produced by a Fluid in Motion, 215 



so greatly accelerated by gravity, that the cohesion of two con- 

 tiguous cross sections of the jet is destroyed, and the jet, ceasing 

 to be continuous, breaks up into separate masses of water. We 

 observe, moreover, that when the continuity begins to be broken, 

 particles of water are thrown out from the centre stream and 

 move downwards in a gradually diverging line inclined at a small 

 angle to the main stream. Now when such a stream passes 

 downwards through a long vertical tube, the area of the cross 

 section of which is not many times as great as that of the 

 orifice from which the stream issues, we find that at a certain dis- 

 tance from the orifice the quantity of spray thrown off from the 

 stream which reaches the sides of the tube is so great that, by 

 the adhesion of the water-particles to the glass, the whole stream 

 is dragged to the sides of the tube; it is not, however, dragged 

 to one side of the tube, as would be the case if the jet were not 

 perfectly vertical, but it is evenly and equally spread out in a 

 dome shape, and for the rest of its course Hows as a tube of water. 

 If the lower orifice of the tube through which it flows is perfectly 

 free, the water as it passes out forms a long conical bag filled 

 with air. 



Above the point where the jet is spread out by the adhesion 

 of the glass there is a good deal of spray ; and although it is 

 insufficient to drag the jet to the sides of the tube, it accumu- 

 lates ; the tube is thus narrowed at certain points, and disks of 

 water are formed which are pushed down by the stream above 

 and force down air beneath them : at the moment of the forma- 

 tion of a disk the conical bag of water at the end of the tube is 

 expanded laterally by the air forced into it, and bursts, deliver- 

 ing up its contents; this bursting occurs with great rapidity 

 under certain conditions. 



The pressure on the lower orifice of the vertical tube through 

 which the stream passes also tends to the formation of disks, 

 because the descending masses of water will have a tendency to 

 be flattened out by the air beneath them ; the greater the pres- 

 sure, the higher within certain limits will disks be formed ; but 

 the quantity of air forced down will be less, because the chance 

 of the rupture of the disks will be increased ; and when this 

 occurs, the air beneath them escapes upwards between the de- 

 scending stream and the sides of the tube. The formation of 

 disks can only happen below a certain point, because the stream 

 above that point ceases to be sufficiently broad to allow of their 

 being produced; the point varies with the nature of the stream, 

 the relative dimensions of the tube and of the orifice from which 

 the stream flows, and with the pressure on the lower orifice of 

 the tube. 



If a stream is very small compared with the tube through 



