1863.] 695 



23. Altogether there are 20 days (out of ahout 170) on which 

 negative electricity has been observed ; but on every one of these 

 days positive electricity was observed also. With the exceptions of 

 December 10th arid 19th, negative electricity has only been observed 

 either during downfall (i. e. rain, hail, sleet, or snow) or immediately 

 before or after it. In these two exceptional instances the sky was 

 entirely covered with nimbi, and the negative electricity observed 

 was weak. In the latter the observation was taken about 9 P.M. ; 

 and the next following observation, taken between 8 and 9 A.M. the 

 next morning, showed positive electricity of unusual strength. 



There are only 2 days (February 4th and March 14th) on which 

 the strength of electricity, when there was no downfall or fog, has 

 been as high as 10, and on these two occasions the temperature of 

 the air was below zero. The average strength of electricity, giving 

 equal weight to all observations, and excepting those taken during 

 downfall or fog, has been 4-2 or4'3, the averages for the respective 

 months being Oct. 3'3, Nov. 3'1, Dec. 4'0, Jan. 4-2, Feb. 5'6, 

 March 5'5. . 



24. With the view of investigating the diurnal range of atmospheric 

 electricity, I have added, for each month, all observations taken 

 during the same hour, and have divided the sums by the numbers of 

 observations. By " observations " I mean, here and during the re- 

 mainder of this section, the numbers representing the mean electrical 

 potential for each group of readings, as entered in the complete 

 tabular statement already given, except when there are two or more 

 such entries for the same hour, in which case their arithmetical mean 

 has been adopted and reckoned merely as one observation. 



Again, dividing the day into three portions before noon, noon to 

 6 P.M., and after 6 P.M., I have divided the sum of all observations 

 taken in the same portion of the day, for each month, by the number 

 of observations ; and I have, in the same manner, found the mean 

 potential "at all hours" by dividing the sum of all observations 

 taken during a month by the number of observations. 



It will be seen that for every month of the six the electricity is 

 weaker after 6 P,M. than in either of the previous portions of the 

 day. (See Table III.) 



25. The average potential at each hour for the 6 months may be 

 found either (1) by dividing the sum of all observations taken at the 



