216 Mr. G. F. Rodwell on the Effects 



which it passes, or if there is sufficient pressure on the orifice of 

 the tube to support a column of water at a height in the tube 

 above that at which the stream is sufficiently broad to allow 

 disks to be formed, the stream meets the water surface in direct 

 collision, and air is then undoubtedly carried down, for the reason 

 proposed by Prof. Magnus*. This physicist observed that small 

 solid bodies, if allowed to fall into water, produce a cavity of 

 greater or less depth, into which air enters ; and the water, if the 

 particles at its surface possess any motion (which they must do 

 from the disturbance caused by the falling body), unites over the 

 cavity and thus air is enclosed. Magnus conceives that a water- 

 jet falling into water acts in a precisely similar manner, the de- 

 tached masses of water forming cavities like solid bodies. We 

 have a frequent example of this action of detached fluid masses : 

 if we wish to pour out beer so that it shall have no froth, we 

 pour it down the side of the glass, the adhesion of which flat- 

 tens the stream into a ribbon, and it enters the fluid in the glass 

 slowly : there is no falling of detached masses here, consequently 

 no air is carried down ; on the other hand, if we wish to produce 

 froth, we pour out the beer from as great a height as possible, 

 and the masses detached by the accelerating force of gravity 

 carve out channels in the liquid, into which air enters and is car- 

 ried down, and afterwards rises to the surface in bubbles. 



Richard f states that a M. Mercadier constructed artificial 

 trompes in which sand, wheat, rye, millet, salt, mercury, and lead 

 shot were severally caused to fall into water in place of a descend- 

 ing stream of that liquid, and a considerable blast of air was 

 obtained thereby. 



The quantity of air carried down by a solid body falling into 

 water is very surprising. I took small lead shot about y^-th of 

 an inch diameter, and weighing *072 gramme apiece, and threw 

 them one at a time from a height of about a foot and a half 

 into a vessel containing water to a depth of 6 inches ; in order 

 to collect the air, a wide-mouthed vessel was filled with water, 

 inverted, and placed with its mouth near the water-surface in the 

 vessel into which the shot was thrown. By throwing in a shot 

 at an angle of about 60 degrees, and as near as possible to the 

 edge of the inverted vessel, the air carried down could be collected, 

 and I thus found that the lead shot caused no less than 190 

 times its own bulk of air at 18° C. to penetrate beneath the 

 water. The same shot falling by its own weight from a height 

 of 4 feet into 6 inches of water appeared to carry down quite as 

 much air, but in this case it is obvious the amount could not be 

 determined. 



* "On the Motion of Fluids," Phil. Mag. for January 1851, p. 8. 

 t Etudes sur I'art d'extraire imme'diatement le Fer de ses Minerals sans 

 converter le me'tal en font e. Paris, 1838. 



