-1859] WRITINGS OF JOSEPH HENEY. 145 



slightly packed, and all above the rim having been removed 

 by means of a ruler, so as to present a uniformly plain sur- 

 face, the box is turned on its side opposite the lamp, and the 

 pasteboard interposed. In a short time the plain surface of 

 snow will be hollowed out beneath the disk, and at the end 

 of half an hour the cavity will be several lines deep at its 

 centre. When the same experiment is repeated by substi- 

 tuting for the lamp an iron ball heated to about 400° F., the 

 phenomena present themselves in a reverse order, that is to 

 say, the melting of the snow would be more abundant where 

 the direct rays impinge on the surface, than where they are 

 intercepted by the interposed disk, and instead of a hollow, 

 a protuberance would be produced at the centre of the shaded 

 portion. If we substitute in this experiment for the black 

 disk of pasteboard one covered with white lead, the heat will 

 not be absorbed, but will be reflected as from the snow itself. 



Another example of the transmission and, as it were, 

 transformation of radiant heat from the sun is afforded in 

 the high temperature produced by the ordinary hot-bed of 

 a garden. The solar rays, consisting of short vibrations, 

 readily pass through the glass cover, and are absorbed by 

 the dark ground, the atoms of which they put into more 

 rapid vibration, and these in turn give rise to new emana- 

 tions, which consisting of long waves are arrested by the glass, 

 and the temperature of the enclosed space is thus constantly 

 increased. It is also on the same principle that the radiant 

 heat of a stove does not pass out into space through the win- 

 dows of a house, though a considerable portion of the radi- 

 ant heat from an open fire would be lost in this way. 



We may apply the foregoing principles to explain the 

 accumulation of heat at the surface of the earth. The trans- 

 parent envelope which covers the surface of our planet is not 

 entirely diathermanous; and though it transmits freely the 

 intense rays of the sun it stops those of the long vibrations. 

 The surface of the earth is then in the condition of the ground 

 under the glass of the hot-bed; it is constantly absorbing 

 and receiving heat of high intensity, and constantly radi- 

 ating heat of intermediate intensity. Let us suppose all 



10-2 



