48 



Or Draining. 



tween the day and night temperature declines, and in the opposite 

 case there is the reverse effect : — 





Rain in 

 Year. 



Mean Difference between 

 Night and Day. 





June. 



July. 



August. 





o 



o 



22-8 



o 



21-2 



o 



21 -1 





25-5 



18-5 



17-3 



16-8 



Truro 



44-0 



13«6 



11-4 



11-8 



Whitehaven .... 



47-1 



12-9 



11-0 





The consequence of this must be, that whereas, for instance, 

 the difference at Chiswick in June amounts to 22°. 8, while that 

 at Whitehaven is but 12*^.9 ; the mean temperature of the twenty- 

 four hours (which must in each case correspond with the middle 

 point between the two extremes), would in the wet district only fall 

 short of the heat of day by 6°. 45 ; while in the dry it would 

 differ from it by no less than 11°. 4. The numbers annexed to 

 the other two places support the same conclusion. Now, when 

 we were comparing the temperatures of Bedford and of Derby, 

 we were speaking, as before observed, of the respective means ; 

 and by the same rule, if there is any difference in point of moisture 

 between the two climates, the greatest heat will exceed the mean 

 in the drier, in a greater degree than in the wetter ; and since the 

 mean at Bedford is 8°. 5 above that of Derby, the greatest heat 

 at the former must therefore exceed that at the latter in a still 

 higher degree. 



In the table given by Mr. Whitley, indeed, to record the fall of 

 rain, it would appear that the result obtained at these two places 

 is identical, namely 27 inches ; in which I cannot help suspecting 

 some inaccuracy. It seems to be almost necessarily refuted by 

 the mere fact of the difference of temperature. It cannot be sup- 

 posed, even according to the rules laid down by Mr. Whitley him- 

 self, that the small interval of latitude alone would suffice entirely 

 to account for the lower range of the thermometer at Derby ; nor 

 can it be credibly ascribed in any great degree to the nature of 

 the soil. What cause, then, remains but a greater proportion of 

 moisture ? This, indeed, may be said to act and re-act upon the 

 climate. The falling rain is probably in most cases itself colder 

 than the lowest strata of the atmosphere, and abstracts at once 

 some heat. When fallen, the process of evaporation begins, and 

 enormously accelerates that abstraction. In proportion as there is 

 naturally any scantier supply of heat to make up the loss, this is 

 the more felt in the decUne of temperature, and the process also 



