io8 



A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. 



Figure 7. — Chart Givinc. d in Terms oi- s. 



Improved Method. 



5. Number of droplets. — The following results were obtained with the 

 definite form of apparatus shown in figure i. A method of estimating the 

 nucleation from a direct count made under the microscope is obtained as follows : 

 Let the plate be so rotated eccentrically as to catch the descending fog particles 

 for a definite interval of time, /. If v be their velocity of subsidence, all par- 

 ticles within a height, li, will be caught, if 



h = vt (i) 



and V = io''dy3.24 (2) 



where the usual value of the viscosity of air has been inserted. Furthermore, 



111 grams are precipitated per cub. cm. by the given exhaustion, and if n be the 



nucleation 



n = dm/^d^. (3) 



Finally, if c is the area of the field seen in the microscope and n' the number of 

 particles falling into this field 



n' = nhc. (4) 



From these equations n is obtained by eliminating v as 



^ 



31 ^ 2.1 1 X n' 



M= ; 



ii_m=''i06. 



The values of the constants usually; adopted were t = 30 sec, c = 144 X 10 " sq. 

 cm., nf^^= 2.8 X io~\ whence \i'm=i.75 X «'. The experiments to test this 



