488 KEPOET — 1880. 



I am not aware that the integral of the above equation has ever before been 

 presented in finite terms ; on which account I make this commimication to the 

 British Association.' 



MONDA Y, A UG UST 30. 

 The following Reports and Papers were read : — 



1. Report of the Committee on Tidal Observations in, the English Channel, Sj'O. 



See Reports, p. 390. 



2. Beporf of the Committee on Luminous Meteors. — See Reports, p. 39. 



Report of the Committee on the qtiestion of Improvements in Astronomical 

 Clocks. — See Reports, p. 56. 



4. On a Septum permeahle to Water and impermeable to Air, with practical 

 applications to a Navigational Depth-gauge. By Professor Sir William 

 Thomson, At.A., F.B.S. 



A. small quantity of Avater in a capillary tube, with both ends in air, acts as a 

 perfectly air-tight plug against diiference of pressure of air at its two ends, equal 

 to the hydrostatic pressure corresponding to the height at which water stands in 

 the same capillaiy tube when it is held upright, with one end under water and the 

 other in air. And if the same capillary tube be held completely under water, it is 

 perfectly permeable to the water, opposing no resistance except that due to viscidity, 

 and permitting a current of water to flow through it with any difierence of pressure 

 at its two ends, however small. In passing it may be remarked that the same 

 capillary tube is, when not plugged by liquid, perfectly permeable to air. 



A plate of glass, or other solid, capable of being perfectly wet by water, with a 

 hole bored through it, acts similarly in letting air pass freely through it when 

 there is no water in the hole; and letting water pass freely through it when it is 

 held under water ; and resisting a difierence of air-pressures at the two sides of it 

 when the hole is plugged by water. The difierence of air-pressures on the two 

 sides which it resists is equal to the hydrostatic pressm-e corresponding to the rise 

 of water in a capillary tube of the same diameter as the narrowest part of the 

 hole. Thus a metal plate with a great many fine perforations, like a very fine rose 

 for a watering-can for flowers, fulfils the conditions stated in the title to this com- 

 munication. So does very fine wire cloth. The finer the holes, the greater is the 

 difference of air-pressures balanced, when they are plugged with water. The 

 shorter the length of each hole the less it resists the passage of water when com- 

 pletely submerged ; and the greater the number of boles, the less is the whole re- 

 sistance to the permeation of water through the membrane. 



Hence, clearly, the object indicated in the title is more perfectly attained the 

 thinner the plate and the smaller and more numerous the holes. Very fine wire 

 cloth woidd answer the purpose better than any metal plate with holes drilled 

 through it ; and very fine closelj'-woven cotton cloth, or cambric, answers better 

 than the finest wire cloth. The impenetrability of wet cloth to air is well known 

 to laundresses, and to every naturalist who has ever chanced to watch their 

 operations. The quality of dry cloth to let air through vdth considerable freedom, 

 and wet cloth to resist it, is well known to sailors, wet sails being sensibly more 



' The original paper is ready for the press, and will shortly be published. 



