146 



THE MECHANICS OF THE EARTHS ATMOSPHERE. 



when the pressure on the rubber tube is periodically increased and 

 diminished for a long time the continuity of the jet is not completely 

 broken. Such a jet presents a very remarkable appearance, which is 

 reproduced as well as possible in Fig. 18. 



The phenomena hitherto described occur with 

 difi'erences of pressure of GO millimetres at the 

 maximum, but very different results are obtained 

 |if larger differences of pressure are used. With 80 

 or 90 millimetres we obtain jets of the greatest 

 sensitiveness. By every small disturbance the con- 

 tinuity of the jet is broken, and it must then form 

 for itself a new path every time. Above 100 milli- 

 metres difference of pressure there are formed only 

 very short jets in the immediate neighborhood of 

 the opening. These at a slight altitude break up 

 into a cloud of individual small drops that under 

 the rapid motion immediately mix with the color- 

 less liquid. 



When (as an experiment) colored liquids were 

 used whose specific gravity differed considerably 

 from that of the colorless water, no regular dis- 

 continuous currents could be obtained; thus in 

 one experiment a solution of salt was added to 

 the colored water, in another experiment some 

 alcohol was added. The salt solution immediately 

 after its discharge fell in thick, irregular, heavy 

 drops back upon the discharge pipe, while the alco- 

 hol moved in very thin threads, frequently broken 

 up, toward the upper free surface of the water. 



From the experiments hitherto described it follows that in fact steady 

 jets form with small differences of pressure. The viscosity therefore 

 does not prevent discontinuous currents. Viscosity appears in general 

 to exert so unimportant an influence upon the cylindrical portion of the 

 jet formation that we are tempted to assume the real possibility of the 

 sliding of moving particles of water past those at rest, as the simpler 

 theory assumes to be the fact, without any consideration of viscosity. 

 If, however, the transition from the finite velocity of the jet to the quiet 

 fluid does not take ])lace within the thickness of a mathematical cylin- 

 drical surface but gradually within a layer of a definite thickness then 

 this thickness can only be extraordinarily small and appears not to 

 change with the time. That on the other hand the viscosity plays an 

 important role in the origin of the jets is already mentioned above. The 

 principal proof of this consists in the invariable formation of spiral sur- 

 faces of rotation into which the jet is transformed. The origin of these 

 assumes that the colorless liquid in the neighborhood of the jet r< ceives 

 a certain velocity in the direction of the jet. 



