18 Sir John Herschol on some Phenomena 



of heat. To conceive this, let us imagine a very large block 

 of stone at the commencement of the summer, to lie on a level 

 surface of ice, in a situation exposed to the direct rays of the 

 sun, where the meantemperature of day and /«|^A;is(eveninsum- 

 mer) but little above the freezing point, but where, however, 

 no fresh snow falls during the whole summer. In the day-time 

 then, while receiving the sun's rays, the upper surface of the 

 stone will be strongly heated, and a wave of heat will be propa- 

 gated slowly downwards through the stone towards the ice, di- 

 minishing in intensity rapidly, however, as it travels, since each 

 superior stratum only divides its excess of temperature with 

 that below. Long before this can reach the ice, however, 

 night comes on. The surface cools below the mean or even 

 below the actual temperature of the air by radiation, and a 

 ivave of cold is propagated (or which comes to the same thing, 

 heat is abstracted from stratum to stratum) by the same laws. 

 This follows close on the wave of heat below, and travels with 

 equal velocity. In consequence, the heated stratum parts with 

 its heat, now both upwards and downwards, and thus the in- 

 tensity of the wave of heat diminishes with much greater ra- 

 pidity as it proceeds downwards. It is manifest, that were 

 the thickness of the stone infinite, the wave of heat being al- 

 ways followed close up by the wave of cold, and a perpetual 

 tendency to an equilibrium of temperature going on between 

 them, they would ultimately reduce each other to then* mean 

 quantity, and (not to take the extreme case of infinity) at 

 some very moderate depth, the fluctuations above and below 

 the mean temperature of the air, as the successive nocturnal 

 and dim'nal waves pass through a particle of the stone there 

 situated, will be rendered very trifling, and may for our pre- 

 sent purpose be regarded as evanescent. Beyond this depth, 

 whatever mass of stone may exist, may be regarded as a slow 

 conducting mass, interposed between a surface of ice constantly 

 maintained at 32°, and a surface of stone constantly maintained 

 at the mean temperature of the air, which by hypothesis is 

 very little above it. Through this, then, the heat will perco- 

 late uniformly but feebly, and the ice below will be very slowly 

 melted, and the more so in proportion to the thickness of the 

 interposed stratum. Let us now consider what happens to the 



