Observations on the Blacking of Garden Walls. 331 



known that surfaces which absorb heat most quickly, part 

 with it more easily by radiation when the source of heat is 

 withdrawn, and cool quicker. In the summer time, when 

 the days are long, the wall will be more intensely heated 

 under the blackened surface, and the night (or time of 

 cooling) being short, it may not have returned to the tem- 

 perature of the air, before it is again subjected to an increase 

 of heat. If the time of cooling were long enough, that part 

 of the wall under the blackened surface, might become 

 actually colder than the part not blackened, and thus the 

 extremes of heat and cold be greater than when the wall was 

 left with its usual surface. In the summer time, however, 

 the wall is not only more intensely heated, but probably 

 retains a great portion of the heat during the night. Horti- 

 culturists will decide which of those two causes is efficient 

 in producing the effect I have stated, or whether both may 

 not co-operate ; it is not for me to presume to do so, though 

 I should be inclined to think, that in this climate, the inten- 

 sity had more influence than the uniformity. 



I have the honour to be, 



your very obedient servant, 



Henry Dawes. 



Shugh, near Windsor, 

 October 16, 1818. 



VOL. III. 



X x 



