June 22, 1906.] 



SCIENCE. 



953 



The advantage of the method is its independ- 

 ence of the drop in pressure if this exceeds a 

 certain value. 



Since the announcement by A. Wood and 

 A. E. Campbell ° of the probability of cosmical 

 radiation as evidenced by the existence of a 

 daily period of the same, showing maximum 

 ionization between 8 and 10 a.m. and 10 p.m. 

 and 1 A.M., minimum ionization at about 2 



rapidly with the pressure difference and hence 

 with the barometer, etc., and great care must 

 be taken with these details. This, however, 

 has been done and the results obtained are 

 given in the following figure. The ordinates 

 show the angular diameter, s, of the successive 

 coronas, from which the number of efficient 

 nuclei, «•, per cubic centimeter may be ob- 

 tained. Observations were made at about 9 



Upper curve: Relative values of the angular diameters of coronas for the same drop of pres- 

 sure, on the days and hours given by the abscissas. The branches a are in agreement with the Wood- 

 Campbell discovery; the branches e show a tendency to inversion; r denotes rain. Loicer curve: 

 nucleations in ten thousands of colloidal nuclei per cubic centimeter of du?t-free air computed 

 from the preceding curve. 



P.M. and 4 a.m., I have taken the subject up 

 again. It seems possible that I overestimated 

 the sensitiveness of the earlier method. I 

 have, therefore, changed it in the present ex- 

 periment, replacing the large terminal coronas 

 by the small coronas very near the fog limit. 

 The observations, in other words, are now 

 made with a drop in pressure but just suffi- 

 cient to produce coronal condensation on the 

 larger colloidal nuclei of dust-free air 

 (8p = 21 cm.). The sizes of coronas vary 



-Nature, 1906, Vol. 73, p. 583. Reference is 

 also due to the work of Burton and McLennan. 



A.M. and 3 p.m. (as near the time of the Wood 

 and Campbell maxima and minima as my 

 duties permitted) on the successive days and 

 hours given by the abscissas. 



The figure shows, in the first place, that 

 minima and maxima of nucleation would gen- 

 erally have to appear at about the time at 

 which Wood and Campbell observed maxima 

 and minima of ionization, respectively; or 

 that an inversion of Wood and Campbell's re- 

 sults is in question, since there is usually in- 

 cremented nucleation in the afternoon as com- 

 pared with the morning. This, however, may 



