34 



" The absorption of incident heat as solar heat and its radiation outwards as terres- 

 trial heat (i. e. heat of a much more absorbable nature) by the solid surface depends very- 

 much on the nature of its substance ; but if the ground be covered with vegetation, the 

 -whole of the incident heat is returned back either by radiation or contact communication, 

 to the air ; and the soil receives no heat where so covered otherwise than oircuitously 

 through the medium of heated air. All these causes acting together, produce a vast dif- 

 ference as respects the temperature of the air in regions of the globe covered by the 

 ocean and those occupied by dry land. In the former, the fluctuations both diurnal and 

 annual are confined within very much narrower limits than in the latter ; and this con- 

 trast which theory indicates, is confirmed by universal observation as the expression of 

 the distiuction between an insula/r and a continental climate, or that of a small island 

 remote from all other land and of the central regions of an extensive continent. If there 

 be one general feature in meteorology more ^prominent than another it is the uniformity 

 of temperature over large bodies of water, as compared to that under similar exposures 

 to the sun on land." 



Terrestrial Radiation. 



" The theory of radiant heat promulgated by Prevost, which all experimental enquiry 

 into the subject, has tended to confirm, lays it down as a principle, that a mutual inter- 

 change of heat is continually taking place between all bodies freely exposed to view of 

 each other, the hotter radiating more than the colder, in the ratio of some function in- 

 creasing with the temperature. The experiments of Dulong and Pefit on the radiation of 

 bodies in vacuo have shown that this function, within the limits of their experiments is 

 of the exponential form, or in other words, that the force of radiation in vacuo increases 

 in geometrical progression as the excess of temperature of the radiant body above that 

 of its envelppe increases in arithmetical. Hence when a hot body is placed in presence of 

 bodies, some colder, some hotter than itself, an equilibrium will rapidly be established, in 

 which its momentary gains and losses of heat to and fro among them aU will balance each 

 other, and its temperature will thenceforward be unchanged. 



" The mean temperature of the earth remaining unchanged, it necessarily follows that 

 it emits by radiation from and through the surface of its atmosphere, on an average, the 

 exact amount of heat it receives from the sun ; i. e. as much as would melt 0.01093 

 inch thickness of ice per minute over one of its great circles, which is equivalent to l-40th 

 inch of water per hour over its whole surface, condensed from its dewpoint. Taking this 

 as the measure of the total average radiation, one-third of it, or l-120th inch, may be 

 taken as radiated off from the atmosphere without even reaching the earth, and the 

 remaining two-thirds, (l-60th inch), may be considered as got rid of by radiation from 

 the surface of the earth. Let us now consider the manner in which this takes place, 

 supposing a clear sky to prevalil : — 



" Conduction through the soil is a very slow process, radiation a very rapid one. So 

 soon, then, as the sun has sunk so low as not to counteract the earth's radiation, the im- 

 mediate surface begins to part with its heat, at first slowly, but as night advances more 

 rapidly, and at length faster than it can percolate from the interior to supply the waste. 

 The surface therefore becomes greatly chilled, and a wave of cold is propagated down- 

 wards, neutralizing and destroying the heat wave rising to meet it, a process which goes 

 on leisurely, and takes its own time. Meanwhile the chilled surface now borrows heat 

 from the air also, to supply its waste ; 1st, by contact communication ; 2nd, by down- 

 ward radiation ; and 3rd, by condensation of vapour when the temperature of the surface 

 air is reduced to the dewpoint, and thus attains that state of equilibrium which the 

 circumstances admit of." 



We will now consider the facts adduced by Dr. Bryce, premising that although cor- 

 rectly expressing the author's views as to facts and figures, it is (as is too often the case 

 with newspaper reports), not nearly as well worded, and not as connected as the original 

 paper. The only full copy, however, unfortunately wandered into that Slough of Despond, 



