162 ESSAY 



than grass very near to them, and similarly ex- 

 posed to the heavens. As the night proceeded, 

 clouds formed and accumulated ; in consequence 

 of which the difference at sunrise, between the 

 temperatures of the grass and the gravel walk, 

 was only 6, and between those of the grass and 

 the mould only 4, the temperature of the grass 

 having in the mean time increased considerably, 

 while that of the other bodies had decreased a 

 little. At another time, shortly before sunrise, 

 a very clear morning having succeeded a cloudy 

 night, I found the gravel walk to be 10 and the 

 garden bed to be 9 warmer than neighbouring 

 grass, which was 8 colder than the air. Both 

 of these examples occurred in summer, and I 

 believe, that such considerable differences will 

 occur in that season only. It was on the first of 

 these two nights, that 10 grains of wool gained 

 only \ a grain of moisture on the mould, and 

 that the same quantity gained no weight on the 

 gravel walk. That the unfitness of the gravel 

 walk, however, to become cold, like its unfitness 

 to attract dew, arose from its situation, and not 

 from the nature of the substance of which it was 

 made, is proved by this circumstance, that river 

 ^sand, placed on the raised board, was on 4 dif- 

 ferent nights, none of them highly favourable for 

 the production of cold, 7, 7, 8, and 8^ degrees 

 colder than the air at the same height. 



