ATMOSPHERIC ELECTRICITY. 67 
negative electricity by means of the small electrophorus which accom- 
panies the instrument. When enough has been given to bring the 
hair a little below the middle of the space between the black dots, 
give no more charge; but remove the charging rod and close the 
aperture immediately. If now the hair is still seen a little below 
the middle of the space between the black dots, turn the screw head 
in such a direction as to raise the attracting disc, and so diminish the 
attraction till the hair is exactly midway between the dots, Watch 
the instrument for a few minutes, and if the hair is seen to rise, as it 
generally will (because of the electricity, which has been given, 
spreading over the inner surface of the glass), turn the micrometer 
screw in the direction to lower the attracting plate, so as to keep the 
hair midway between the dots. 
(3) The insulation will generally improve for several hours, and 
sometimes for several days, after the instrument is first charged. The 
instrument may be considered to be in a satisfactory state if the earth 
reading does not diminish by more than 3U divisions per 24 hours, 
If the maker has been fortunate with respect to the quality of the 
substance of the glass jar, the earth reading may not sink by more 
than 30 divisions per week, when the pumice is sufficiently moistened 
with strong and pure sulphuric acid. Recharge with negative elec- 
tricity occasionally so as to keep the earth reading between 1000 
and 2000. 
II. To keep the instrument in order. Watch the pumice care- 
fully, looking at it every day. If it begins to look moist, remove the 
cover, take out the screws holding the lead cup, remove the pumice and 
dry it on a shovel over a fire. When cool, put prepared sulphuric acid 
on it, replace it in the instrument, and re-electrify according to No. I. 
Never leave the pumice unwatched, in the instrument, for as long as 
a week, WHEN THE INSTRUMENT IS TO BE OUT OF USE FOR A WEEK 
OR LONGER, TAKE THE PUMICE OUT OF IT. 
III. To use the portable electrometer for observing atmospheric 
electricity : 
(1) The place of observation, if on board ship, must be as far 
removed from spars and rigging as possible. In a sailing ship or 
rigged steamer the best position for the electrometer generally is over 
the weather quarter when under way, or anywhere a few feet above 
the tafferel when at anchor. On shore or on the ice a position not 
less than 20 yards from any prominent object (such as a hut or a 
rock or mass of ice or ship), standing up to any considerable height 
above the general level, should be chosen. Whether on board ship 
or in an open boat or on shore or on the ice, the electrometer may be 
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