PHENOMENA CONNECTED WITH CLOUDY CONDENSATION. 213 



and the suddeu increase in density is probably due to the temperature 

 falling- low enough for the films of the drops to have a repulsive action, 

 sufficient to prevent them from coalescing. 



■). Rough nozzles and obstnictious hi front of jets. — If we use a nozzle 

 of irregular form, or having roughened edges, it is found that it gives 

 a dense condensation at a lower pressure than a nozzle of circular sec- 

 tion with smoothbore and thin, even edges. This is owing to the irreg- 

 ularities in the nozzle, producing eddies in the jet, and mixing a greater 

 amount of air with the steam, so cooling it more and supi)lying it with 

 a greater number of nuclei. It, in fact, acts in the same direction as 

 increase of pressure, and aids pressure in producing its results with 

 a less velocity of steam. 



An obstruction in front of the jet acts in a similar manner, if we 

 have a jet of steam of such a pressure that at the temperature of the 

 air it gives oidy the ordinary form of condensation. If now we place 

 an obstruction in front of the jet so as to produce eddies, the conden- 

 sation at once becomes dense. Wind has also a somewhat similar 

 effect. The reason of the increased density in these cases is the same 

 as for the jets issuing from irregular nozzles. They all assist the f>res- 

 sure in intensifying the density of the condensation by lowering the 

 temperature of the jet, increasing the number of nuclei and quicken- 

 ing the rate of condensation. 



The seat of the sensitiveness of the jet. — The seat of maximum sensi- 

 tiveness to all influences tending to change the condensation from 

 ordinary to dense is near the origin of the jet close to the nozzle. The 

 different influences, however, affect the jet to different distances from its 

 origin. The most limited in the range of its action is cold, which only 

 produces the dense condensation when it acts near the nozzle, whereas 

 some of the other influences have some effect, though a gradually 

 decreasing one, to a distance of li or 3 centimeters from the nozzle. 



The following experiment illustrates the limited range of the action 

 of cold: A piece of ice about 2 centimeters thick was selected, and a 

 small hole bored through it. The ice was then held so the steam jet passed 

 thiough the opening. While the ice was held at a distance of 1 centi- 

 meter from the nozzle, almost no eflect was produced, though nuich 

 cold air from the ice was mixing with the jet. But when the ice was 

 brought nearer the origin of the jet, so that the nozzle almost entered 

 the plane of the ice, the dense condensation immediately appeared. 



The range of sensitiveness of the jet to change of condensation by 

 obstructions is also very limited. It is only when the obstruction acts 

 near the nozzle that its eflect is great. For instance, the blade of the 

 knife resting on the nozzle with its back or edge pointing in the direc- 

 tion of the jet, and depressed so as to deflect the jet sligljtly, causes 

 the jet to become very dense. But if the knife acts on the jet at a dis- 

 tance of only 1 centimeter from the nozzle very little increase in density 

 is produced. 



