250 A. deF. Palmer^ Jr. — Condensation in the Stemn Jet. 



its plane contained the vertical axis of the jet and was perpen- 

 dicular to that of the camera. 



Fig. 3 (Plate lY) is a reproduction of one of the series of 

 photographs, and though quite inferior to the negative in point 

 of definition and detail, gives a very good idea of the general 

 form of the jet. The values of P in centimeters of mercury 

 and of D in millimeters, corresponding to this series, are given 

 in the following table, the numbers being taken from left to 

 right. 



No. 



1. 



2. 



3. 



4. 



5. 



6. 



1. 



8. 



9. 



JO. 



p 



43-8 



42-3 



40-8 



37-8 



35-3 



30-3 



19-6 



36-8 



28-3 



13-8 



D 



6-0 



6-0 



6-7 



7-1 



7-1 



7-3 



9-1 



7-7 



7-4 



7-1 



Fifty series of ten exposures each were made and the dis- 

 tances D corresponding to values of Pup to about sixty-five 

 centimeters of mercury very carefully measured on the nega- 

 tives. Above this point the lower limit of condensation was 

 always in coincidence with, or possibly below, the plane of the 

 nozzle, showing that at very high pressures the condensation 

 phenomena take place entirely within the nozzle. The result 

 of these observations is shown graphically on the chart fig. 4, 

 the abscissas being values of P to the scale one-half and the 

 ordinates values of D to the same scale. All the points here 

 represented have very nearly equal weight, and though errors 

 of a few tenths of a millimeter might easily be made in the 

 determination of D, the large variations in this quantity corre- 

 sponding to the same pressure cannot be accounted for by 

 errors of observation. Neither can they be explained by varia- 

 tions in the temperature and dust content of the air surround- 

 ing the jet, since the greatest change in this temperature dur- 

 ing all the series of observations did not exceed two or three 

 degrees centigrade as indicated on a mercury thermometer 

 placed near the jet, and since many of the largest variations 

 correspond to exposures the interval between which did not 

 exceed a few seconds. 



These considerations, together with the appearance of the 

 image of the jet on the ground glass of the camera, led me to 

 the opinion that the rate of condensation varied periodically, 

 and hence that D, for moderate pressures at least, was a periodic 

 function of the time. For the further study of this point I 

 constructed a shutter having an electrical connection with the 

 laboratory clock such that exposures of about one-tenth second 

 could be repeated at intervals of one second. The actuating 

 pressure being kept constant, a series of some thirty such expo- 

 sures were made on the same plate and the distances D corre- 

 sponding to each carefully determined. Eight such series of 

 observations were made corresponding to different values of the 

 pressure and the result of one of these is represented in fig. 5, 



I 



