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 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 thickly lined with dry branches, straw, &c. The roof is constructed 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.