BADIENT HEAT AND AQUEOUS VAPOR : RAINS IN AUSTRALIA. 333 



supply for springs. These are the benefits resulting from the presence 

 of forests upon mountains and inclined slopes exposed to torrential rains. 



[The researches of M. Becquerel tended to show, that when the air is 

 otherwise calm, and the sun is shining warmly, there is a current of air 

 flowing out from forests into the fields adjacent. At night, the tendency 

 is inward, this alternate movement inward and outward being analogous 

 to land and sea breezes in the insular climates of warm latitudes, and, in 

 fact, due to the same cause. 



M. Tissandier, in a note addressed to the French Academy of Sciences, 

 October 10, 1873, records an observation made by him in a balloon, to 

 the effect that in passing over a large body of forest, the hygrometer 

 indicated a decided increase of moisture in the air, which disappeared 

 as he passed from thence over the open fields. This would seem to indi- 

 cate that irom over great masses of forest there is an increase in the 

 amount of vapor from evaporation, or a relative increase from a reduc- 

 tion of temperature, or both. He also noticed, as was known before, 

 that the hygrometer indicates more dryness as we rise above the sur- 

 face.] 



EFFECT OF CAEBONIC-ACn) GAS AND OF AQUEOUS VAPOR UPON THE 

 RADIATION OF TERRESTKIAX, HEAT. 



The presence of these gases in the atmosphere, near the surface, has 

 been shown by the experiments of Prof. John Tyndall to afford great 

 protection by preventing the radiation of terrestrial heat, while they 

 aflbrd almost no obstruction to the transmission of the direct heat. In 

 this effect they resemble the action of snow, ice, and many other bodies 

 which are tramcalescent, as to heat of high intensity; but impervious 

 to heat of low intensity as that given off by moderately warm bodies, 

 such as the soil warmed by the.sun.^ 



It is a common remark that the absence of dew on a summer night is 

 a prognostication of rain. This seemingly contradictory statement, that 

 there is less dew when there is most moisture in the air, may be recon- 

 ciled with reason, if we admit that the radiation of heat by the earth is 

 hindered by the stratum of moist air that lies upon its surface, and that 

 as a consequence the surface does not cool down to the dew-point. 



EFFECT OF VEGETATION UPON THE RAIN-FALL, AS OBSERVED IN AUS- 

 TRALIA AND TASMANIA. 



An observer, in writing upon the climate of these countries, remarks 

 that the influence of winds, great as it may be, is not the only one which 

 increases or diminishes the fall of rain ; that of vegetation is nearly 

 equal to it. The refrigerating power of plants, acquired through the noc- 

 turnal radiation of heat, and their feeble absorption of heat during the 

 day, is exemplified in a striking manner by a comparison of the quan- 

 tities of rain condensed by the mountain districts — the one richly wooded, 

 the other but scantily clothed with vegetation. On Middlesex Plains, 

 a dependency of the Circular Head Company, 2,700 feet above the sea, 

 the rain is less than on the Hampshire Hills, which average 1,800 feet 

 above the sea. The rich arborescent vegetation found on the latter, and 

 the partial barrenness of the former, thus difierently influence the con- 

 densation of the floating vapors. 



The influence of vegetation on the amount of rain is still better exem- 



' This subject is further considered in Prof. T. Starry Hunt's Chemical and Geological 

 Essays (1875), p. 48. 



