Pressure on a given Surface. 209 



In the experiment of Mr. Rodwell (p. 105), in which a stream 

 of water descending freely from the orifice of a vertical pipe 

 withdraws water from the orifice of a horizontal pipe placed in 

 contact with the stream near the point of issue, the " lateral 

 action " is explained by " a strain upon the particles which acts 

 against the lateral force of the stream ; " but surely this is only 

 removing the difficulty one step further, for what causes the 

 " strain." To this I would simply reply, " Cohesion : as soon as 

 the velocity of the issuing molecules exceeds that at which they 

 will slide freely over each other, the pressure at the orifice of the 

 horizontal pipe becomes negative, and fluid escapes until the 

 negative pressure is balanced by the weight of the column vir- 

 tually raised." Precisely the same remarks will apply to the 

 steam-jet, except that when the lateral tube contained air only, 

 the force in action was adhesion of the molecules of air to those of 

 vapour. The "inner cone" mentioned in p. 107 I presume to 

 be the space occupied by "dry" steam. 



Before quitting this part of the subject, permit me to notice a 

 remark of Professor Magnus in the paper alluded to. After de- 

 scribing the water-bellows (in which water descending by its own 

 gravity in a large vertical pipe about 13 feet long draws in air 

 through a number of orifices, which being carried dow T n by the 

 stream into a water-closed chamber, produces a stream of air 

 sufficient for blowing a furnace), he observes that " the real phy- 

 sical cause of the descent of the air is still totally unknown." 

 Surely in this case the negative pressure of the water against 

 any given point of the tube arises from the antagonism between 

 the cohesion of the molecules above to the fluid at rest, and the 

 downward pressure of the molecules below, due to their acquired 

 energy. These opposite forces acting on any two vertically con- 

 tiguous molecules will tend to separate them, and thus to pro- 

 duce a negative pressure on the horizontally adjacent molecules, 

 which is transferred to the inner surface of the pipe ; and any 

 air-bubbles that enter the orifices near the top of the pipe enable 

 the molecules below them to acquire more vis viva than those 

 above, and thus the negative pressure becomes augmented as the 

 fluid descends. 



In regard to Mr. RodwelPs first experiment, there is a remark- 

 able resemblance between his lycopodium- figure and those ob- 

 served by Strehlke* when a drop of fluid covered with lycopo- 

 dium is deposited on a vibrating plate near a nodal line ; he 

 also observed that when drops were similarly placed on opposite 

 sides of the line, the gyratory motion was (as might be expected) 

 in opposite directions. 



The second memoir to which I proposed to allude is that of 

 * PoggendorfPs Annalen, vol. xl. p. 146. 



Phil Mag. S. 4. Vol. 33. No. 222. March 1867. P 



