224 On the Effects produced by a Fluid in Motion. 



air can be carried down by a stream of water falling through 

 a tube. 



1 . If the area of the cross section of the tube through which 

 the water falls be not much greater than that of the orifice from 

 which the water flows, disks will be formed in the tube, and, 

 being pushed down by the descending stream, will force down 

 the air beneath them. 



2. If the area of the cross section of the tube through which 

 the water falls be much greater than that of the orifice from 

 which the water flows, so that disk action is prevented, or if the 

 pressure on the lower end of the tube be competent to support a 

 column of water in the tube at such a distance from the orifice 

 from which the water flows that the descending stream has not 

 widened sufficiently to allow of the formation of disks, air will 

 be carried beneath the water-surface on account of the formation 

 of cavities, according to the theory of Magnus. 



3. If there is not a great depth of water in the vessel which 

 supplies the descending stream, or if (the depth not of necessity 

 being small) rotatory motion is from any cause imparted to the 

 water, air will enter through a cavity formed above the orifice 

 from which the descending stream issues, and extending into the 

 descending stream. 



4. If the area of the cross section of the orifice from which 

 the water flows be as great, or nearly as great, as that of the 

 tube through which the water falls, and if at the same time the 

 orifices for the admission of air do not exceed a certain area 

 compared with that of the orifice from which the water flows, 

 air will enter at the rupture of the stream, produced at the 

 orifices by the accelerated motion of the water below those 

 orifices. 



The cause of the descent of air in the different modifications 

 of the trompe is not due to any one action of a stream of water ; 

 air is carried down by all four of the modes of action mentioned 

 above. 



Generally only one mode obtains in one form of the machine; 

 but there may be two modes acting simultaneously, the parti- 

 cular mode or modes being determined (a) by the relation of the 

 area of the cross section of the trompe-tube to that of the orifice 

 from which the stream flows ; (b) by the head of water above 

 the orifice from which the stream flows ; (c) by the fact of whether 

 there are causes which induce rotatory motion in the water before 

 it leaves the cistern ; (d) by the form of the orifice from which 

 the stream flows; (e) by the manner in which air is allowed 

 access to the interior of the tube; and lastly, (/) by the amount 

 of pressure on the lower orifice of the tube. 



The first and fourth modes least seldom obtain ; the second 



