220 WELLS'S NATUEAL PHILOSOPHY. 



can not preserve ice more effectually in summer than by enveloping it iu its 

 fjlds. Firemen exposed to the intcuso heat of furnaces and steam-boilers, iu- 

 variablj' protect themselves with flannel garments. 



Cargoes of ice shipped to the tropics, are generally packed for preservation 

 in sawdust : a casing of sawdust is also one of the most effectual means of 

 preventing the escape of heat from the surfaces of steam-boilers and steam- 

 pipes. Straw, from its fibrous character, is an excellent non-conductor of 

 heat, and is for this reason extensively used by gardeners for incasing plants 

 and trees which are exposed to the extreme cold of winter. 



Snow protects the soil in winter from the effects of cold in 

 protect the the same way that fur and wool protect animals, and cloth- 

 earth from jjjg, man. Snow is made up of an infinite number of little 

 cold? ° . ..... 



crj^stals, which retam among their interstices a large amourit 



of air, and thus contribute to render it a non-conductor of heat. A covering 

 of snow also prevents the earth from throwing off its heat by radiation. The 

 temperature of the earth, therefore, when covered with snow, rarely descends 

 much below the freezing-point, even wlien the air is fifteen or twenty de- 

 grees colder.* Thus roots and fibers of trees and plants, ai'e protected from a 

 destructive cold. 



499. Clothing is considered warm or cool ac- 



circumstances cordlng as it impedcs or facilitates the passage 

 «idfrua"trarm of hcat to or from the surface of our bodies. 

 The finer the cloth, the more slowly it con- 

 ducts heat. Fine cloths, therefore, are warmer than 

 coarse ones. 



Woolen substances are worse conductors of heat than cotton, cotton than 

 silk, and silk than linen. A flannel shirt more effectually intercepts heat 

 than cotton, and a cotton than a linen one. 



The sheets of a bed feel colder than the blankets, because they are better 

 conductors of heat, and carry off the heat more rapidly from the body, the 

 actual temperature of both, however, is the same. For the same reason, a 

 linen handkerchief is cooler and more agreeable to the face than a cotton one. 



Cellars feel cool in summer, and warm in winter, because the external air 



• " Few can realize the protecting value of the warm coverlet of snow. No eider-down 

 Jn the cradle of an infant is tucked in more kindly than the sleeping-dress of winter about 

 the feeble flower-life of the Arctic regions. The first warm snows of August and Septem- 

 ter, falling on a thickly-blended carpet of grasses, heaths and willows, enshrine the 

 flowery growths which nestle around them in a non-conducting air-chamber; and as 

 each successive snow increases the thickness of the cover, we have, before the intense cild 

 of winter sets in, a light cellular bed, covered by drift, six, eight, or ten feet deep, iu 

 which tlie plant retains its vitality. The frozen sub-soil does not encroach upon this nar- 

 row cover of vegetition. I have found, in mid-winter, in the high latitude of 7S°, the 

 Burface so nearly moist as to be friable to the touch ; and on the ice-floes commencing 

 with a Burface temperature of 30° below zero, I found, at two feet deep, a temperature of 

 8° below zero, at four feet 2° above zero, and at eight feet 26° above zero. My experi- 

 jnents prove that the conducting power of snow is proportioned to its compression by 

 winds, ruins, drifts, and congelutiou." — De. Kane'b Secund Arctic Espedition. 



