a^HOMSOJ^.— ATMOSPHEKIC ELECTRICITY. 21 



clean and free from dust or fibres, and the pumice dry. To pre- 

 pare it for use : 



(1.) Remove from the top the cover carrying the pumice. 

 Drop upon the pumice a small quantity of the prepared sulphuric 

 acid supplied with the instrument, distributing it as well as may 

 be over the whole surface of the stone. There ought not to be 

 so much acid as to show almost any visible appearance of moisture 

 when once it has soaked into the pumice. Replace the cover 

 without delay, and screw it firmly in its proper position, and 

 then leave the instrument for half an hour or an hour, or any 

 longer time that may be convenient to allow the inner surface of 

 the glass to be well dried through the drying effect of the acidu- 

 lated pumice on the air within. 



(2.) Turn the micrometer screw till the reading is 2,000. 

 (There are 100 divisions on the circle which turns with the screw 

 on the top outside, and the numbers on the vertical scale inside 

 show full turns of the screw. Thus each division on the vertical 

 scale inside corresponds to 100 divisions on the circle ; and 20 

 on the vertical scale is read " 2,000.") Introduce the charging rod 

 and give a charge of negative electricity by means of the small 

 electrophorus which accompanies 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 thie 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 30 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. Re- 

 charge with negative electricity occasionally so as to keep the 

 earth reading between 1,000 and 2,000. 



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 the galley 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. 



