WALLS, ESPALIER-RAILS, AND TRELLIS-WORK. 



145 



sunshine, the one operation neutralizes the other. If, indeed, we could 

 ensure a powerful absorption from a bright sun during the day, and 

 retain the radiation by a canvas or other screen during the night, a 



considerable increase of temperature might probably be the result; 

 but the number of cloudy days in our climate in proportion to those 

 of bright sunshine is not favourable to such an experiment. 



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