142 ESSAY 



found to have increased 16 grains in weight, 

 but that on the gravel walk only 9, and that on 

 the garden mould only 8 On another night y 

 during the time that 10 grains of wool, laid 

 upon grass, acquired 2^ grains of moisture, the 

 same quantity gained only 3- a grain upon the 

 bed of garden mould, and a like quantity, 

 placed upon the gravel walk, received no acces- 

 sion of weight whatever. 



Two objections will probably be made against 

 the accuracy of these, as well as my other ex- 

 periments with wool. One is, that wool placed 

 on grass may, by a kind of capillary attraction 

 receive dew previously formed on the grass, in 

 addition to its own. To this I answer, that 

 wool in a china saucer, placed on the grass, ac- 

 quired very nearly as much weight, as an equal 

 parcel immediately touching the grass. The 

 second objection is, that a part of the increased 

 weight in the wool might arise from its im- 

 bibing moisture, as a hygroscopic substance. I 

 do not deny, that some weight was given to the 

 wool in this way; but it may be safely affirmed, 

 that this quantity must have been very small. 

 For, on very cloudy nights, apparently the best 

 fitted to increase the weight of hygroscopic 

 substances, wool upon the raised board would, 

 in the course of many hours, acquire little or 

 no weight j and in London, 1 have never found 



