hands, and that no bricklayer is to touch a brick which has not 

 been made in some definite district. Should he do so, he is a 

 marked man, and will stand but little chance of getting even 

 a day's work. 



The power of the double wall may be seen in many ways. 

 For example, in the old days of coaching, when one had to pass 

 hour after hour on the roof of the coach, it was known by 

 practical experience that double body linen, and two pairs of 

 stockings, worn one over the other, formed the best preparation 

 for the journey. The reason was, that air became entangled 

 between the layers of fabric, and acted as a non-conductor of 

 heat. 



Another mode of utilising the principle of the double wall is 

 seen in the refrigerators which add so much to the comfort of 

 the household in a hot summer. The one principle of these 

 refrigerators is, to keep a layer of air between the ice and the 

 surrounding atmosphere. The same principle may be used in a 

 reverse way, and heat be preserved instead of repelled. Those 



