688 [June 18, 



as the variation generally amounts to two or three degrees every 

 minute. 



9. Of late each observation has consisted of an earth-reading, 

 followed by five air-readings, taken at intervals of a minute. During 

 the first three months of observation the number of consecutive 

 readings was irregular, and the interval between them was not always 

 the same. The loss of charge in ten minutes is practically insensible, 

 and is sometimes more than counterbalanced by changes in the zero 

 of torsion produced by the strain upon the fibre during the observa- 

 tion. The only case in which any considerable los's occurs is when 

 the electricity of the air is very strong negative (that of the jar being 

 positive), causing a very high air-reading. The jar holds very well 

 when the reading is 360 (corresponding to 60 units) ; and I gene- 

 rally give it a charge of about this amount, renewing it when it has 

 fallen a little below 250 (or 50 units). 



10. The Station Electrometer stands on the shelf at the distance 

 of about a foot from the can. In taking earth-readings, the elec- 

 trode is connected with one of the brass foot-screws which support 

 the instrument, the connexion being made by a brass wire. In taking 

 air-readings, the electrode is connected with the can by means of a 

 copper wire. The square roots are generally taken on the spot by a 

 table which serves for all whole degrees from to two revolutions. 



11. The Station Electrometer has never been removed from its 

 place except during very cold weather, when the sulphuric acid at 

 the bottom of the jar sometimes became frozen over, so that the 

 needle was prevented from turning ; and it was necessary to remove 

 the instrument to a warm room, sometimes to thaw the ice, and 

 sometimes to prevent it from freezing. The ice in question is, I am 

 informed, not frozen water, but a definite compound of sulphuric 

 acid and water. It was most liable to form when the acid had been 

 for a long time unchanged, but I have seen a little of it when the 

 acid had been less than a week in the jar. It never formed unless 

 the temperature of the instrument was as low as about 20. 



12. On the first occasion of its formation, being taken by surprise, 

 and not using proper precautions, I broke the glass fibre which sup- 

 ported the needle : this was on the 3rd of December. A new fibre 

 was put in on the 6th, and the sights were adjusted to suit the new 

 fibre on the 8th, the observations of atmospheric electricity being 





