3fn PLEASANT WA YS IN SCIENCE. 



the subject of dew in the commencement of the present cen- 

 tury. His observations were made in a garden three miles 

 from Blackfriars Bridge. 



Wells exposed little bundles of wool, weighing, when 

 dry, ten grains each, and determined by their increase in 

 weight the amount of moisture which had been deposited 

 upon them. At first, he confined himself to comparing the 

 amount of moisture collected on different nights. He found 

 that although it was an invariable rule that cloudy nights 

 were unfavourable to the deposition of dew, yet that on some 

 of the very clearest and most serene nights, less dew was 

 collected than on other occasions. Hence it became evident 

 that mere clearness was not the only circumstance which 

 favoured the deposition of dew. In making these experi- 

 ments, he was struck by results which appeared to be 

 anomalous. He soon found that these anomalies were 

 caused by any obstructions which hid the heavens from his 

 wool-packs : such obstructions hindered the deposition of 

 dew. He tried a crucial experiment. Having placed a 

 board on four props, he laid a piece of wool on the board, 

 and another under it During a clear night, he found that 

 the difference in the amount of dew deposited on the two 

 pieces of wool was remarkable : the upper one gained four- 

 teen grains in weight, the lower one gained only four grains. 

 He made a little roof over one piece of wool, with a sheet of 

 pasteboard ; and the increase of weight was reduced to two 

 grains, while a piece of wool outside the roof gained no less 

 than sixteen grains in weight 



Leaving these singular results unexplained for a while, 

 Dr. Wells next proceeded to test the temperature near his 

 wool-packs. He found that where dew is most copiously 

 produced, there the temperature is lowest Now, since it is 

 quite clear that the deposition of dew was not the cause of 

 the increased cold for the condensation of vapour is a pro- 

 cess producing heat it became quite clear that the forma- 

 tion of dew is dependent on and proportional to the loss of 

 heat 



