an Elster and Geitel Electrical Dissipation Apparatus. 487 



case of a + , and for 5 out of 9 in the case of a_. In only one 

 out of 12 summer months was there an association of high 

 values of a + and a_ with high pressure. Tables VII. and 

 VIII. agree in indicating a tendency for high values of 

 a_/a + to be associated with highpressure ; but Table V. gives 

 no support to this conclusion except in equinox. The 

 association of high potential with high pressure is marked. 

 According to Table V. it is reduced in winter, but still 

 winter contributes 9 months out of the 28 in which the 

 association ship appears. 



Tables VI. , VII., and VIII. agree in associating high 

 values of a + and a_ and low values of P with the prevalence 

 of much cloud. The apparent influence of cloud seems 

 remarkably alike in a + and a_ according to Table VI. It 

 appears greatest in winter, and is distinctly larger in the 

 case of dissipation than in that of potential. 



§ 10. The question of the influence of cloud is complicated 

 by the facts that there are a number of different types, 

 representing different meteorological conditions, and that 

 the relative frequency of the various types varies with the 

 season of the year. An attempt was made to ascertain 

 whether the electrical phenomena associated with the different 

 types of cloud differed. The difficulty at once presented 

 itself that upper and lower clouds are usually both present, 

 and that not infrequently there is more than one type repre- 

 sented, both in the lower and in the upper. There were, 

 however, some types which occurred alone in a sufficient 

 number of instances to warrant the hope that conclusions of 

 fair reliability might be obtained. These were stratus 

 (including ordinary low stratus and alto-stratus), cumulus 

 (at all levels including fracto-cumulus, but not strato- 

 cumulus), and cirro-stratus (at high levels). In a good 

 many months there were no representatives of one or other 

 of these types, and in many other months there were only 

 one or two representatives. Thus the method of deriving 

 mean values of dissipation and potential for days of each 

 species of cloud in each month was unworkable. The plan 

 adopted was to examine each day by itself, and classify 

 separately the corresponding a_,a + , and P, according as the 

 value was above or below the mean value for the month. 



A difficulty, however, presented itself in the interpretation 

 of the results. None of the three quantities a_, a + , or P 

 has its values occurring symmetrically with respect to its 

 arithmetic mean. Taking all the observations, the number 

 of occasions having values larger than the mean is very 

 decidedly less than the number having values below the mean. 



