198 On Ducks — Drt/, Wet, and Floating. 



pared a nidus for the insect, and that the real cause of the 

 malady was the dense population, the consequent increase 

 of coal fires and smoke, and deterioration of the air, by 

 which it was rendered less fit for vegetable respiration. 

 Whether the newly planted trees have become acclimatized 

 I know not. 



The aphides of the potato may similarly be regarded as 

 the result of a diseased condition of the plant ; ancf I have 

 no fear that the health of new plants, raised from seed in a 

 new colony, would be readily afi'ected by them. 



XIII. 



On Ducks — Dry, Wet, and Floating. Bg His Excellency 

 Sir William T. Denison, F.R.S., F.G.S., &c. {Uead 

 nth July, 1849.] 



Thinking it desirable that the Papers of the Royal Society 

 should not be merely a record of new facts or observations 

 upon matters connected with tlie progress of the arts, but 

 that they should also contain such of the results of the 

 experience of the members as may be likely to prove of 

 practical use to the community, I have embodied in the 

 present paper some general observations on the subject of 

 the construction of dry docks, which will serve as an 

 introduction to the detailed drawings and descriptions of 

 some floating docks with which I was furnished some years 

 ago by some friends in America, and which I now present 

 to the Royal Society. 



