DOUBLE WALLS. 179 



cooking-pots are now well known, where half-cooked meat can 

 be inserted in the morning, and at luncheon-time be turned out 

 quite hot and perfectly cooked. The fact is, that the vessels in 

 question are covered with a very thick layer of felt. The felt, 

 however, is only a device for entangling air, and a double wall 

 would answer the purpose as well, if not better. 



The now well-known fire-resisting safes are made on this 

 principle, and after they have been for hours in a raging fire, 

 and the outer case has become red-hot, the interior is quite 

 safe, the papers uninjured, and even a watch continuing to go. 



Then there is the ordinary Ice-house, a sketch of which is 

 given in the illustration. A pit is first dug in the ground, and 

 thicklv lined with dry branches, straw, &c. The roof is con- 

 structed in the same manner, only the non-conducting power 

 is increased by a thick coating of earth over the sticks and 

 straw. The door, which is approached by a shelving cutting, is 

 similarly protected, the covering only being removed when the 

 door is opened. 



I once made a very effective refrigerator out of two hampers, 

 putting a small hamper inside a large one, and packing the 

 space between them with straw. 



IN Nature we find many examples of this principle, which 

 enables the inhabitants to bid defiance to frost. 



A familiar example may be found in the cocoon of the 

 common Silk-worm (Bombyx mori), and indeed in that of almost 

 any silk-producing insect. When the caterpillar is about to 

 make its cocoon, it begins by a number of rather strong threads 

 attached to different points, and making a sort of scaffolding, so 

 to speak, for the cocoon itself. Upon these is spun a slight 

 outer cocoon of very loose and vague texture the "floss silk" 

 of commerce, and within that is the cocoon proper, in which the 

 insect lies enclosed. It will be seen, therefore, that there are 

 really three cocoons, one within the other, namely, the scaffold 

 cocoon, the floss cocoon, and the silk cocoon itself, so that the 

 inmate is protected from variations of temperature. 



The cocoon of the emperor-moth, which has already been 

 described, is made on the same principle. 



There are several caterpillars which are social in their early 

 stages, and which construct a common habitation. The Little 



