1863.] 689 



taken with the Portable Electrometer in the meantime. On several 

 subsequent occasions the Portable Electrometer was used in con- 

 sequence of ice having formed in the Station Electrometer. On all 

 other occasions the observations have been taken with the Station 

 Electrometer, except when the electricity was too strong or too 

 variable to be conveniently measured by it. On one occasion, in the 

 first month of the observations (Oct. 22nd), in showery weather, 

 the earth-reading being one revolution and 19, the negative elec- 

 tricity of the air was so strong that six revolutions were not nearly 

 sufficient to bring the needle to the sight. Electricity of such 

 strength is very common during showers either of rain, hail, or snow, 

 being always positive during snow, generally negative during hail, 

 and generally negative (in some instances alternating with positive) 

 during heavy rain. On such occasions I have generally had recourse 

 to the " Portable," which is less sensitive. 



13. On some occasions, when the electricity of the air has been 

 strong, it has been so violent in its fluctuations from second to 

 second, that the needle of the " Station " was quite unmanageable 

 from the extent and rapidity of its vibrations. On such occasions, 

 which are denoted by the word "vibration" in the column "Re- 

 marks," I had recourse to the Portable Electrometer, whose needle, 

 in the same state of the atmosphere, frequently exhibited no vibra- 

 tion. Sometimes, however, the needle of the " Portable " has been 

 violently agitated, and on these occasions, which only occurred 

 when the electricity observed was very strong and of opposite kind to 

 that in the jar, I have always found that the jar was discharging. 

 On a few occasions, especially on the 6th and 7th of January, the 

 needle of the " Station," though not trembling as in the cases above 

 alluded to, has been remarkably unsteady, the air-reading changing 

 by several degrees every two or three seconds. 



14. In the Table of Comparisons of Electrometers, "Station I." 

 denotes the Station Electrometer with its first fibre ; " Station II.' 5 

 denotes the Station Electrometer with its second fibre; and "Por- 

 table " denotes the Portable Electrometer. The most probable values 

 of the ratios of their indications, as shown by their comparisons, may 



be assumed to be 



Portable : Station I. : : 1 : 8-5 



Portable : Station II. : : 1 : 3'1 

 Hence Station I. : Station II. : : 1 : -365. 



